'

THE ORIGIN OF VERTEBRATES

WALTER HOLBROOK GASKELL

THE

ORIGIN OF VERTEBRATES

THE

ORIGIN OF VERTEBRATES

BY

WALTER HOLBROOK GASKELL

M.A., M.D. (CANTAB.), LL.D. (EDIN. AND McGILL UNIV.) ; F.R.S. ; FELLOW OF TRINITY
HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE ; HONORARY FELLOW
OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER
OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC.

LONGMANS, GREEN, AND CO

39 PATERNOSTER ROW, LONDON

NEW YORK, BOMBAY, AND CALCUTTA

1908

All rights reserved

CONTENTS

PAGE

Introduction 1

CHAPTER I
The Evidence of the Central Nervous System

Theories of the origin of vertebrates — Importance of the central nervous system
— Evolution of tissues — Evidence of Paleontology — Reasons for choosing
Ammoccetes rather than Amphiosus for the investigation of this problem —
Importance of larval forms — Comparison of the vertebrate and arthropod
central nervous systems — Antagonism between cephalization and alimenta-
tion — Life-history of lamprey, not a degenerate animal — Brain of Ammo-
ccetes compared with brain of arthropod — Summary . . ■ . . 8

CHAPTER II

The Evidence of the Organs of Vision

Different kinds of eye — Simple and compound retinas — Upright and inverted
retinas — Median eyes — Median or pineal eyes of Ammoccetes and their
optic ganglia — Comparison with other median eyes — Lateral eyes of verte-
brates compared with lateral eyes of crustaceans — Peculiarities of the
lateral eye of the lamprey — Meaning of the optic diverticula — Evolution
of vertebrate eyes — Summary 68

CHAPTER III

The Evidence of the Skeleton

The bony and cartilaginous skeleton considered, not the notochord — Nature of
the earliest cartilaginous skeleton — The mesosomatic skeleton of Ammo-
ccetes ; its topographical arrangement, its structure, its origin in muco-
cartilage — The prosomatic skeleton of Ammoccetes ; the trabeculse and
parachordals, their structure, their origin in white fibrous tissue — The
mesosomatic skeleton of Limulus compared with that of Ammoccetes ;
similarity of position, of structure, of origin in muco-cartilage — The
prosomatic skeleton of Limulus ; the entosternite, or plastron, compared
with the trabeculse of Arnmocoetes ; similarity of position, of structure, of
origin in fibrous tissue — Summary 119

31233

vi CONTENTS

CHAPTER IV
The Evidence of the Respiratory Apparatus

r.\r, i.

Branchiae considered as internal branchial appendages — Innervation of branchial
segments — Cranial region older than spinal — Three-root system of cranial
nerves : dorsal, lateral, ventral — Explanation of van Wijhe's segments —
Lateral mixed root is appendage-nerve of invertebrate — The branchial
chamber of Amniocoetes — The branchial unit, not a pouch but an appendage
— The origin of the branchial musculature — The branchial circulation — The
branchial heart of the vertebrate — Not homologous with the systemic heart
of the arthropod — Its formation from two longitudinal venous sinuses —
Summary 148

CHAPTER V
The Evidence of the Thyroid Gland

The value of the appendage-unit in non-branchial segments — The double nature
of the hyoid segment — Its branchial part — Its thyroid part — The double
nature of the opercular appendage — Its branchial part — Its genital part-
Unique character of the thyroid gland of Ammoccetes — Its structure-
Its openings — The nature of the thyroid segment — The uterus of the
scorpion — Its glands — Comparison with the thyroid gland of Ammoccetes—
Cephalic generative glands of Limulus — Interpretation of glandular tissue
filling up the brain-case of Ammoccetes — Function of thyroid gland —
Relation of thyroid gland to sexual functions — Summary .... 185

CHAPTER VI

The Evidence of the Olfactory Apparatus

Fishes divided into Amphirhime and Monorhins — Nasal tube of the lamprey
—Its termination at the infundibulum — The olfactory organs of the scorpion
group — The camerostome — Its formation as a tube — Its derivation from a
pair of antennae — Its termination at the true mouth — Comparison with the
olfactory tube of Ammoccetes — Origin of the nasal tube of Ammoccetes from
the tube of the hypophysis — Direct comparison of the hypophysial tube with
the olfactory tube of the scorpion group — Summary 218

CHAPTER VII

The Prosomatic Segments of Limulus and its Allies

Comparison of the trigeminal with the prosomatic region — The prosomatic
appendages of the Gigantostraca — Their number and nature — Endognaths
and ectognath — The metastoma — The coxal glands — Prosomatic region of
Eurypterus compared with that of Ammoccetes — Prosomatic segmentation
shown by marks on carapace — Evidence of ccelomic cavities in Limulus —
Summary 233

CONTENTS vii

CHAPTER VIII

The Segments belonging to the Trigeminal Nerve-Group

PAGE

The prosoruatic segments of the vertebrate — Number of segments belonging to
the trigeminal nerve-group — History of cranial segments — Eye-muscles and
their nerves — Comparison with the dorso-ventral somatic muscles of the
scorpion — Explanation of the oculomotor nerve and its group of muscles —
Explanation of the trochlear nerve and its dorsal crossing — Explanation
of the abducens nerve — Number of segments supplied by the trigeminal
nerves — Evidence of their motor nuclei — Evidence of their sensory ganglia
— Summary 257

CHAPTER IX

The Prosomatic Segments op Ammoccetes

The prosomatic region in Ammoccetes — The suctorial apparatus of the adult
Petromyzon — Its origin in Ammoccetes — Its derivation from appendages —
The segment of the lower lip or the metastomal segment — The tentacular
• segments — The tubular muscles — Their segmental arrangement — Their
peculiar innervation — Their correspondence with the system of veno-peri-
cardial muscles in Limulus — The old mouth or palseostoma — The pituitary
gland — Its comparison with the coxal gland of Limulus — Summary . . 286

CHAPTER X

The Relationship of Ammocostes to the most Ancient Fishes

— the Ostracodermata

The nose of the Osteostraci — Comparison of head-shield of Ammoccetes and of
Cephalaspis — Ammoccetes only living representative of these ancient fishes
— Formation of cranium — Closure of old mouth — Rohon's primordial
cranium — Primordial cranium of Phrynus and Galeodes — Summary . . 326

CHAPTER XI

The Evidence of the Auditory Apparatus and the Organs of

the Lateral Line

Lateral line organs — Function of this group of organs — Poriferous sense-organs
on the appendages in Limulus — Branchial sense-organs — Prosomatic sense-
organs — Flabellum — Its structure and position — Sense-organs of mandibles
— Auditory organs of insects and arachnids — Poriferous chordotonal organs —
Balancers of Diptera — Resemblance to organs of flabellum — Racquet-organs
of Galeodes — Pectens of scorpions — Large size- of nerve to all these special
sense-organs — Origin of parachordals and auditory capsule— Reason why
VHth nerve passes in and out of capsule — Evidence of Ammoccetes —
Intrusion of glandular mass round brain into auditory capsule — Intrusion
of generative and hepatic mass round brain into base of flabellum —
Summary 355

viii CONTENTS

CHAPTER XII
The Region op the Spinal Cord

PAGE

Difference between cranial and spinal regions — Absence of lateral root — Meristic
variation — Segmentation of coelorn — Segmental excretory organs — Develop-
ment of nepbric organs ; pronepbric, mesonepbric, metanepbric — Excretory
organs of Ampbioxus— Solenocytes — Excretory organs of Brancbipus and
Peripatus, appendicular and somatic — Comparison of coelom of Peripatus
and of vertebrate — Pronepbric organs compared to coxal glands — Origin of
vertebrate body-cavity (metaccele) — -Segmental duct — Summary of formation
of excretory organs — Origin of somatic trunk-musculature — Atrial cavity
of Ampbioxus — Pleural folds — Ventral growtb of pleural folds and somatic
musculature — Pleural folds of Cephalaspidse and of Trilobita — Meaning of
tbe ductless glands — Alteration in structure of excretory organs which bave
lost tbeir duct in vertebrates and in invertebrates — Formation of lympbatic
glands — Segmental coxal glands of arthropods and of vertebrates — Origin of
adrenals, pituitary body, tbymus, tonsils, thyroid, and other ductless glands
— Summary 385

CHAPTER XIII
The Notochord and Alimentary Canal

Relationship between notocbord and gut — Position of unsegmented tube of
notocbord — Origin of notocbord from a median groove — Its function as an
accessory digestive tube — Formation of notocbordal tissue in invertebrates
from closed portions of tbe digestive tube — Digestive power of tbe skin of
Ammoccetes — Formation of new gut in Ammoccetes at transformation —
Innervation of the vertebrate gut — The three outflows of efferent nerves
belonging to v tbe organic system — The original close contiguity of the
respiratory chamber to the cloaca — The elongation of the gut — Conclusion 433

CHAPTER XIV

The Principles of Embryology

The law of recapitulation — Vindication of this law by tbe theory advanced in
this book — The germ-layer theory — Its present position — A physiological
not a morphological conception — New fundamental law required — Com-
position of adult body — Neuro-epitbelial syncytium and free-living cells —
Meaning of the blastula — Derivation of the Metazoa from the Protozoa —
Importance of the central nervous system for Ontogeny as well as for
Phylogeny — Derivation of free-living cells from germ-cells — Meaning of
coelom — Formation of neural canal — Gastrula of Ampbioxus and of Lucifer
— Summary 455

CONTENTS ix

CHAPTER XV
Final Remarks

PAGE

Problems requiring investigation — ■

Giant nerve-cells and giant nerve-fibres ; tbeir comparison in fisbes and
artbropods ; blood- and lymph-corpuscles ; nature of the skin ; origin of
system of unstriped muscles ; origin of the sympathetic nervous system ;
biological test of relationship.

Criticisms of Balanoglossus theory — Theory of parallel development — Importance

of the theory advocated in this book for all problems of Evolution . . 488

Bibliography and Index of Authors 501

General Index 517

" GO ON AND PROSPER ; THERE IS NOTHING SO
USEFUL IN SCIENCE AS ONE OF THOSE EARTH-
QUAKE HYPOTHESES, WHICH OBLIGE ONE TO FACE
THE POSSIBILITY THAT THE SOLIDEST-LOOKING
STRUCTURES MAY COLLAPSE."

Letter from Prof. Huxley to
the Author. June 2, 1889.

THE

ORIGIN OF VERTEBRATES

INTRODUCTION

In former days it was possible for a man like Johannes Muller
to be a leader both in physiology and in comparative anatomy.
Nowadays all scientific knowledge has increased so largely that
specialization is inevitable, and every investigator is confined more
and more not only to one department of science, but as a rule to
one small portion of that department. In the case of such cognate
sciences as physiology and comparative anatomy this limiting of the
scope of view is especially deleterious, for zoology without physiology
is dead, and physiology in many of its departments without com-
parative anatomy can advance but little. Then, again, the too
exclusive study of one subject always tends to force the mind into
a special groove— into a line of thought so deeply tinged with the
prevalent teaching of the subject, that any suggestions which arise
contrary to such teaching are apt to be dismissed at once as heretical
and not worthy of further thought ; whereas the same suggestion
arising in the mind of one outside this particular line of thought
may give rise to new and valuable scientific discoveries.

Nothing but good can, in my opinion, result from the incursion
of the non-specialist into the realm of the specialist, provided that
the former is in earnest. Over and over again the chemist has
given valuable help to the physicist, and the physicist to the
chemist, so closely allied are the two subjects ; so also is it with
physiology and anatomy, the two subjects are so interdependent
that a worker in the one may give valuable aid towards the solution
of some large problem which is the special territory of the other.

It has been a matter of surprise to many how it came about that

B

2 THE ORIGIN OF VERTEBRATES

I, a worker in the physiological laboratory at Cambridge ever since
Foster introduced experimental physiology into English-speaking
nations, should have devoted so much time to the promulgation of
a theory of the origin of vertebrates — a subject remote from phy-
siology, and one of the larger questions appertaining to comparative
anatomy. By what process of thought was I led to take up the
consideration of a subject apparently so remote from all my previous
work, and so foreign to the atmosphere of a physiological laboratory ?

It may perhaps be instructive to my readers to see how one
investigation leads to another, until at last, nolens volens, the worker
finds himself in front of a possible solution to a problem far removed
from his original investigation, which by the very magnitude and
importance of it forces him to devote his whole energy and time to
seeing whether his theory is good.

In the years 1880-1884 I was engaged in the investigation of
the action of the heart, and the nature of the nerves which regulate
that action. In the course of that investigation I was struck by the
ease with which it was possible to distinguish between the fibres of
the vagus and accelerator nerves on their way to the heart, owing to
the medullation of the former and the non-medullation of the latter.
This led me to an investigation of the accelerator fibres, to find out
how far they are non-medullated, and so to the discovery that the
rami commicnicantes connecting together the central nervous system
and the sympathetic are in reality single, not double, as had
hitherto been thought ; for the grey ramus communicans is in
reality a peripheral nerve which supplies the blood-vessels of the
spinal cord and its membranes, and is of the same nature as the
grey accelerators to the heart.

This led to the conclusion that there is no give and take
between two independent nervous systems, the cerebro-spinal and
the sympathetic, as had been taught formerly, but only one nervous
system, the cerebro-spinal, which sends special medullated nerve-
fibres, characterized by their smallness, to the cells of the sympathetic
system, from which fibres pass to the periphery, usually non-
medullated. These fine medullated nerves form the system of
white rami communicantcs, and have since been called by Langley
the preganglionic nerves. Further investigation showed that such
white rami are not universally distributed, but are confined to the
thoracico-lumbar region, where their distribution is easily seen in

INTRODUCTION 3

the ventral roots, for the cells of the sympathetic system arc entirely
efferent in nature, not afferent ; therefore, the fibres entering into them
from the central nervous system leave the spinal cord by ventral, not
dorsal roots.

Following out this clue, I then found that in addition to this
thoracico-lumbar outflow of efferent ganglionated visceral nerves,
there are similar outflows in the cranial and sacral regions, belong-
ing in the former case especially to the vagus system of nerves, and
in the latter to the system of nerves which pass from the sacral
region of the cord to the ganglion-cells of the hypogastric plexus,
and from them supply the bladder, rectum, etc. To this system of
nerves, formerly called the nervi erigcntes, I gave the name pelvic
splanchnics, in order to show their uniformity with the abdominal
splanchnics. These investigations led to the conclusion that the
organic system of nerves, characterized by the possession of efferent
nerve- cells situated peripherally, arises from the central nervous
system by three distinct outflows — cranial, thoracico-lumbar, and
sacral, respectively. To this system Langley has lately given the
name ' autonomic' These three outflows are separated by two gaps
just where the plexuses for the anterior and posterior extremities
come in.

This peculiar arrangement of the white rami communicantes set
me thinking, for the gaps corresponded to an increase of somatic
musculature to form the muscles of the fore and hind limbs, so that
if, as seemed probable, the white rami communicantes arise segmentally
from the spinal cord, then a marked distinction must exist in
structure between the spinal cord in the thoracic region, where the
visceral efferent nerves are large in amount and the body muscu-
lature scanty, and in the cervical or lumbar swellings, where the
somatic musculature abounds, and the white rami communicantes
scarcely exist.

I therefore directed my attention in the next place to the
structure of the central nervous system in the endeavour to asso-
ciate the topographical arrangement of cell-groups in this system
with the outflow of the different kinds of nerve-fibres to the
peripheral organs.

This investigation forcibly impressed upon my mind the
uniformity in the arrangement of the central nervous system as far
as the centres of origin of all the segmental nerves are concerned,

4 THE ORIGIN OF VERTEBRATES

both cranial and spinal, and also the original segmental character of
this part of the nervous system.

I could not, therefore, help being struck by the force of the
comparison between the central nervous systems of Vertebrata and
Appendiculata as put forward again and again by the past gene-
• ration of comparative anatomists, and wondered why it had been
discredited. There in the infundibulum was the old oesophagus,
there in the cranial segmental nerves the infracesophageal ganglia,
there in the cerebral hemispheres and optic and olfactory nerves the
supracesophageal ganglia, there in the spinal cord the ventral chain
of ganglia. But if the infundibulum was the old oesophagus, what
then ? The old oesophagus was continuous with and led into the
cephalic stomach. What about the infundibulum ? It was continuous
with and led into the ventricles of the brain, and the whole thing
became clear. The ventricles of the brain were the old cephalic
stomach, and the canal of the spinal cord the long straight intestine
which led originally to the anus, and still in the vertebrate embryo
opens out into the anus. Not having been educated in a morpho-
logical laboratory and taught that the one organ which is homologous
throughout the animal kingdom is the gut, and that therefore the
Efut of the invertebrate ancestor must continue on as the gut of
the vertebrate, the conception that the central nervous system has
grown round and enclosed the original ancestral gut, and that the
vertebrate has formed a new gut did not seem to me so impossible
as to prevent my taking it as a working hypothesis, and seeing to
what it would lead.

This theory that the so-called central nervous system of the
vertebrate is in reality composed of two separate parts, of which
the one, the segmented part, corresponds to the central nervous
system of the highest invertebrates, while the other, the unseg-
mented tube, was originally the alimentary canal of that same
invertebrate, came into my mind in the year 1887. The following
year, on June 23, 1888, I read a paper on the subject before the
Anatomical Society at Cambridge, which was published in the Journal
of Anatomy and Physiology, vol. 23, and more fully in the Journal of
Physiology, vol. 10. Since that time I have been engaged in testing
the theory in every possible way, and have published the results of
my investigations in a series of papers in different journals, a list of
which I append at the end of this introductory chapter.

INTRODUCTION 5

It is now twenty years since the theory first came into my mind,
and the work of those twenty years has convinced me more and more
of its truth, and yet during the whole time it has heen ignored by
the morphological world as a whole rather than criticized. Whatever
may have been the causes for such absence of criticism, it is clear
that the serial character of its publication is a hindrance to criticism
of the theory as a whole, and I hope, therefore, that the publication
of the whole of the twenty years' work in book-form will induce
those who differ from my conclusions to come forward and show me
where I am wrong, and why my theory is untenable. Any one
who has been thinking over any one problem for so long a time
becomes obsessed with the infallibility of his own views, and is not
capable of criticizing his own work as thoroughly as others would
do. I have been told that it is impossible for one man to consider
so vast a subject with that thoroughness which is necessary, before
any theory can be accepted as the true solution of the problem. I
acknowledge the vastness of the task, and feel keenly enough my
own shortcomings. For all that, I do feel that it can only be of
advantage to scientific progress and a help to the solution of this
great problem, to bring together in one book all the facts which I
have been able to collect, which appeal to me as having an important
bearing on this solution.

In this work I have been helped throughout by Miss R. Alcock.
It is not too much to say that without the assistance she has given
me, many an important link in the chain of evidence would have
been missing. With extraordinary patience she has followed, section
by section, the smallest nerves to their destination, and has largely
helped to free the transformation process in the lamprey from the
mystery which has hitherto enveloped it. She has drawn for me
very many of the illustrations scattered through the pages in this
book, and I feel that her aid has been so valuable and so continuous,
lasting as it does over the whole period of the work, that her name
ought fittingly to be associated with mine, if perchance the theory of
the Origin of Vertebrates, advocated in the pages of this book, gains
acceptance.

I am also indebted to Mr. J. Stanley Gardiner and to Dr. A.
Sheridan Lea for valuable assistance in preparing this book for the
press. I desire to express my grateful thanks to the former for
valuable criticism of the scientific evidence which I have brought

6 THE ORIGIN OF VERTEBRATES

forward in this hook, and to the latter for his great kindness in
undertaking the laborious task of correcting the proofs.

LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CON-
CERNING THE ORIGIN OF VERTEBRATES.

1888. "Spinal and Cranial Nerves." Proceedings of the Anatomical Society,

June, 1888. Journal of Anatomy and Physiology, vol. xxiii.

1889. " On the Relation between the Structure, Function. Distribution, and

Origin of the Cranial Nerves ; together with a Theory of the Origin
of the Nervous System of Vertebrata." Journal of Physiology, vol. x.,
p. 153.

1889. •• On the Origin of the Central Nervous System of Vertebrates."

Brain, vol. xii.. p. 1.

1890. '• On the Origin of Vertebrates from a Crustacean-like Ancestor."

Quarterly Journal of Microscopical Science, vol. xxxi.. p. 379.

1895. "The Origin of Vertebrates." Proceedings of the Cambridge Philo-

sophical Society, vol. ix., p. 19.

1896. Presidential Address to Section I. at the meeting- of the British

Association for the Advancement of Science in Liverpool. Report
of the British Association, 1896, p. 942.
1899. " On the Meaning of the Cranial Nerves." Presidential Address to the
Neurological Society for the year 1899. Brain, vol. xxii., p. 329.

A series of papers on " The Origin of Vertebrates, deduced from the
study of Ammocoetes," in the Journal of Anatomy and Physiology, as
follows : —

1898. Part I. " The Origin of the Brain," vol. xxxii., p. 513.

II. " The Origin of the Vertebrate Cranio-facial Skeleton,"
vol. xxxii., p. 553.
III. " The Origin of the Branchial Segmentation," vol. xxxiii..
p. 154.

1899. .. IV. " The Thyroid, or Opercular Segment : the Meaning of the

Facial Nerve," vol. xxxiii.. p. 638.

1900. .. V. " The Origin of the Pro-otic Segmentation : the Meaning

of the Trigeminal and Eye-muscle Nerves," vol. xxxiv..

p. 465.
1900. .. VI. " The Old Mouth and the Olfactory Organ : the Meaning

of the First Nei*ve," vol. xxxiv., p. 514.
19oo. „ VII. " The Evidence of Prosomatic Appendages in Ammocoetes,

as given by the Course and Distribution of the Trigeminal

Nerve," vol. xxxiv., p. 537.

1900. .. VIII. "The Pakeontological Evidence: Ammocoetes a Cepha-

laspid," vol. xxxiv., p. 562.

1901. .. IX. "The Origin of the Optic Apparatus: the Meaning of the

Optic Nerves," vol. xxxv., p. 224.

INTRODUCTION

I

1902. Part X. " The Origin of the Auditory Organ : the Meaning- of the

Vlllth Cranial Nerve," vol. xxxvi., p. 164.

1903. ., XI. '" The Origin of the Vertebrate Body-cavity and Excretory

Organs : the Meaning of the Somites of the Trunk and
of the Ductless Glands," vol. xxxvii., p. 168.

1905. .. XII. " The Principles of Embryology," vol. xxxix., p. 371.

1906. .. XIII. " The Origin of the Notochord and Alimentary Canal,"

vol. xl., p. 305.

CHAPTER I

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM

Theories of the origin of vertebrates. — Importance of the central nervous
system. — Evolution of tissues. — Evidence of Palaeontology. — Reasons for
choosing- Animocoetes rather than Amphioxus. — Importance of larval forms.
— Comparison of the vertebrate and arthropod central nervous systems. —
Antagonism between cephalization and alimentation. — Life-history of
lamprey : not a degenerate animal.— Brain of Animocoetes compared with
brain of arthropod. — Summary.

At the present time it is no longer a debatable question whether or
no Evolution has taken place. Since the time of Darwin the accu-
mulation of facts in its support has been so overwhelming that all
zoologists look upon this question as settled, and desire now to find
out the manner in which such evolution has taken place. Here two
problems offer themselves for investigation, which can be and are
treated separately — the one dealing with the question of those laws
of heredity and variation which have brought about in the past and
are still causing in the present the evolution of living beings, i.e. the
causes of evolution ; the other concerned with the relationship of
animals, or groups of animals, rather than with the causes which
have brought about such relationship, i.e. the sequence of evolution.

It is the latter problem with which this book deals, and, indeed,
not with the whole question at all, but only with that part of it
which concerns the origin of vertebrates.

This problem of the sequence of evolution is of a twofold character :
first, the finding out of the steps by which the higher forms in
any one group of animals have been evolved from the lower ; and
secondly, the evolution of the group itself from a lower group.

In any classification of the animal kingdom, it is clear that large
groups of animals exist which have so many common characteristics
as to necessitate their being placed in one larger group or kingdom ;

THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 9

thus zoologists are able to speak definitely of the Vertebrata, Arthro-
poda, Annelida, Echinodermata, Porifera, Ccelenterata, Mollusca,
etc. In each of these groups affinities can be traced between the
members, so that it is possible to speak of the progress from lower
to higher members of the group, and it is conceivable, given time to
work out the details, that the natural relationships between the
members of the whole group will ultimately be discovered.

Thus no one can doubt that a sequence of the kind has taken
place in the Vertebrata as we trace the progress from the lowest fishes
to man, and already the discoveries of palaeontology and anatomy
give us a distinct clue to the sequence from fish to amphibian, from
amphibian to reptile, from reptile to mammal on the one hand, and
to bird on the other. That the different members of the vertebrate
group are related to each other in orderly sequence is no longer a
matter of doubt ; the connected problems are matters of detail, the
solution of which is certain sooner or later. The same may be said
of the members of any of the other great natural groups, such as
the Arthropoda, the Annelida, the Echinodermata, etc.

It is different, however, when an attempt is made to connect
two of the main divisions themselves. It is true enough that there
is every reason to believe that the arthropod group has been evolved
from the segmented annelid, and so the whole of the segmented
invertebrates may be looked on as forming one big division, the
Appendiculata, all the members of which will some day be arranged
in orderly sequence, but the same feeling of certainty does not exist
in other cases.

In the very case of the origin of the Appendiculata we are con-
fronted with one of the large problems of evolution — the origin of
segmented from non-segmented animals — the solution of which is not
yet known.

Theories of the Origin of Vertebrates.

The other large problem, perhaps the most important of all, is the
question of the relationship of the great kingdom of the Vertebrata :
from what invertebrate group did the vertebrate arise ?

The great difficulty which presents itself in attempting a solution
of this question is not so much, as used to be thought, the difficulty
of deriving a group of animals possessing an internal bony and

IO

THE ORIGIN OF VERTEBRATES

cartilaginous skeleton from a group possessing an external skeleton
of a calcareous or chitinous nature, but rather the difficulty caused by
the fundamental difference of arrangement of the important internal
organs, especially the relative positions of the central nervous system
and the digestive tube.

Now, if we take a broad and comprehensive view of the inver-
tebrate kingdom, without arguing out each separate case, we find that

D

B

Fig. 1. — Arrangement of Organs in the Vertebrate (A) and Arthropod (B)'
Al, gut; IT, heart; C.N.S., central nervous system; V, ventral side; D, dorsal side.

it bears strongly the stamp of a general plan of evolution derived
from a co;lenterate animal, whose central nervous system formed a
ring surrounding the mouth. Then when the radial symmetry was
given up, and an elongated, bilateral, segmented form evolved, the
central nervous system also became elongated and segmented, but,
owing to its derivation from an oral ring, it still surrounded the
mouth-tube, or oesophagus, and thus in its highest forms is divided into
supra- oesophageal and infra-oesophageal nervous masses. These latter

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM II

nervous masses are of necessity ventral to the digestive tube, because
the mouth of the ccelenterate is on the ventral side. The striking
characteristic, then, of the invertebrate kingdom is the situation of a
large portion of the central nervous system ventrally to the alimentary
canal and the piercing of the nervous system by a tube — the oeso-
phagus — leading from the mouth to the alimentary canal. The
equally striking characteristic of the vertebrate is the dorsal position
of the central nervous system and the ventral position of the ali-
mentary canal combined with the absence of any piercing of the
central nervous system by the oesophagus.

So fundamentally different is the arrangement of the important
organs in the two groups that it might well give rise to a feeling of
despair of ever hoping to solve the problem of the Origin of Verte-
brates; and, to my mind, this is the prevalent feeling among
morphologists at the present time. Two attempts at solution have
been made. The one is associated with the name of Geoffrey St.
Hilaire, and is based on the supposition that the vertebrate has
arisen from the invertebrate by turning over on its back, swimming
in this position, and so gradually converting an originally dorsal
surface into a ventral one, and vice versa ; at the same time, a new
mouth is supposed to have been formed on the new ventral side,
which opened directly into the alimentary canal, while the old
mouth, which had now become dorsal, was obliterated.

The other attempt at solution is of much more recent date, and is
especially associated with the name of Bateson. It supposes that
bilaterally symmetrical, elongated, segmented animals were formed
from the very first in two distinct ways. In the one case the diges-
tive tube pierced the central nervous system, and was situated dorsally
to its main mass. In the other case the segmented central nervous
system was situated from the first dorsally to the alimentary canal,
and was not pierced by it. In the first case the highest result of
evolution led to the Arthropoda ; in the second case to the Vertebrata.

Neither of these views is based on evidence so strong as to cause
universal acceptance. The great difficulty in the way of accepting
the second alternative is the complete absence of any evidence, either
among animals living on the earth at the present day or among those
known to have existed in the past, of any such chain of intermediate
animal forms as must, on this hypothesis, have existed in order to
link together the lower forms of life with the vertebrates.

12

THE ORIGIN OF VERTE B RATES

It has been supposed that the Tunicata and the Enteropneusta
{Balanoglossus) (Fig. '2) are members of this missing chain, and that

in Amphioxus the ver-
tebrate approaches in
organization to these
low invertebrate forms.
The timicates, indeed,
are looked upon as de-
generate members of an
early vertebrate stock,
which may give help in
picturing the nature of
the vertebrate ancestor
but are not themselves

in the direct line of
Fig. 2.— Larval Balanoglossus (from the Royal

Natural History). descent. Balanoglossus

is supposed to have
arisen from the Echinodermata, or at all events to have affinities
with them, so that to fill up the enormous gap between the
Echinodermata and the Vertebrata on this theory there is absolutely
nothing living on the earth except Balanoglossus, Bhabdopleura,
and Cephalodiscus. The characteristics of the vertebrate upon
which this second theory is based are the notochord, the respiratory
character of the anterior part of the alimentary canal, and the tubular
nature of the central nervous system ; it is claimed that in Balano-
glossus the beginnings of a notochord and a tubular central nervous
system are to be found, while the respiratory portion of the gut is
closely comparable to that of Amphioxus.

The strength of the first theory is essentially based on the com-
parison of the vertebrate central nervous system with that of tho
segmented in vertel irate, annelid or arthropod. In the latter the
central nervous system is composed of —

1. The supra-cesophageal ganglia, which give origin to the nerves
of the eyes and antennules, i.e. to the optic and olfactory nerves,
for the first pair of antenna? are olfactory in function. These are
connected with the infra-cesophageal ganglia by the oesophageal
commissures which encircle the oesophagus.

2. The infra-cesophageal ganglia and the two chains of ventral
ganglia, which are segmentally-arranged sets of ganglia. Of these,

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM I *

each pair gives rise to the nerves of its own segment, and these
nerves are not nerves of special sense as are the supra-cesophageal
nerves, but motor and sensory to the segment ; nerves by the agency
of which food is taken in and masticated, respiration is effected, and
the animal moves from place to place.

In the vertebrate the central nervous system consists of —

1. The brain proper, from which arise only the olfactory and optic

nerves.

DORSAL

Spinal canal

Neureateric canal

H JM I | II p r'

Spinal Cord « Seomenlal Nerves

U/un^tulum VENTRAL

DORSAL

• ■■'aiopKaju. VENTRAL

Fig. 3. — Vertebrate Central Nervous System compared with the Central
Nervous System and Alimentary Canal of the Arthropod.

A. Vertebrate central nervous system. 8. Inf. Br., supra-infundibular brain;
I. Inf. Br., infra-infundibular brain and cranial segmental nerves; C.Q., corpora
quadrigemina ; Cb., cerebellum; C.C., crura cerebri; C.S., corpus striatum; Fn..
pineal gland.

B. Invertebrate central nervous system. <S'. (27s. G., supra-cesophageal ganglia ;
I. (Es. G., infra-cesopbageal ganglia; QSs. Com., oesophageal commissures.

2. The region of the mid-brain, medulla oblongata, and spinal
cord ; from these arises a series of nerves segmentally arranged,
which, as in the invertebrate, gives origin to the nerves governing
mastication, respiration, and locomotion.

Further, the vertebrate central nervous system possesses the
peculiarity, found nowhere else, of being tubular, and the tube is
of a striking character. In the spinal region it is a small, simple
canal of uniform calibre, which at the front end dilates to form the
ventricles of the region of the brain. From that part of this dilated

14 THE ORIGIN OF VERTEBRATES

portion, known as the third ventricle, a narrow tube passes to the
ventral surface of the brain. This tube is called the infundibulum,
and, extraordinary to relate, lies just anteriorly to the exits of the
third cranial or oculomotor nerves ; in other words, it marks the
termination of the series of spinal and cranial segmental nerves.
Further, on each side of this infundibular tube are lying the two
thick masses of the crura cerebri, the strands of fibres which connect
the higher brain-region proper with the lower region of the medulla
oblongata and spinal cord. Not only, then, are the nerve-masses
in the two systems exactly comparable, but in the very place where
the oesophageal tube is found in the invertebrate, the infundibular
tube exists in the vertebrate, so that if the words infundibular and
oesophageal are taken to be interchangable, then in every respect
the two central nervous systems are comparable. The brain proper
of the vertebrate, with its olfactory and optic nerves, becomes the
direct descendant of the supra-cesophageal ganglia ; the crura cerebri
become the oesophageal commissures, and the cranial and spinal
segmental nerves are respectively the nerves belonging to the infra-
cesophageal and ventral chain of ganglia.

This overwhelmingly strong evidence has always pointed directly
to the origin of the vertebrate from some form among the segmented
group of invertebrates, annelid or arthropod, in which the original
oesophagus had become converted into the infundibulum, and a new
mouth formed. So far, the position of this school of anatomists was
extremely sound, for it is impossible to dispute the facts on which
it is based. Still, however, the fact remained that the gut of the
vertebrate lies ventrally to the nervous system, while that of the
invertebrate lies dorsally ; consequently, since the infundibulum was
in the position of the invertebrate oesophagus, it must originally have
entered into the gut, and since the vertebrate gut was lying ventrally
to it, it could only have opened into that gut in the invertebrate stage
by the shifting of dorsal and ventral surfaces. From this argument
it followed that the remains of the original mouth into which the in-
fundibulum, i.e. oesophagus, opened were to be sought for on the dorsal
side of the vertebrate brain. Here in all vertebrates there are two
spots where the roof of the brain is very thin, the one in the region of
the pineal body, and the other constituting the roof of the fourth ven-
tricle. Both of these places have had their advocates as the position of
the old mouth, the former being upheld by Owen, the latter by Dohrn.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 5

The discovery that the pineal body was originally an eye, or,
rather, a pair of eyes, has perhaps more than anything else proved
the impossibility of accepting this reversal of surfaces as an explana-
tion of the genesis of the vertebrate from the annelid group. For
whereas a pair of eyes close to the mid-dorsal line is not only likely
enough, but is actually found to exist among large numbers of
arthropods, both living and extinct, a pair of eyes situated close
to the mid-ventral line near the mouth is not only unheard of in
nature, but so improbable as to render impossible the theory which
necessitates such a position.

Yet this very discovery gives the strongest possible additional
support to the close identity in the plan of the central nervous
system of vertebrate and appendiculate.

A truly paradoxical situation ! The very discovery which may
almost be said to prove the truth of the hypothesis, is the very one
which has done most to discredit it, because in the minds of its
authors the only possible solution of the transition from the one
group to the other was by means of the reversal of surfaces.

Still, as already said, even if the theory advanced to explain the
facts be discredited, the facts remain the same ; and still to this day
an explanation is required as to why such extraordinary resemblances
should exist between the two nervous systems, unless there is a
genetic connection between the two groups of animals. An ex-
planation may still be fouud, and must be diligently sought for,
which shall take into account the strong evidence of this relation-
ship between the two groups, and yet not necessitate any reversal
of surfaces. It is the object of this book to consider the possibility
of such an explanation.

What are the lines of investigation most likely to meet with
success ? Is it possible to lay down any laws of evolution ? It
is instructive to consider the nature of the investigations which
have led to the two theories just mentioned, for the fundamental
starting-point is remarkably different in the two cases. The one
theory is based upon the study of the vertebrate itself, and especially
of its central nervous system, and its supporters and upholders have
been and are essentially anatomists, whose chief study is that of
vertebrate and human anatomy. The other theory is based upon the
study of the invertebrate, and consists especially of an attempt to
find in the invertebrate some structure resembling a notochord, such

1 6 THE ORIGIN OF VERTEBRATES

organ being considered by them as the great characteristic of the
vertebrate ; indeed, so much is this the case, that a large number
of zoologists speak now of Chordata rather than of Vertebrata, and
in order to emphasize their position follow Bateson, and speak of the
Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the
Enteropneusta as Hemi-chordata, and even of Actinotrocha (to use
Masterman's term), as Diplo-chordata.

The upholders of this theory lay no stress on the nature of the
central nervous system in vertebrates, they are essentially zoologists
who have made a special study of the invertebrate rather than of
the vertebrate.

Of these two methods of investigating the problem, it must be
conceded that the former is more likely to give reliable results.
By putting the vertebrate to the question in every possible way, by
studying its anatomy and physiology, both gross and minute, by
inquiring into its past history, we can reasonably hope to get a
clue to its origin, but by no amount of investigation can we tell
with any certainty what will be its future fate ; we can only guess
and prophesy in an uncertain and hesitating manner. So it must be
with any theory of the origin of vertebrates, based on the study of
one or other invertebrate group. Such theory must partake rather
of the nature of prophecy than of deduction, and can only be placed
on a firm basis when it so happens that the investigation of the
vertebrate points irresistibly to its origin from the same group ; in
fact, " never prophesy unless you know."

The first principle, then, I would lay down is this : In order to
find out the origin of vertebrates, inquire, in the first place, of the
vertebrate itself.

Impoetance of the Central Nervous System.

Does the history of evolution pick out any particular organ or
group of organs as more necessary than another for upward progress ?
If so, it is upon that organ or group of organs that special stress must
be laid.

Since Darwin wrote the " Origin of Species," and laid down that
the law of the ' survival of the fittest ' is the factor upon which evolu-
tion depends, it has gradually dawned upon the scientific mind that
' the iittest ' may be produced in two diametrically opposite ways :

THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 1 7

either by progress upwards to a superior form, or by degeneration to
a lower type of animal. The principle of degeneration as a factor
in the formation of groups of animals, which are thereby enabled
to survive, is nowadays universally admitted. The most striking
example is to be found in the widely distributed group of Tunicata,
which live, in numbers of instances, a sedentary life upon the rocks,
have the appearance of very low forms of animal life, propagate
by budding, have lost all the characteristics of higher forms, and
yet are considered to be derived from an original vertebrate stock.
Such degenerate forms remain degenerate, and are never known to
regenerate and again to reach the higher stage of evolution from
which they arose. Such forms are of considerable interest, but
cannot help, except negatively, to decide what factor is especially
important for upward progress.

At the head of the animal race at the present day stands man,
and in mankind itself some races are recognized as higher than others.
Such recognition is given essentially on account of their greater
brain-power, and without doubt the great characteristic which puts
man at the head is the development of his central nervous system,
especially of the region of the brain. Not only is this point most
manifest in distinguishing man from the lower animals, but it applies
to the latter as well. By the amount of convolution of the brain,
the amount of grey matter in the cerebral hemispheres, the enlarge-
ment and increasing complexity of the higher parts of the central
nervous system, the anthropoid apes are differentiated from the lower
forms, and the higher mammals from the lower. In the recent work
of Elliot Smith, and of Edinger, most conclusive proof is given that
the upward progress in the vertebrate phylum is correlated with the
increase of brain-power, and the latter writer shows how steady and
remarkable is the increase in substance and in complexity of the
brain-region as we pass from the fishes, through the amphibians and
reptiles, to the birds and mammals.

The study of the forms which lived on the earth in past ages con-
firms and emphasizes this conclusion, for it is most striking to see
how small is the cranium among the gigantic Dinosaurs ; how in the
great reptilian age the denizens of the earth were far inferior in brain-
power to the lords of creation in after-times.

What applies to the vertebrate phylum applies also to the inver-
tebrate groups. Here also an upward progress is recognized as we

c

1 8 THE ORIGIN OF VERTEBRATES

pass from the sponges to the arthropods — a progress which is mani-
fested, first by the concentration of nervous material to form a central
nervous system, and then by the increase in substance and complexity
of that nervous system to form a higher and a higher type, until the
culmination is reached in the nervous system of the scorpions and
spiders. No upward progress is possible with degeneration of the
central nervous system, and in all those cases where a group owes its
existence to degeneration, the central nervous system takes part in
the degeneration.

This law of the paramount importance of the growth of the central
nervous system for all upward progress in the evolution of animals
receives confirmation from the study of the development of individuals,
especially in those cases where a large portion of the life of the
animal is spent in a larval condition, and then, by a process of trans-
formation, the larva changes into the adult form. Such cases are
well known among Arthropoda, the familiar instance being the change
from the larval caterpillar to the adult imago. Among Vertebrata,
the change from the tadpole to the frog, from the larval form of
the lamprey (Ammocwtes) to the adult form (Petromyzon), are well-
known instances. In all such cases the larva shows signs of having
attained a certain stage in evolution, and then a remarkable trans-
formation takes place, with the result that an adult animal emerges,
whose organization reaches a higher stage of evolution than that of
the larva.

This transformation process is characterized by a very great
destruction of the larval tissues and a subsequent formation of new
adult tissues. Most extensive is the destruction in the caterpillar
and in the larval lamprey. But one organ never shares in this process
of histolysis, and that is the central nervous system ; amidst the
ruins of the larva it remains, leading and directing the process of
re-formation. In the Arthropoda, the larval alimentary canal may
be entirely destroyed and eaten up by phagocytes, but the central
nervous system not only remains intact but increases in size, and by
the concentration and cephalization of its infra-cesophageal ganglia
forms in the adult a central nervous system of a higher type than
that of the larva.

So, too, in the transformation of the lamprey, there is not the
slightest trace of any destruction in the central nervous system, but
simply a development and increase in nervous material, which

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 9

results in the formation of a brain region more like that of the higher
vertebrates than exists in Animocoetes.

In these cases the development is upward — the adult form is of a
higher type than that of the larva. It is, however, possible for the
reverse to occur, so that the individual development leads to degene-
ration, not to a higher type. Instances are seen in the Tunicata, and
in various parasitic arthropod forms, such as Lernaea, etc. In these
cases, the transformation from the larval to the adult form leads to
degradation, and in this degradation the central nervous system is
always involved.

It is perhaps a truism to state that upward progress is necessarily
accompanied by increased development of the central nervous system ;
but it is necessary to lay special stress upon the importance of the
central nervous system in all problems of evolution, because there is,
in my opinion, a tendency at the present time to ignore this factor to
too great an extent.

The law of progress is this — The race is not to the swift, nor to
the strong, but to the wise.

This law carries with it the necessary corollary that the imme-
diate ancestor of the vertebrate must have had a central nervous
system nearly approaching that of the lowest undegenerated verte-
brate. Among all the animals living on the earth at the present
time, the highest invertebrate group, the Arthropoda, possesses
a central nervous system most closely resembling that of the
vertebrate.

The law, then, of the paramount importance of a steady develop-
ment of the central nervous system for the upward progress of the
animal kingdom, points directly to the arthropod as the most probable
ancestor of the vertebrate.

Evolution of Tissues.

In the whole scheme of evolution we can recognize, not only an
upward progress in the organization of the animal as a whole, but
also a distinct advance in the structure of the tissues composing an
individual, which accompanies that upward progress. Thus it is
possible to speak of an evolution of the supporting tissues from the
simplest form of connective tissue up to cartilage and thence to bone;
of the contractile tissues, from the simplest contractile protoplasm

20 THE ORIGIN OF VERTEBRATES

to unstriped muscle, and thence to the highest forms of striated
muscle ; of the nervous connecting strands, from undifferentiated to
fine strands, then to thicker, more separated ones, resembling non-
medullated fibres, and finally to well-differentiated separate fibres,
each enclosed in a medullated sheath.

In the connective tissue group, bone is confined to the vertebrates,
cartilage is found among invertebrates, and the closest resemblance
to vertebrate embryonic or parenchymatous cartilage is found in the
cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus,
more than any other invertebrate, possesses a fibrous connective
tissue resembling that of vertebrates.

In the muscular group, Biedermann, who has made a special
study of the physiology of striated muscle, says that among inver-
tebrates the striated muscle of the arthropod group resembles most
closely that of the vertebrate.

In the nervous group the resemblance between the nerve-fibres
of Limulus and Ammoccetes, both of which are devoid of any marked
medullary sheath, is very apparent, and Eetzius points out that the
only evidence of medullation, so characteristic of the vertebrates, is
found in a species of prawn (Palamion). In all these cases the
nearest resemblance to the vertebrate tissues is to be found in the
arthropod.

The Evidence of Paleontology.

Perhaps the most important of all the clues likely to help in the
solution of the origin of vertebrates is that afforded by Geology, for
although the geological record is admittedly so imperfect that we
can never hope by its means alone to link together the animals at
present in existence, yet it does undoubtedly point to a sequence in
the evolution of animal forms, and gives valuable information as to
the nature of such sequence. In different groups of animals there
are times when the group can be spoken of as having attained its
most flourishing period. During these geological epochs the dis-
tribution of the group was universal, the numbers were very great,
the number of species was at the maximum, and some of them had
attained a maximal size. Such races were at that time dominant,
and the struggle for existence was essentially among members of the
same group. At the present time the dominant race is man, and the

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 21

struggle for existence is essentially between the members of that
race, and not between them and any inferior race.

The effect of such conditions is, as Darwin has pointed out, to
cause great variation in that group ; in consequence of that variation
and that dominance the evolution of the next higher group is brought
about from some member of the dominant group. Thus the present
age is the outcome of the Tertiary period, a time when giant mammals
roamed the earth and left as their successors the mammals of the
present day ; a time of dominance of quadruped mammals ; a time
of which the period of maximum development is long past, and we
now see how the dominance of the biped mammal, man, is accom-
panied by the rapid diminution and approaching extermination of
the larger mammals. No question can possibly arise as to the im-
mediate ancestor of the biped mammal ; he undoubtedly arose from
one of the dominant quadrupedal mammals.

Passing along to the next evidence of the rocks, we find an age of
reptiles in the Mesozoic period. Here, again, the number and
variety is most striking ; here, again, the size is enormous in com-
parison with that of the present-day members of the group. This
was the dominant race at the time when the birds and mammals
first appeared on the earth, and anatomists recognize in these extinct
reptilian forms two types ; the one bird-like, the other more mamma-
lian in character. From some members of the former group birds
are supposed to have been evolved, and mammals from members of
the other group. There is no question of their origin directly from
lower fish-like forms ; the time of their appearance on the earth,
their structure, all point irresistibly to the same conclusion as we
have arrived at from the consideration of the origin of the biped
from the quadruped mammal, viz. that birds and mammals arose, in
consequence of the struggle for existence, from some members of the
reptilian race which at that time was the dominant one on earth.

Passing down the geological record, we find that when the reptiles
first appear in the Carboniferous age there is abundant evidence of
the existence of numbers of amphibian forms. At this time the
giant Labyrinthodonts flourished. Here among the swamps and
marshes of the coal-period the prevalent vertebrate was amphibian
in structure. Their variety and number were very great, and at that
period they attained their greatest size. Here, again, from the
geological record we draw the same conclusion as before, that the

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FIRST FISHES
MARINE SCOPPIONS

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Fig. 4.— Plan of Geological Strata. (From Lankester.)

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 23

reptiles arose from the race which was then predominant on the earth
— the Amphibia.

Again, another point of great interest is seen here, and that is
that these Labyrinthodonts, as Huxley has pointed out, possess
characters which bring them more closely than the amphibians of
the present day into connection with the fishes; and further, the
fish-like characters they possessed are those of the Ganoids, the
Marsipobranchs, the Dipnoans, and the Elasmobranchs, rather than
of the Teleosteans.

Now, it is a striking fact that the ancient fishes at the time when
the amphibians appeared had not reached the teleostean stage. The
ganoids and elasmobranchs swarmed in the waters of the Devonian
and Carboniferous times. Dipnoans and marsipobranchs were there,
too, in all probability, but teleosteans do not appear until the
Mesozoic period. The very kinds of fish, then, which swarmed in
the seas at that time, and were the predominant race before the
Carboniferous epoch, are those to which the amphibians at their first
appearance show the closest affinity. Here, again, the same law
appears ; from the predominant race at the time, the next higher
race arose, and arose by a most striking modification, which was the
consequence of altering the medium in which it lived. By coming
out of the water and living on the land, or, rather, being able to live
partly on land and partly in the water, by the acquisition of air-
breathing respiratory organs or lungs in addition to, and instead of,
water-breathing organs or gills, the amphibian not only arose from
the fish, but made an entirely new departure in the sequence of
progressive forms.

This was a most momentous step in the history of evolution —
one fraught with mighty consequences and full of most important
suggestions.

From this time onwards the struggle for existence by which
upward progress ensued took place on the land, not in the sea, and,
as has been pointed out, led to the evolution of reptiles from am-
phibians, birds and quadrupedal mammals from reptiles, and man
from quadrupeds. In the sea the fishes were left to multiply and
struggle among themselves, their only opponents being the giant
cephalopods, which themselves had been evolved from a continual
succession of the Mollusca. For this reason the struggle for existence
between the fishes and the higher race evolved from them did not

24 THE ORIGIN OF VERTEBRATES

take place until some members of that higher race took again to the
water, and so competed with the fish-tribe in their own element.

Another most important conclusion to be derived from the
uprising of the Amphibia is that at that time there was no race
of animals living on the land which had a chance against them. No
race of land-living animals had been evolved whose organization
enabled them to compete with and overcome these intruders from
the sea in the struggle for existence. For this reason that the
whole land was their own, and no serious competition could arise
from their congeners, the fish, they took possession of it, and increased
mightily in size ; losing more and more the habit of going into the
water, becoming more and more truly terrestrial animals. Hence-
forth, then, in trying to find out the sequence of evolution, we must
leave the land and examine the nature of the animals living in the
sea ; the air-breathing animals which lived on the land in the Upper
Silurian and Devonian times cannot have reached a stage of organi-
zation comparable with that of the fishes, seeing how easily the
amphibians became dominant.

We arrive, then, at the conclusion that the ancestors of the fishes
must have lived in the sea, and applying still the same principles
that have held good up to this time, the ancestors of the fishes must
have arisen from some member of the race predominant at the time
when they first appeared, and also the earliest fishes must have much
more closely resembled the ancestral form than those found in later
times or at the present day.

What, then, is the record of the rocks at the time of the first
appearance of fish-like forms ? What kind of fishes were they, and
what was the predominant race at the time ?

We have now reached the Upper Silurian and Lower Devonian
times, and most instructive and suggestive is the revelation of the
rocks. Here, when the first vertebrates appeared, the sea was peopled
with corals, brachiopods, .early forms of cephalopods, and other in-
vertebrates ; but, above all, with the great tribe of trilobites (Fig. 6)
and their successors. From the trilobites arose, as evidenced by
their larval form, the king-crab group, called the Xiphosura (Fig. 5).
Closely connected with them, and forming intermediate stages
between trilobites and king-crabs, numerous forms have been dis-
covered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc.
(Fig. 5 and Fig. 12). From them also arose the most striking group

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 25

of animals which existed at this period — the giant sea-scorpions, or
Gigantostraca. This group was closely associated with the king-
crabs, and the two groups together are classified under the title
Merostomata.

The appearance of these sea-scorpions is given iti Figs. 7 and 8,
representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They

Fig. 5 (from H. Woodward). — 1. Limulus polyphemus (dorsal aspect). 2. Lunulas,
young, in trilobitc stage. 3. Prestwichia rotundata. 4. Prestivichia Birtwelli.
5. Hemiaspis limuloides. 6. Pseudoniscus acitlcatus.

must have been in those days the tyrants of the deep, for specimens
of Pterygotus have been found over six feet in length.

At this time, then, by every criterion hitherto used, by the
multitude of species, by the size of individual species, which at this
period reached the maximum, by their subsequent decay and final
extinction, we must conclude that these forms were in their zenith,
that the predominant race at this time was to be found in this group
of arthropods. Just previously, the sea swarmed with trilobites, and
right into the period when the Gigantostraca flourished, the tril obites

26

THE ORIGIN OF VERTEBRATES

are still found of countless forms, of great difference in size. The
whole period may be spoken of as the great trilobite age, just as the
Tertiary times form the mammalian age, the Mesozoic times the
reptilian age, etc. From the trilobites the Gigantostraca and
Xiphosura arose, as evidenced by the embryology of Limulus, and,
therefore, in the term trilobite age would be included the whole of
those peculiar forms which are classified by the names Trilobita,

Fig. 6.— A Trilobite (Dalma-
tites) (after Pictet). Dorsal

Fig. 7. — Euryplerus remises (after
Nieskowski). Dorsal view.

view.

Gigantostraca, Xiphosura, etc. Of all these the only member alive
at the present time is Limulus, or the King-Crab.

As, however, the term ' trilobite ' does not include the members
of the king-crab or sea-scorpion groups, it is advisable to use some
other term to represent the whole group. They cannot be called
crustaceans or arachnids, for in all probability they gave origin to
both ; the nearest approach to the Trilobite stage of development at
the present time is to be found perhaps in Branchipus (Fig. 10) and
Apus (Fig. 9), just as the nearest approach to the Eurypterid

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 27

form is Limulus. Crustaceans such as crabs and lobsters are of
much later origin, and do not occur in any quantity until the late

Fig. 8. — A, Pterygotus Osiliensis (from Schmidt). B, Stylonurus Logani (from
Woodward). C, Slimonia acuminata (from Woodward).

Mesozoic period. The earliest found, a kind of prawn, occurs in the
Carboniferous age.

Korschelt and Heider have accordingly suggested the name
Palceostraca for this whole group, and Protostraca for the still earlier

28

THE ORIGIN OF VERTEBRATES

arthropod-like animals which gave origin to the trilobites themselves.
This name I shall adopt, and speak, therefore, of the Palasostraca as

the dominant race at the time when
vertebrates first appeared.

If, then, there is no break in the
law of evolution here, the race which
was predominant at the time when
the vertebrate first appeared must
have been that from which the first
fishes arose, and these fishes must
have resembled, not the crustacean
proper, or the arachnid proper, but a
member of the palreostracan group.
Moreover, just as the Labyrinthodonts
show special affinities to the fishes
which were then living, so we should
expect that the forms of the earliest
fish would resemble the arthropodan
type dominant at the time more
closely than the fish of a later era.

At first sight it seems too great
an absurdity even to imagine the
possibility of any genetic connection between a fish and an arthropod,
for to the mind's eye there arises immediately the picture of a
salmon or a shark and a lobster or a spider. So different in appear-

Fig. 9. — Apus (from the Royal
Natural History). Dorsal view.

Fig. 10.— Branchipus stagnalis. (From Claus.)

ance are the two groups of animals, so different their methods of
locomotion, that it is apparently only an inmate of a lunatic asylum

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 29

who could possibly suggest such a connection. Much more likely
is it that a fish-like form should have been developed out of a smooth,
wrio-a-lincr worm-like animal, and it is therefore to the annelids that
the upholders of the theory of the reversal of surfaces look for the
ancestor of the vertebrate.

We must endeavour to dismiss from our imagination such forms
as the salmon and shark as representatives of the fish-tribe, and the
lobster and spider of the arthropods, and try to picture the kind of
animals living in the seas in the early Devonian and Upper Silurian
times, and then we find, to our surprise, that instead of the contrast
between fishes and arthropods being so striking as to make any
comparison between the two seem an absurdity, the difficulty in the
last century, and even now, is to decide in many cases whether a
fossil is an arthropod or a fish.

I have shown what kind of animal the palaeostracan was like.
What information is there of the nature of the earliest vertebrate ?

The most ancient fishes hitherto discovered have been classified
by Lankester and Smith Woodward into the three orders, Hetero-
straci, Osteostraci, and Antiarcha. Of these the Heterostraci contain
the genera Pteraspis and Cyathaspis, and are the very earliest
vertebrates yet discovered, being found in the Lower Silurian. The
Osteostraci are divided into the Cephalaspidae, Tremataspida?, etc.,
and are found in the Upper Silurian and Devonian beds. The
Antiarcha, comprising Pterichthys and Bothriolepis, belong to the
Devonian and are not found in Silurian deposits. This, then, is the
order of their appearance— Pteraspis, Cephalaspis, and Pterichthys.

In none of these families is there any resemblance to an ordinary
fish. In no case is there any sign of vertebra? or of jaws. They, like
the lampreys, were all agnathostomatous. Strange indeed is their
appearance, and it is no wonder that there should have been a
difficulty in deciding whether they were fish or arthropod. Their great
characteristic is their buckler- plated cephalic shield, especially con-
spicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give
the dorsal shields of Pteraspis, Auchenaspis, Pterichthys, and
Bothriolepis.

In 1904, Drevermann discovered a mass of Pteraspis Dunensis
embedded in a single stone, showing the same kind of head-shield
as P. rostrcda, but the rostrum was longer and the spine at the
extremity of the head-shield much longer and more conspicuous.

;o

THE ORIGIN OF VERTEBRATES

Fig. 11. — Ptcraspis duncnsis (from Drevermann). Dorsal view of body and spine
on the right side. Head-end, showing long rostrum on the left side.

Fig. 12. — Bunodes lunula. (From
Schmidt.)

Fig. 13. — Auclicnaspis (Tkyestes) verru-
cosus, natural size. (From Woodward.)

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 3 1

The whole shape of the animal as seen in this photograph recalls the
shape of a Hemiaspid rather than of a fish. It is, then, natural
enough for the earlier observers to have looked upon such a fossil as
related to an arthropod rather than a fish.

In Figs. 12 and 13 I have placed side by side two Silurian fossils
which are found in the same geological horizon. They are both life
size and possess a general similarity of appearance, yet the one is a

Fig. 14.— Dorsal Head-shield of Thy
estes (Auchenaspis) verrucosus. (From
Rohon.)

Fro., narial opening; i.e., lateral eyes; gl.,
glabellum or plate over brain; Occ, oc-
cipital region.

Fig. 15. — Ptcricthys.

Cephalaspidian fish known by the name of Auchenasjris or Tlu/estes
verrucosa, the other a Palreostracan called Bunodes lunula.

In a later chapter I propose to discuss the peculiarities and the
nature of the head-shields of these earliest fishes, in connection with
the question of the affinities of the animals which bore them. At
this point of my argument I want simply to draw attention to the
undoubted fact of the striking similarity in appearance between the

32

THE ORIGIN OF VERTEBRATES

earliest fishes and members of the Palaaostraca, the dominant race of
arthropods which swarmed in the sea at the time : a similarity which
could never have been suspected by any amount of investigation

Fig. 16. — Bothriolepis. (After Patten.)
An., position of anus.

among living forms, but is immediately revealed when the ages
themselves are questioned.

I have not reproduced any of the attempted restorations of these
old forms, as usually given in the text-books, because all such restora-
tions possess a large element of fancy, due to the personal bias of the
observer. I have put in Eohon's idea of the general shape of Tre-
mataspis (Fig. 17) in order to draw attention to the lamprey-like
appearance of the fish according to his researches (cf. Fig. 18).

Fig. 17. — Kestoration of Tremataspis. (After Kohon, slightly modified.)

*&s(

Fig. 18. — Ammoecetes.

The argument, then, from geology, like that from comparative
anatomy and from the consideration of the importance of the central
nervous system in the upward development of the animal race, not
only points directly to the arthropod group as the ancestor of the

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 33

vertebrate, but also to a distinct ancient type of arthropod, the
Palseostracan, the only living example of which is the King- Crab or
Limulus ; while the nearest approach to the trilobite group among
living arthropods are Branchipus and Apus. It follows, therefore, that
for the following up of this clue, Limulus especially must be taken into
consideration, while Branchipus and Apus are always to be kept in mind.

Ammoccetes rather than Amphioxus is the Best Subject for

Investigation.

It is not, however, Limulus that must be investigated in the first
instance, but the vertebrate itself; for it can never be insisted on too
often that in the vertebrate itself its past history will be found, but
that Limulus cannot reveal the future of its race. What vertebrate
must be chosen for investigation ? Reasons have been given why
oUr attention should be fixed upon the king-crab rather than on the
lobster on the invertebrate side ; what is the most likely animal on
the vertebrate side ?

From the evidence already given it is manifest that the earliest
mammal belonged to the lowest group of mammals ; that the birds
on their first appearance presented reptilian characteristics, that the
earliest reptiles belonged to a low type of reptile, that the amphibians
at their first appearance were nearer in type to the fishes than were
the later forms. As each of these groups advances in number and
power, specialization takes place in it, and the latest developed
members become further and further removed in type from the
earliest. So also it must have been with the origin of fishes : here
too, in the quest for information as to the structure and nature of
the first-formed fishes, we must look to the lowest rather than to
the highest living members of the group.

The lowest fish-like animal at present living is Amphioxus, and
on this ground it is argued that the original vertebrate must have
approached in organization to that of Amphioxus ; it is upon the
comparison between the structure of Amphioxus and that of Balano-
glossus, that the theory of the origin of vertebrates from forms like
the latter animal is based. For my own part, I think that in the
first instance, at all events, Amphioxus should be put on one side,
although of course its structure must always be kept in mind, for

the following reasons

D

34 THE ORIGIN OF VERTEBRATES

Amphioxus, like the tunicates, does not possess the character-
istics of other vertebrates. In all vertebrates above these forms
the great characteristic is a well-defined brain-region from which
arise nerves to organs of special sense, the eyes and nose. la
Amphioxus no eyes exist, for the pigmented spot at the anterior
extremity of the brain-region is no eye but only a mass of pig-
ment, and the so-called olfactory pit is a very rudimentary and
inferior organ of smell. In connection with the nearly complete
absence of these two most important sense-organs, the most im-
portant part of the central nervous system, the region corresponding
to the cerebral hemispheres, is also nearly completely absent.

Now, the history of the evolution of the central nervous system in
the animal race points directly to its formation as a concentrated
mass of nervous material at the anterior extremity of the body, in
consequence of the formation of special olfactory and visual organs
at that extremity. As already stated, the concentration of nervous
material around the mouth as an oral ring was its beginning. In
connection with this there arose special sense-organs for the guidance
of the animal to its food which took the form of olfactory and optic
organs. With the shifting from the radial to the elongated form
these sense-organs remained at the anterior or mouth-end of the
animal, and owing to their immense importance in the struggle for
existence, that part of the central nervous system with which they
were connected developed more than any other part, became the
leader to which the rest of the nervous system was subservient, and
from that time onwards the development of the brain-region was
inevitably associated with the upward progress of animal life.

To those who believe in Evolution and the Darwinian theory of
the survival of the fittest, it is simply inconceivable that a soft-bodied
animal living in the mud, blind, with a rudimentary brain and rudi-
mentary olfactory organs, such as is postulated when we think of
Balanoglossus and Amphioxus, should hold its own and come victorious
out of the struggle for existence at a time when the sea was peopled
with powerful predaceous scorpion- and crab-like armour-plated
animals possessing a well-developed brain, good eyes and olfactory
organs, and powerful means of locomotion. Wherever in the scale of
animal development Amphioxus may ultimately be placed, it cannot
be looked upon as the type of the earliest formed fishes such as
appeared in Silurian times.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 35

The next lowest group of living fishes is the M arsipobranchii which
include the lampreys and hag-fishes. To these naturally we must turn
for a clue as to the organization of the earliest fish, for here we find
all the characteristics of the vertebrates represented : a well-formed
brain-region, well-developed eyes and nose, cranial nerves directly
comparable with those of other vertebrates, and even the commence-
ment of vertebra?.

Among these forms the lamprey is by far the best for investiga-
tion, not only because it is easily obtainable in large quantities, but
especially because it passes a large portion of its existence in a larval
condition, from which it emerges into the adult state by a wonderful
process of transformation, comparable in extent with the transforma-
tion of the larval caterpillar into the adult imago. So long does the
lamprey live in this free larval condition, and so different is it in
the adult stage, that the older anatomists considered that the two
states were really different species, and gave the name of Am-
rnoccetes branchialis to the larval stage, while the adult form was
called Petromyzon planeri, or Petromyzon fluviatilis.

This long-continued free-living existence in the larval or Am-
moccetes stage makes the lamprey, more than any other type of
lowly organized fish, invaluable for the present investigation, for
throughout the animal kingdom it is recognized that the larval
form approaches nearer to the ancestral type than the adult form,
whether the latter is progressive or degenerate. Not only are the
tissues formed during the stages which are passed through in a
free-living larval form, serviceable tissues comparable to those
of adult life, but also these stages proceed at so much slower a rate
than do those in the embryo in utero or in the egg, as to make
the larval form much more suitable than the embryo for the investi-
gation of ancestral problems. It is true enough that the free life of
the larva may bring about special adaptations which are not of an
ancestral character, as may also occur during the life of the adult ;
but the evidence is very strong that although some of the peculi-
arities of the larva may be due to such ccenogenetic factors, yet on
the whole many of them are due to ancestral characters, which dis-
appear when transformation takes place, and are not found in the
adult.

Thus if it be supposed that the amphibian arose from the fish,
the tadpole presents more resemblance to the fish than the frog. If

36 THE ORIGIN OF VERTEBRATES

it be supposed that the arthropod arose from the segmented worm,
the caterpillar bears out the suggestion better than the adult imago.
If it be supposed that the tunicate arose from a stock allied to the
vertebrate, it is because of the peculiarities of the larva that such a
supposition is entertained. So, too, if it be supposed that the fish
arose from a member of the arthropod group, the larval form of the
fish is most likely to give decisive information on the point.

For all these reasons the lowest form of fish to be investigated,
in the hopes of finding out the nature of the earliest formed fish, is
not Amphioxus, but Ammoccetes, the larval form of the lamprey — a
form which, as I hope to satisfy my reader after perusal of subse-
quent pages, more nearly resembles the ancient Cephalaspidian fishes
than any other living vertebrate.

Comparison of Central Nervous Systems of Vertebrate and
Arthropod without Eeversal of Surfaces.

So far different lines of investigation all point to the origin of the
vertebrate from arthropods, the group of arthropods in question being
now extinct, the nearest living representative being Limulus ; also to
the fact that of the two theories of the origin of vertebrates, that
one which is based on the resemblance between the central nervous
systems of the Vertebrata and the Appendiculata (Arthropoda and
Annelida) is more in accordance with this evidence than the other,
which is based mainly on the supposed possession of a notochord
among certain animals.

How is it, then, that this theory has been discredited and lost
ground ? Simply, I imagine, because it was thought to necessitate
the turning over of the animal. Let us, then, again look at the
nervous system of the vertebrate, and see whether there is any such
necessity.

As previously mentioned, the comparison of the two central
nervous systems showed such close resemblances as to force those
anatomists who supported this theory to the conclusion that the
infundibular tube was in the position of the original oesophagus ;
they therefore looked for the remains of a mouth opening in the
dorsal roof of the brain, but did not attempt to explain the extra-
ordinary fact that the infundibular tube is only a ventral offshoot
from the tube of the central nervous system. Yet this latter tube

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 37

is one, if not the most striking, of the peculiarities which distinguish
the vertebrate ; a tubular central nervous system such as that of the
vertebrate is totally unlike any other nervous system, and the very
fact that the two nervous systems of the vertebrate and arthropod
are so similar in their nervous arrangements, makes it still more
extraordinary that the nervous system should be grouped round a
tube in the one case and not in the other.

Now, in the arthropod the oesophagus leads directly into the
stomach, which is situated in the head-region, and from this a straight
intestine passes directly along the length of the body to the anus,
where it terminates. The relations of mouth, oesophagus, alimentary
canal, and nervous system in these animals are represented in the
diagram (Fig. 3).

Any tube, therefore, such as that of the infundibulum, which
would represent the oesophagus of such an animal, must have opened
into the mouth on the ventral side, and into the stomach on the
dorsal side, and the lining epithelium of such an oesophagus must
have been continuous with that of the stomach, and so of the whole
intestinal tract.

Supposing, then, the animal is not turned over, but that the dorsal
side still remains dorsal and ventral ventral, then the original mouth-
opening of the oesophagus must be looked for on the ventral surface
of the vertebrate brain in the region of the pituitary body or hypo-
physis, and on the dorsal side the tube representing the oesophagus
must be continuous with a large cephalically dilated tube, which
ought to pass into a small canal, to run along the length of the body
and terminate in the anus.

This is exactly what is found in the vertebrate, for the infun-
dibular tube passes into the third ventricle of the brain, which forms,
with the other ventricles of the brain, the large dilated cephalic
portion of the so-called nerve tube, and at the junction of the medulla
oblongata and spinal cord, this dilated anterior part passes into the
small, straight, central canal of the spinal cord, which in the embryo
terminates in the anus by way of the neurenteric canal. If the
animal is regarded as not having been turned over, then the con-
clusion that the infundibulum was the original oesophagus leads
immediately to the further conclusion that the ventricles of the verte-
brate brain represent the original cephalic stomach, and the central
canal of the spinal cord the straight intestine of the arthropod ancestor.

38 THE ORIGIN OF VERTEBRATES

For the first time a logical, straightforward explanation is thus
given of the peculiarities of the tube of the central nervous system,
with its extraordinary termination in the anus in the embryo, its
smallness in the spinal cord, its largeness in the brain region, and its
offshoot to the ventral side of the brain as the infundibular channel.
It is so clear that, if the infundibular tube be looked on as the old
cesophagus, then its lining epithelium is the lining of that oesophagus ;
and the fact that this lining epithelium is continuous with that of
the third ventricle, and so with the lining of the whole nerve-tube,
must be taken into account and not entirely ignored as has hitherto
been the case. If, then, we look at the central nervous system of
the vertebrate in the light of the central nervous system of the
arthropod without turning the animal over, we are led immediately
to the conclusion that what has hitherto been called the vertebrate
nervous system is in reality composed of two parts, viz. a nervous
part comparable in all respects with that of the arthropod ancestor,
which has grown over and included into itself, to a greater or less
extent, a tubular part comparable in all respects with the alimentary
canal of the aforesaid ancestor. If this conclusion is correct, it is
entirely wrong to speak of the vertebrate central nervous system as
being tubular, for the tube does not belong to the nervous system,
but was originally a simple epithelial tube, such as characterizes the
cesophagus, cephalic stomach, and straight intestine of the arthropod.

Here, then, is the crux of the position — either the so-called
nervous tube of the vertebrate is composed of two separate factors,
consisting of a true non-tubular nervous system and a non-nervous
epithelial tube, these two elements having become closely connected
together; or it is composed of one factor, an epithelial tube which
constitutes the nervous system, its elements being all nervous
elements.

If this latter hypothesis be accepted, then it is necessary to
explain why parts of that tube, such as the roof of the fourth
ventricle, the choroid plexuses of the various ventricles, which are
parts of the original roof inserted into the ventricles, are not com-
posed of nervous material, but form simple single-layered epithelial
sheets, which by no possibility can be included among functional
nervous structures. The upholders of this hypothesis can only
explain the nature of these thin epithelial parts of the nervous tube
in one of two ways ; either the tube was originally formed of nervous

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 39

material throughout, and for some reason parts of it have lost their
nervous function and thinned down ; or else these thin epithelial
parts are on their way to become nervous material, are still in an
embryonic condition, and are of the nature of epiblast-epithelium,
from which the central nervous system originally arose.

The first explanation is said to be supported by embryology, for
at first the nerve-tube is formed in a uniform manner, and then
later, parts of the roof appear to thin out and so form the thin epi-
thelial parts. If this were the right explanation, then it ought to
be found that in the lowest vertebrates there is greater evidence of
a uniformly nervous tube than in the higher members of the group :
while conversely, if, on the contrary, as we descend the vertebrate
phylum, it is found that more and more of the tube presents the
appearance of a single layer of epithelium, and the nervous material
is limited more and more to certain parts of that tube, then the
evidence is strong that the tubular character of the central nervous
system is not due to an original nervous tube, but to a non-nervous
epithelial tube with which the original nervous system has become
closely connected.

The comparison of the brain region of the different groups of
vertebrates (Fig. 19) is most instructive, for it demonstrates in the
most conclusive manner how the roof of the nervous tube in that
region loses more and more its nervous character, and takes on the
appearance of a simple epithelial tube, as we descend lower and
lower ; until at last, in the brain of Ammoccetes, as represented in
the figures, the whole of the brain- roof, from the region of the
pineal eye to the commencement of the spinal cord, is composed of
fold upon fold of a thin epithelial membrane forming an epithelial
bag, which is constricted in only one place, where the fourth cranial
nerve crosses over it.

Further, the brain of Ammoccetes (Fig. 20) shows clearly not only
that it is composed of two parts, an epithelial tube and a nervous
system, but also that the nerve-masses are arranged in the same
relative position with respect to this tube as are the nerve-masses in
the invertebrate with respect to the cephalic stomach and cesophagus.
This evidence is so striking, so conclusive, that it is impossible to
resist the conclusion that the tube did not originate as part of the
central nervous system, but was originally independent of the central
nervous system, and has been invaded by it.

4o

THE O RIG IX OF VERTEBRATES

MAMMALIA.

REPTILIA.

AMPHIBIA

TELEOSTEA

AMMOCCETES

Fig.19.— Comi'abisok of Vertebeate Bbalns.

CB., cerebellum ; FT., pituitary body ; PK., pineal body; C. STB., corpus striatum ;
G.H.B., right ganglion habenulse. I., olfactory; II., optic nerves.

CER

GHR

INF

CER

VII+VIII

Fig. 20. — Brain of
Ammoccetes.

A, dorsal view; B, late-
ral view; C, ventral
view.

Vll+Vlil

(B) xVff

-. v"\. •"*•

"Mi'

M •

■:■■&$

•v&.

.*>

- ' '*4j£i

.3)

§

It

•■-ii

3

C.E.H., cerebral hemi-
spheres ; G.H.R.,
right ganglion habe-
nulse ; PN., right
pineal eye ; CH„,
CH 2 , choroid plex-
uses ; I.— XII. cra-
nial nerves ; C.P.,
Conus post-commis-
suralis.

42 THE ORIGIN OF VERTEBRATES

The second explanation is hardly worth serious consideration, for
it supposes that the nervous system, for no possible reason, was laid
down in its most important parts — the brain-region — as an epithelial
tube with latent potential nervous functions ; that even up to the
highest vertebrate yet evolved these nervous functions are still in
abeyance over the whole of the choroid plexuses and the roof of the
fourth ventricle. Further, it supposes that this prophetic epithelial
tube originally developed into true nervous material only in certain
parts, and that these parts, curiously enough, formed a nervous
system absolutely comparable to that of the arthropod, while the
dormant prophetic epithelial part was formed so as just to mimic,
in relation to the nervous part, the alimentary canal of that same
arthropod.

The mere facts of the case are sufficient to show the glaring
absurdity of such an explanation. This is not the way Nature works ;
it is not consistent with natural selection to suppose that in a low
form nervous material can be laid down as non-nervous epithelial
material in order to provide in some future ages for the great increase
in the nervous system.

Every method of investigation points to the same conclusion,
whether the method is embryological, anatomical, or pathological.

First, take the embryological evidence. On the ground that the
individual development reproduces to a certain extent the phylo-
genetic development, the peculiarities of the formation of the central
nervous system in the vertebrate embryo ought to receive an appro-
priate explanation in any theory of phylogenetic development.
Hitherto such explanation has been totally lacking ; any suggestion
of the manner in which a tubular nervous system may have been
formed takes no account whatever of the differences between different
parts of the tube ; its dilated cephalic end with its infundibular
projection ventrally, its small straight spinal part, and its termination
in the anus. My theory, on the other hand, is in perfect harmony
with the embryological history, and explains it point by point.

From the very first origin of the central nervous system there
is evidence of two structures— the one nervous, and the other an
epithelial surface-layer which ultimately forms a tube ; this was
first described by Scott in Petromyzon, and later by Assheton in the
frog. In the latter case the external epithelial layer is pigmented,
while the underlying nervous layer contains no pigment ; a marked

THE EVIDENCE OE THE CENTRAL NERVOUS SYSTEM 43

and conspicuous demarcation exists, therefore, between the two layers
from the very beginning, and it is easy to trace the subsequent fate
of the two layers owing to this difference of pigmentation. The pig-
mented cells form the lining cells of the central canal, and becoming
elongated, stretch out between the cells of the nervous layer ; while
the latter, on their side, invade and press between the pigmented
cells. In this case, owing to the pigmentation of the epithelial layer,
embryology points out in the clearest possible manner how the
central nervous system of the vertebrate is composed of two struc-
tures — an epithelial non-nervous tube, on the outside of which the
central nervous system was originally grouped ; how, as develop-
ment proceeds, the elements of these two structures invade each
other, until at last they become so involved together as to give rise
to the conception that we are dealing with one single nerve tube.
It is impossible for embryology to give a clearer clue to the past
history than it does in this case, for it actually shows, step by step,
how the amalgamation between the central nervous system and the
old alimentary canal took place.

Further, consider the shape of the tube when it is first formed,
how extraordinary and significant that is. It consists of a simple
dilated anterior end leading into a straight tube, the lumen of which
is much larger than that of the ultimate spinal canal, and terminates
by way of the neurenteric canal in the anus.

Why should the tube take this peculiar shape at its first forma-
tion ? No explanation is given or suggested in any text-book of
embryology, and yet it is so natural, so simple : it is simply the shape
of the invertebrate alimentary canal with its cephalic stomach and
straight intestine ending in the anus. Again embryology indicates
most unmistakably the past history of the race. How are the
nervous elements grouped round this tube when it is first formed ?
Here embryology shows that a striking difference exists between the
part of the tube which forms the spinal cord and the dilated cephalic
part. Fig. 21, A (2), represents the relation between the nervous
masses and the epithelial tube in the first instance. At this stage
the nervous material in the spinal cord lies laterally and ventrally
to this tube, and at a very early stage the white anterior commissure
is formed, joining together these two lateral masses ; as yet there is
no sign of any posterior fissure, the tube with its open lumen extends
right to the dorsal surface.

44

THE ORIGIN OF VERTEBRATES

The interpretation of this stage is that in the invertebrate ancestor
the nerve-masses were situated laterally and ventrally to the
epithelial tube, and were connected together by commissures on the
ventral side of the tube (Fig. 21, A (1)) ; in other words, the chain of
ventral ganglia and their transverse commissures lying just ventrally
to the intestine, which are so characteristic of the arthropod nervous
system, is represented at this stage.

Subsequently, by the growth dorsalwards of nervous material to
form the posterior columns, the original epithelial tube is compressed
dorsally and laterally to such an extent that those parts lose all signs
of lumen, the one becoming the posterior fissure and the others the

3 J

2

Fig. 21. — A, Method of Formation of the Vertebrate Spinal Cord from the
Ventral Chain of Ganglia and the Intestine of an Arthropod, repre-
sented in 1 ; B, Method of Formation of the Vertebrate Medulla
Oblongata from the Infra-ossophageal Ganglia and the Cephalic
Stomach of an Arthropod.

substantia gelatinosa Rolandi on each side. The original tube is thus
reduced to a small canal formed by its ventral portion only (Fig. 21,
A (3)). In this way the spinal cord is formed, and the walls of the
original epithelial tube are finally visible only as the lining of the
central canal (Fig. 21, A (4)).

When we pass to the brain-region, to the anterior dilated
portion of the tube, embryology tells a different story. Here, as in
the spinal cord, the nervous masses are grouped at first laterally and
ventrally to the epithelial tube, as is seen in Fig. 21, B (2), but owing
to the large size of its lumen here, the nervous material is not
able to enclose it completely, as in the case of the spinal cord ;

THE FA 7 IDE NCR OF THE CENTRAL NERVOUS SYSTEM 45

consequently there is no posterior fissure formed ; but, on the contrary,
the dorsal roof, not enclosed by the nerve-masses, remains epithelial,
and so forms the membranous roof of the fourth ventricle and of the
other ventricles of the brain (Fig. 21, B (3)). In the higher animals,
owing to the development of the cerebrum and cerebellum, this
membranous roof becomes pushed into the larger brain cavity, and
thus forms the choroid plexuses of the third and lateral ventricles.
In the lower vertebrates, as in Ammoccetes and the Dipnoi, it still
remains as a dorsal epithelial roof and forms a most striking
characteristic of such brains.

In this part of the nervous system, then, the nervous material is
all grouped in its original position on the ventral side of the tube ;
and yet it is the same nervous material as that of the spinal cord,
all the elements are there, giving origin here to the segmental cranial
nerves just as lower down they give rise to the segmental spinal
nerves, connecting together the separate segments each with the other
and all with the higher brain-centres — the supra-infundibular centres
— just as they do in the spinal region.

Why should there be this striking difference between the
formation of the infra-infundibular region of the brain and that of
the spinal cord ? Do the advocates of the origin of vertebrates from
Balanoglossus give the slightest reason for it ? They claim that their
view also provides a tubular nervous system for the vertebrate, but
give not the slightest sign or indication as to why the nervous
material should be grouped entirely on the ventral side of an
epithelial tube in the infra-infundibular region and yet surround
it in the spinal cord region. And the explanation is so natural,
so simple : embryology does its very best to tell us the past history
of the race, if only we look at it the right way.

The infra-infundibular nervous mass is naturally confined to the
ventral side of the epithelial tube, because it represents the infra-
cesophageal ganglia, situated as they are on the ventral side of the
cephalic stomach, and, owing to the size of the stomach, they could
not enclose it by dorsal growth, as they do in the case of the forma-
tion of the spinal cord (Fig. 21, B (1)). Still these nervous masses
have grown dorsalwards, have commenced to involve the walls of
the cephalic stomach even in the lowest vertebrate, as is seen in
Ammoccetes, in which animal a ventral portion of the epithelial
bag has been evidently compressed and its lumen finally obliterated

46

THE ORIGIN OF VERTEBRATES

I

by the growth of the nerve-masses on each side of it. Throughout
the whole vertebrate kingdom this obliterated portion still leaves

its mark as the raphe or seam,
which is so characteristic of
the infra-infundibular portion
of the brain.

Here, again, it is seen how
simple is the explanation of a
peculiarity which has always
puzzled anatomists — why
should there be this seam in
the infra-infundibular portion
of the brain and not in the
supra-infundibular or in the
spinal cord ? The correspond-
ing compression in the upper
brain-region forms the lateral
ventricles, as is seen in the
accompanying figure of the
brain of Ammoccetes (Fig. 22).
In yet another instance it is
seen how markedly the nervous
masses are arranged in the
same position with respect to
the central tube as are the
nerve ganglia with respect to

Fig. 22. — Horizontal Section through
the Brain of Ammoccetes.

Cr., membranous cranium ; I, olfactory
nerves; l.v., lateral ventricles; gl., glan-
dular tissue which fills up the cranial
cavity.

the intestinal tube in the case
of the invertebrate. Thus in birds a portion of the spinal cord
in the lumbo-sacral region presents a very different appearance

from the rest of the cord ; it is
known as the rhomboidal sinus,
and a section of the cord of an
adult pigeon across this region is
given in Fig. 23. As is seen, the
nervous portions are entirely con-
Pig. 23.— Section through Rhomboidal fi ne d to two masses connected

together by the white anterior
commissures which are situated laterally and ventrally to a
median gelatinous mass ; the small central canal is visible and

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 47

the whole dorsal area of the cord is taken up by a peculiar non-
nervous wedge-shaped mass of tissue. At its first formation this
portion of the cord is formed exactly in the same manner as the rest
of the cord ; instead, however, of the nervous material invading the
dorsal part of the tube to form the posterior fissure, it has been from
some cause unable to do so, the walls of the original non-nervous
tube have become thickened dorsally, been transformed into this
peculiar tissue, and so caused the peculiar appearance of the cord
here. The nervous parts have not suffered in their development ;
the mechanism for walking in the bird is as well developed as in
any other animal ; their position only is different, for they still retain
the original ventro-lateral position, but the non-nervous tube, the
remains of the old intestine, has undergone a peculiar gelatinous
degeneration just where it has remained free from invasion by the
nervous tissue.

Throughout the whole of that part of the nervous system which
gives origin to the cranial and spinal segmental nerves, the evidence
is absolutely uniform that the nervous material was originally
arranged bilaterally and ventrally on each side of the central tube,
exactly in the same way as the nerve-masses of the infra-oesophageal
and ventral chain of ganglia are arranged with respect to the cephalic
stomach and straight intestine of the arthropod. But, in addition, we
find in the vertebrate nervous masses, the cerebral hemispheres, the
corpora quadrigemina and the cerebellum situated on the dorsal side
of the central tube in the brain-region ; this nervous material is,
however, of a different character to that which gives origin to the
spinal and cranial segmental nerves. How is the presence of these
dorsal masses to be explained on the supposition that the dilated
anterior part of the nerve-tube was originally the cephalic stomach
of the arthropod ancestor ? The cerebral hemispheres are simple
enough, for they represent the supra-cesophageal ganglia, which of
necessity, as they increased in size, would grow round the anterior
end of the cephalic stomach and become more and more dorsal in
position.

The difficulty lies rather in the position of the cerebellum and
corpora quadrigemina, and the solution is as simple as it is
conclusive.

Let us again turn to embryology and see what help it gives. In
all vertebrates the dilated anterior portion of the nerve-tube does not,

48 THE ORIGIN OF VERTEBRATES

as it grows, increase in size uniformly, but a constriction appears on
its dorsal surface at one particular place, so as to divide it into an
anterior and posterior vesicle ; then the latter becomes divided into
two portions by a second constriction. In this way three cerebral
vesicles are formed ; these three primary cerebral vesicles indicate
the region of the fore-brain, mid-brain, and hind-brain respectively.
Subsequently the first cerebral vesicle becomes divided into two to
form the prosencephalon and thalamencephalon, while the third
cerebral vesicle is also divided into two to form the region of the
cerebellum and medulla oblongata.

These constrictions are in the position of commissural bands of
nervous matter ; of these the limiting nervous strands between the
thalamencephalon and mesencephalon and between the mesencephalon
and the hind-brain are of primary importance. The first of these
commissural bands is in the position of the posterior commissure
connecting the two optic thalami. In close connection with this are
found, on the mid-dorsal region, the two pineal eyes with their optic
ganglia, the so-called ganglia habenulce. From these ganglia a
peculiar tract of fibre, known as Meynert's bundle, passes on each
side to the ventral infra-infundibular portion of the brain. In other
words, the first constriction of the dilated tube is due to the presence
and growth of nervous material in connection with the median pineal
eyes. Here in precisely the same spot, as will be fully explained
in the next chapter, there existed in the arthropod ancestor a pair
of median eyes situated dorsally to the cephalic stomach, the pre-
existence of which explains the reason for the first constriction.

The second primary constriction separating the mid-brain from
the hind-brain is still more interesting, for it is coincident with the
position of the trochlear or fourth cranial nerve. In all vertebrates
without exception this nerve takes an extraordinary course ; all other
nerves, whether cranial or spinal, pass ventralwards to reach their
destination. This nerve passes dorsalwards, crosses its fellow mid-
dorsally in the valve of Vieussens, where the roof of the brain is
thin, and then passes out to supply the superior oblique muscle of the
eye of the opposite side. The two nerves form an arch constricting
the dilated tube at this place. In the lowest vertebrate ( Ammoccetes)
the constriction formed by this nerve-pair is evident not only in the
embryonic condition as in other vertebrates, but during the whole
larval stage. As Fig. 20, A and B, shows, the whole of the dorsal

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 49

region of the brain up to the region of the pineal eye and ganglion
habenultc is one large membranous bag, except for the single con-
striction where the fourth nerve on each side crosses over. The
explanation of this peculiarity is given in Chapter VII., and follows
simply from the facts of the arrangement of that musculature in the
scorpion-group which gave rise to the eye-muscles of the vertebrate.

In Ammocoetes both cerebellum and posterior corpora quad-
rigemina can hardly be said to exist, but upon transformation a
growth of nervous material takes place in this region, and it is seen
that this commencing cerebellum and the corpora quadrigemina arise
from tissue that is present in Ammocoetes along the course of the
fourth nerve.

Here, then, again Embryology does its best to tell us how the
vertebrate arose. The formation of the two primary constrictions
in the dilated anterior vesicle whereby the brain is divided into
fore-brain, mid-brain, and hind-brain is simply the representation
ontogenetically of the two nerve-tracts which crossed over the
cephalic stomach in the prevertebrate stage, in consequence of
the mid-dorsal position of the pineal eyes and of the insertion of
the original superior oblique muscles.

The subsequent constriction by which the prosencephalon is
separated from the thalamencephalon is in the position of the
anterior commissure, that commissure which connects the two supra-
infundibular nerve-masses, and is one of the first-formed commis-
sures in every vertebrate. This naturally is simply the commissure
between the two supra-oesophageal ganglia; anterior to it, in the
middle line, equally naturally, the anterior end of the old stomach
wall still exists as the lamina terminalis.

The other division in the hind-brain region, which separates the
region of the cerebellum from the medulla oblongata, is due to the
growth of the cerebellum, and indicates its posterior limit. In such
an animal as the lamprey, where the cerebellum is only commencing,
this constriction does not occur in the embryo.

From such simple beginnings as are seen in Ammocoetes, the
higher forms of brain have been evolved, to culminate in that of man,
in which the massive cerebrum and cerebellum conceals all sio-n of
the dorsal membranous roof, those parts of the simple epithelial tul >e
which still remain being tucked away into the cavities to form the
various choroid plexuses.

£

50

THE ORIGIN OF VERTEBRATES

In the whole evolution from the brain of Ammocoetes to that of
man, the same process is plainly visible, viz. growth and extension
of nervous material over the epithelial tube; extension dorsally and
posteriorly of the supra-infundibular nervous masses (as seen in
Fig. 19), combined with a dorsal growth of parts of the infra-
infundibular nervous masses to form the cerebellum and posterior
corpora quadrigemina.

Espceially instructive is the formation of the cerebellum. It
consists at first of a small mass of nervous tissue accompanying the

fourth nerve, then by the growth of that mass
surrounding and constricting a fold of the
membranous roof, the worm of the cerebellum
is formed, as in the dog-fish. This very con-
striction causes the membrane to be thrown
into a lateral fold on each side, as seen in
Fig. 24, and in the dog-fish the nervous material
on each side, known as the fimbriae, is already
commencing to grow from the ventral mass of
the medulla oblongata to surround these lateral
membranous folds. These fimbriae develop more
and more in higher forms, and thus form the
cerebellar hemispheres.

Not only does comparative anatomy confirm
the teachings of embryology, but also pathology
gives its quota in the same direction.

One of the striking facts about malforma-
tions and disease of the central nervous system
is the frequency of cystic formations ; spina
bifida is a well-known instance. These cysts are merely epithelial
non-nervous cysts formed from the epithelium of the central canal,
dilficult to understand if the whole nerve tube is one and entirely
nervous, either actually or potentially, but natural and easy if we
are really dealing with a simple epithelial tube on the outside of
which the nervous material was originally grouped. The cystic
formation belongs naturally enough to this tube, not to the nervous
system.

Again, where animals such as lizards have grown a new tail,
owing to the breaking off of the original one, it is found that the
central canal extends into this new tail for some distance, but not

Fig. 24. — Cebebel-
lum of Dog-fish.

v, worm of cerebel-
lum; IV., membra-
nous roof of fourth
ventricle continuous
with the membra-
nous folds on each
side. Through these
the fimbrise (fb.) can
be dimly seen.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 51

the nervous material surrounding it ; all the nerves supplying the
new tail arise from the uninjured spinal cord above, the central
canal with its lining layer of epithelial cells alone grows into the
new-formed appendage.

To all intents and purposes the same thing is seen in the termi-
nation of the spinal cord in a bird-embryo; more and more, as the
end of the tail is approached, does the nervous matter of the spinal
cord grow less and less, until at last a naked central canal with
its lining epithelium is alone left to represent the so-called nerve-
tube.

All these different methods of investigation lead irresistibly to
the one conclusion that the tubular nature of the central nervous
system has been caused by the central nervous system enclosing to a
greater or less extent a pre-existing, non-nervous, epithelial tube.

This must always be borne strictly in mind. The problem, there-
fore, which presents itself is the comparison of these two factors
separately, in order to find out the relationship of the vertebrate to
the invertebrate. The nervous system without the tube must be
compared to other nervous systems, and the tube must be considered
apart from the nervous system.

The Principle of Concentration and Cefhalizatiox.

The central nervous system of the vertebrate resembles that of
all the Appendiculata in the fact that it is composed of segments
joined together which give origin to segmental nerves. There is,
however, a great difference between the two systems : the division
into separate segments is not obvious to the eye in the vertebrate
nervous system, while in the invertebrate we can see that it is
composed of a series of separate pairs of ganglia joined together
longitudinally by nervous strands known as connectives and trans-
versely by the nerve-commissures. Such a simple segmented system
is found in the segmented worms, and in the lower arthropods, such
as Branchipus, no great advance has been made on that of the annelid.
In the higher forms, however, a greater and greater tendency to fusion
of separate ganglia exists, especially in the head-region, so that the
infra- (esophageal ganglia, which, in the lower forms are as separate
as those of the ventral chain, in the higher forms are fused together
to form a single nervous mass.

52 THE ORIGIN OF VERTEBRATES

This is the great characteristic of the advancement of the central
nervous system among the Invertebrata, its concentration in the
region of the head. It may be called the principle of cephalization,
and is characteristic not only of higher organization in a group, but
also of the adult as distinguished from the larval form. Thus in the
imago greater concentration is found than in the caterpillar.

The segmented annelid type of nervous system consists of a
supra-oesophageal ganglion, composed of the fused ganglia belonging
to the pre-oral segments, and an infra-cesophageal chain of separate
ganglia. With the concentration and modification around the
mouth of the most anterior locomotor appendages to form organs
for prehension and mastication of food, a corresponding concentra-
tion and fusion of the ganglia belonging to these segments takes
place, so that finally, in the higher annelids, and in most of the great
arthropod group, a fusion of a number of the most anterior ganglia
has taken place to form the infra-cesophageal ganglion-mass.

The infra-cesophageal ganglia which are the first to fuse are
those which supply the most anterior portion of the animal with
nerves, and include always those anterior appendages which are
modified for mastication purposes. To this part the name pivsoma
has been given ; in many cases it forms a well-defined, distinct
portion of the animal.

Succeeding this prosoma or masticatory region, there occurs in
all gill-bearing arthropods a respiratory region, in many cases more
or less distinctly defined, which has received the name of mcsosoma.
The rest of the body is called the metasoma.

In accordance with this nomenclature the central nervous system
of many of the Arthropoda may be divided as follows : —

1. Pre-oral, or supra-oesophageal ganglia.

2. Infra-oral, or infra-cesophageal ganglia and ventral chain,
which consist of three groups : prosomatic, mesosomatic, and meta-
somatic ganglia.

The infra-cesophageal ganglion- mass, then, in most of the Arthro-
poda may be spoken of as formed by the fusion of the prosomatic or
mouth-ganglia, the mesosomatic and metasomatic remaining separate
and distinct. The number of ganglia which have fused may be
observed by examination of the embryo, in which it is easy to see
indications of the individual ganglia or ncuromercs, although all
such indication has disappeared in the adult ; thus the infra-ceso-

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 53

phageal ganglia of the cray-fish have been shown to be constituted
of six prosomatic ganglia.

In Fig. 25 I give figures of the central nervous system (with the
exception of the abdominal or metasomatic ganglia) of Branchipus,
Astacus, Limulus, Scorpio, Androctonus, Thelyphonus, and Ammo-
ccetes. In all the figures the supra-cesophageal ganglia are lined
horizontally, and their nerves shown, viz. optic (lateral eyes (II) and
median eyes (II')), olfactory (I) (first antenna?, camerostome, nose);
then come the prosomatic ganglia (dotted), with their nerves (A)
supplying the mouth parts, and the second antenna? or chelicera? ;
then the mesosomatic (lined horizontally), with their nerves (B)
supplying respiratory appendages. These figures show that the con-
centrated brain mass around the oesophagus of an arthropod which
has arrived at the stage of Astacus, is represented by the supra-
ossophageal ganglia and the fused prosomatic ganglia.

The next stage in the evolution of the brain is seen in the
gradual in lusion of the mesosomatic ganglia, one after the other,
into the infra-cesophageal mass of the already fused prosomatic
ganglia. "With this fusion is associated the loss of locomotion in
these mesosomatic appendages, and their entire subservience to the
function of respiration. Dana urges that cephalization is a conse-
quence of functional alteration in the appendages, from organs of
locomotion to those of mastication and respiration. Whether this be
true or not, it is certainly a fact that in Limulus, the ganglion
supplying the first mesosomatic appendage has fused with the
prosomatic, infra-cesophageal mass. It is also a fact that the proso-
matic appendages are the organs of mastication, their basal parts
being arranged round the mouth so as to act as foot-jaws, while the
mesosomatic appendages, though still free to move, have been
reduced to such an extent as to consist mainly of their basal parts,
which are all respiratory in function, except in the case of the first
pair, where they carry the terminal ducts of the genital organs. In
the next stage, that, of the scorpion, in which the mesosomatic
appendages have lost all power of free locomotion, and have become
internal branchiae, another mesosomatic ganglion has fused with the
brain mass, while in Androctonus two of the branchial mesosomatic
ganglia have fused ; and finally, in Thelyphonus and Phrynus, all
the mesosomatic ganglia have coalesced with the fused prosomatic
ganglia, while the metasomatic ganglia have themselves fused

54

THE O RIG IS OF VERTEBRATES

ANDROCTONUS

AMMOCCETES

Fig. 25. — Comparison of Invertebrate Brains from Branchipus to

Ammoccetes.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 55

together in the caudal region to form what is known as the caudal
brain.

The brain in these animals may be spoken of as composed of
three parts — (1) the fused supra-cesophageal ganglia, (2) the fused
prosomatic ganglia, and (3) the fused mesosomatic ganglia. Such a
brain is strictly homologous with the vertebrate brain, which also is
built up of three parts — (1) the part in front of the notochord, the
prechordal or supra-infundibular brain, which consists of the cerebral
hemispheres, together with the basal and optic ganglia and corre-
sponds, therefore, to the supra-cesophageal mass, with its olfactory
and optic divisions lying in front of the oesophagus ; (2 and 3) the
epichordal brain, composed of (2) a trigeminal and (3) a vagus divi-
sion, of which the first corresponds strictly to the fused prosomatic
ganglia, and the second to the fused mesosomatic ganglia. Further,
just as in the embryo of an arthropod it is possible, with more or
less accuracy, to see the number of neuromeres or original ganglia
which have fused to form the supra- and infra- oesophageal portions
of its brain, so also in the embryo of a vertebrate we are able at
an early stage to gain an indication, more or less accurate, of the
number of neuromeres which have built up the vertebrate brain.
The further consideration of these neuromeres, and the evidence they
afford as to the number of the prosomatic and mesosomatic ganglia
which have formed the epichordal part of the vertebrate brain, must
be left to the chapter on the segmentation of the cranial nerves.

The further continuation of this process of concentration of
separate segments, together with the fusion of the nervous system
with the tube of the alimentary canal, leads in the simplest manner
to the formation of the spinal cord of the vertebrate from the meta-
somatic ganglia of the ventral chain of the arthropod.

The Antagonism between Cephalization and Alimentation.

This concentration of the nervous system in the head- region,
together with an actual increase in the bulk of the cephalic nervous
masses, constitutes the great principle upon which the law of upward
progress or evolution in the animal kingdom is based, and it illus-
trates in a striking manner the blind way in which natural selection
works; for, as already explained, the central nervous system arose as
a ring round the mouth, in consequence of which, with the progressive

56

THE ORIGIN OF VERTEBRATES

evolution of the animal kingdom, the oesophagus necessarily pierced
the central nervous system at the cephalic end. At the same time,
the very fact that the evolution was progressive necessitated the
concentration and increase of the nervous masses in this very same
oesophageal region.

Progress on these lines must result in a crisis, owing to the
inevitable squeezing out of the food-channel by the increasing nerve-
mass ; and, indeed, the fact that such a crisis had in all probability
arisen at the time when vertebrates first appeared is apparent when
we examine the conditions at the present time.

Those invertebrates whose central nervous system is most con-
centrated at the cephalic end belong to the arachnid group, among
which are included the various living scorpion-like animals, such as
Thelyphonus, Androctonus, etc.

As already mentioned, the giants of the Palaeostracan age were

Pterygotus, Slimonia, etc., all animals of the scorpion-type — in fact,

A sea - scorpions. Now, all these

,S "•'.. animals, spiders and scorpions,

without exception, are blood -
suckers, and in all of them the
concentrated cephalic mass of ner-
vous material surrounds an oeso-
phagus the calibre of which is so
small that nothing but a fluid
pabulum can be taken into the
alimentary canal ; and even for
that purpose a special suctorial
apparatus has in some species
been formed on the gastric side
of the oesophagus for the purpose
of drawing blood through this

B

Fig. 26. — Transverse Section
through the brain of a young
Thelyphonus.

exceedingly narrow tube.

increasing

In Fig. 25 this
antagonism between brain-power
and alimentation, as we pass from
such a form as Branchipus to the
scorpion, is illustrated, and in Fig. 26 the relative sizes of the
oesophagus and the brain-mass surrounding it is shown. The section
shows that the food channel is surrounded by the white and grey

-4, supra-oesophageal ganglia; B, infra
oesophageal ganglia; Al, cesopkagus.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 57

matter of the brain as completely as the central canal of the spinal
cord of the vertebrate is surrounded by the white and grey nervous
material.

Truly, at the time when vertebrates first appeared, the direction
and progress of variation in the Arthropoda was leading, owing to
the manner in which the brain was pierced by the oesophagus, to a
terrible dilemma — either the capacity for taking in food without
sufficient intelligence to capture it, or intelligence sufficient to capture
food and no power to consume it.

Something had to be done — some way had to be found out of this
difficulty. The atrophy of the brain meant degeneration and the
reduction to a lower stage of organization, as is seen in the Tunicata.
The further development of the brain necessitated the establish-
ment of a new method of alimentation and the closure of the old
oesophagus, its vestiges still remaining as the infundibular canal of
the vertebrate, meant the enormous upward stride of the formation
of the vertebrate.

At first sight it might appear too great an assumption even to
imagine the possibility of the formation of a new gut in an animal so
highly organized as an arthropod, but a little consideration will, I
think, show that such is not the case.

In the higher animals we are accustomed to speak of certain
organs as vital and necessary for the further existence of the animal ;
these are essentially the central nervous system, the respiratory
system, the circulatory system, and the digestive system. Of these
four vital systems the first cannot be touched without the chance
of degeneration ; but that is not the case with the second. The
passage from the fish to the amphibian, from the water-breathing
to the air-breathing animal, has actually taken place, and was effected
by the modification of the swim-bladder to form new respiratory
organs — the lungs ; the old respiratory organs — the gills — becoming
functionless, but still persisting in the embryo as vestiges. The
necessity arose in consequence of the passage of the animal from
water to land, and with this necessity nature found a means of over-
coming the difficulty ; air-breathing vertebrates arose, and from the
very fact of their being able to extend over the land-surfaces,
increased in numbers and developed in complexity in the manner
already sketched out.

For a respiratory system all that is required is an arrangement

58 THE ORIGIN OF VERTEBRATES

by means of which blood should be brought to the surface, so as to
interchange its gases with those of the external medium ; and it is
significant to find that of all vertebrates the Amphibia alone are
capable of an effective respiration by means of the skin.

As to the circulatory system, it is exceedingly easily modified.
An animal such as Amphioxus has no heart ; in some the heart is
systemic, in others branchial ; in some there are more than one heart ;
in others there are contractile veins in addition to a heart. There
is no difficulty here in altering and modifying the system according
to the needs of the individual.

For a digestive system all that is required is an arrangement for
the digestion and absorption of food, a mechanism which can arise
easily if some of the cells of the skin possess digestive power. Now
Miss Alcock has shown that some of the surface-cells of crustaceans
secrete a fluid which possesses digestive powers, and she has also
shown that certain of the cells in the skin of Ammocoetes possess
digestive power.

The difficulty, then, of forming a new digestive system in the
passage from the arthropod to the vertebrate is very much the same
as the difficulty in forming a new respiratory system in the passage
from the water-breathing fish to the air-breathing amphibian — a
change which does not strike us as inconceivable, because we know it
has taken place.

The whole argument so far leads to the conclusion that vertebrates
arose from ancient forms of arthropods by the formation of a new
alimentary canal, and the enclosure of the old canal by the growing
central nervous system. If this conclusion is true, then it follows
that we possess a well-defined starting-point from which to compare
the separate organs of the arthropod with those of the vertebrate,
and if, in consequence of such working hypothesis, each organ of the
arthropod is found in the vertebrate in a corresponding position and
of similar structure, then the truth of the starting-point is proved as
fully as can possibly be expected by deductive methods. It is, in
fact, this method of comparative anatomy which has proved the
descent of man from the ape, the frog from the fish, etc.

Let us, then, compare all the organs of such a low vertebrate as
Ammocoetes with those of an arthropod of the ancient type.

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 59

Life History of the Lamprey — not a Degenerate Animal.

The striking peculiarity of the lamprey is its life-history. It
lives in fresh water, spending a large portion of its life in the mud
during the period of its larval existence : then comes a somewhat
sudden transformation-stage, characterized, as in the lepidopterous
larva, by a process of histolysis, by which many of the larval tissues
are destroyed and new ones formed, with the result that the larval
lamprey, or Ammoccetes, is transformed into the adult lamprey, or
Petromyzon. This transformation takes place in August, at all
events in the neighbourhood of Cambridge, and later in the year the
transformed lamprey migrates to the sea, grows in size and maturity,
and returns to the river the following spring up to its spawning beds,
where it spawns and forthwith dies. How long it lives in the Ammo-
coetes stage is unknown ; I myself have kept some without transfor-
mation for four years, and probably they live in the rivers longer
than that before they change from their larval state. It is absolutely
certain that very much the longest part of the animal's life is spent
in the larval stage, and that with the maturity of the sexual organs
and the production of the fertilized ova the life of the individual ends.

Now, the striking point of this transformation is that it produces
an animal more nearly comparable with higher vertebrates than is
the larval form ; in other words, the transformation from larva to
adult is in the direction of upward progress, not of degeneration.
It is, therefore, inaccurate to speak of the adult lamprey as
degenerate from a higher race of fishes represented by its larval form
— Ammoccetes. Its transformation does not resemble that of the
tunicates, but rather that of the frog, so that, just as in the case of
the tadpole, the peculiarities of its larval form may be expected to
afford valuable indications of its immediate ancestry. The very
peculiarities to which attention must especially be paid are those
discarded at transformation, and, as will be seen, these are essentially
characteristic of the invertebrate and are not found in the higher
vertebrates. In fact, the transformation of the lamprey from the
Ammoccetes to the Petromyzon stage may be described as the casting
off of many of its ancestral invertebrate characters and the putting
on of the characteristics of the vertebrate type. It is this double
individuality of the lamprey, together with its long-continued
existence in the larval form, which makes Ammoccetes more

60 THE ORIGIN OF VERTEBRATES

valuable than any other living vertebrate for the study of the stock
from which vertebrates sprang.

Many authorities hold the view that the lamprey, like Amphioxus,
must he looked upon as degenerate, and therefore as no more suitable
for the investigation of the problem of vertebrate ancestry than is
Amphioxus itself. This charge of degeneracy is based on the state-
ment that the lamprey is a parasite, and that the eyes in Ammoccetes
are under the skin. The whole supposition of the degeneracy of the
Cyclostomata arose because of the prevailing belief of the time that
the earliest fishes were elasmobranchs, and therefore gnathosto-
matous. From such gnathostomatous fishes the cyclostomes were
supposed to have descended, having lost their jaws and become
suctorial in habit in consequence of their parasitism.

The charge of parasitism is brought against the lamprey because
it is said to suck on to fishes and so obtain nutriment. It is, how-
ever, undoubtedly a free-swimming fish ; and when we see it coming
up the rivers in thousands to reach the spawning-beds, and sucking
on to the stones on the way in order to anchor itself against the
current, or holding on tightly during the actual process of spawning,
it does not seem justifiable to base a charge of degeneration upon a
parasitic habit, when such so-called habit simply consists in holding
on to its prey until its desires are satisfied. If, of course, its suctorial
mouth had arisen from an ancestral gnathostomatous mouth, then
the argument would have more force.

Dohrn, however, gives absolutely no evidence of a former
gnathostomotous condition either in Petromyzon or, in its larval
state, Ammoccetes. He simply assumes that the Cyclostomata are
degenerated fishes and then proceeds to point out the rudiments of
skeleton, etc., which they still possess. Every point that Dohrn
makes can be turned round ; and, with more probability, it can be
argued that the various structures are the commencement of the
skeletal and other structures in the higher fishes, and not their
degenerated remnants. Compare the life-history of the lamprey
and of the tunicate. In the latter case we look upon the animal as
a degenerate vertebrate, because the larval stage alone shows verte-
brate characteristics ; when transformation has taken place, and the
adult form is reached, the vertebrate characteristics have vanished,
and the animal, instead of reaching a higher grade, has sunk lower
in the scale, the central nervous system especially having lost all

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 1

resemblance to that of the vertebrate. In the former case a trans-
formation also takes place, a marvellous transformation, characterized
by two most striking facts. On the one hand, the resulting animal
is more like a higher vertebrate, for, by the formation of new
cartilages, its cranial skeleton is now comparable with that of the
higher forms, and the beginnings of the spinal vertebrae appear ; by
the increased formation of nervous material, its brain increases in
size and complexity, so as to compare more closely with higher
vertebrate brains ; its eyes become functional, and its branchiae are
so modified, simultaneously with the formation of the new alimentary
canal in the cranial region, that they now surround branchial pouches
which are directly comparable to those of higher vertebrates. On
the other hand, the transformation process is equally characterized
by the throwing off of tissues and organs, one and all of which are
comparable in structure and function with corresponding structures in
the Arthropoda — the thyroid of the Ammoccetes, the tentacles, the
muco-cartilage, the tubular muscles, all these structures, so striking
in the Ammoccetes stage, are got rid of at transformation. Here is
the true clue. Here, in the throwing off of invertebrate characters,
and the taking on of a higher vertebrate form, especially a higher
brain, not a lower one, Petromyzon proclaims as clearly as is possible
that it is not a degenerate elasmobranch, but that it has arisen from
Ammocoetes-like ancestors, even though Myxine, Amphioxus, and
the tunicates be all stages on the downward grade from those same
Ammoccetes-like ancestors.

As to the eyes, they are functional in the adult form and as service-
able as in any fish. There is no sign of degeneracy; it is only possible
to speak of a retarded development which lasts through the larval stage.

Comparison of Brain of Ammocqites with that of an

Arthropod,

Seeing that the steady progress of the development of the central
nervous system is the most important factor in the evolution of
animals, it follows that of all organs of the body, the central nervous
system must be most easily comparable with that of the supposed
ancestor. I will, therefore, start by comparing the brain of
Ammocoetes with that of arthropods, especially of Limulus and of
the scorpion-group.

62

THE ORIGIN OF VERTEBRATES

The supra-infundibular portion of the brain in vertebrates
corresponds clearly to the supra-cesophageal portion of the inverte-
brate brain in so far that in both cases here is the seat of the will.
Voluntary action is as impossible to the arthropod deprived of its
supra-cesophageal ganglia as to the vertebrate deprived of its cere-
brum. It corresponds, also, in that from it arise the nerves of sight
and smell and no other nerves ; this is also the case with the supra-
cesophageal ganglia, for from a portion of these ganglia arise the nerves
to the eyes and the nerves to the first antennte, of which the latter
are olfactory in function. Thus, in the accompanying figure, taken
from Bellonci, it is seen that the supra-cesophageal ganglia consist

Sup. Segment Ant I

Ant II

Inf. Segment

Fig. 27. — The Brain op Sphceroma serratum. (After Bellonci.)

Ant. I. and Ant. II., nerves to 1st and 2nd antenna?, f.br.r., terminal fibre layer of
retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n.,
optic nerve-fibres forming an optic cbiasma.

of a superior segment corresponding to the cerebrum, a middle
segment from which arise the nerves to the lateral eyes and to the
olfactory antennas, corresponding to the basal ganglia of the brain
and the optic lobes, and, according to Bellonci, of an inferior segment
from which arise the nerves to the second pair of antennae. This
last segment is not supra-cesophageal in position, but is situated on
the oesophageal commissures. It has been shown by Lankester and
Brauer in Limulus and the scorpion to be in reality the first ganglion
of the infra-cesophageal series, and not to belong to the supra-
cesophageal group.

Further, in Limulus, in the scorpion-group, and in all the extinct

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6

O

Eurypteridce— in fact, in the Palaaostraca generally — there are two
median eyes in addition to the lateral eyes, which were innervated
from these ganglia.

In Ammoccetes, then, if the supra-infundibular portion of the
brain really corresponds to the supra-cesophageal of the paleeostracan
group, we ought to find, as indeed is the case, an optic apparatus
consisting of two lateral eyes and two median eyes, innervated from
the supra-infundibular brain-mass, and an olfactory apparatus built
up on the same lines as in the scorpion-group, also innervated from
this region. If, in addition, it be found that those two median eyes
are degenerate eyes of the same type as the median eyes of Limulus
and the scorpion-group, then the evidence is so strong as to amount
to a proof of the correctness of the theory. This evidence is precisely
what has been obtained in recent years, for the vertebrate did possess
two median eyes in addition to the two lateral ones, and these two
median eyes are degenerate eyes of the type found in the median
eyes of arthropods and are not of the vertebrate type. Moreover, as
ought also to be the case, they are most evident, and one of the
pair is most nearly functional in the lowest perfect vertebrate,
Anmiocoetes.

Of all the discoveries made in recent years, the discovery that
the pineal gland of the vertebrate brain was originally a pair of
median eyes is by far the most important clue to the ancestry of
the vertebrate, for not only do they correspond exactly in position
with the median eyes of the invertebrates, but, being already
degenerate and functionless in the lowest vertebrate, they must have
been functional in a pre-vertebrate stage, thus giving the most direct
clue possible to the nature of the pre-vertebrate stage. It is
especially significant that in Limulus they are already partially
degenerated. What, then, ought to.be the structure and relation to
the brain of the median and lateral eyes of the vertebrate if they
originated from the corresponding organs of some one or other member
of the paheostracan group ?

This question will form the subject of the next chapter.

Summary.

The object of this book is to attempt to find out from what group of inverte-
brates the vertebrate arose ; no attempt is made to speculate upon the causes of
variation by means of which evolution takes place.

64 THE O RIG IX OF VERTEBRATES

A review of the animal kingdom as a whole leads to the conclusion that the
upward development of animals from an original coelenterate stock, in which
the central nervous system consists of a ring of nervous material surrounding
the mouth, has led. in consequence of the elaboration of the central nervous
system, to a general plan among the higher groups of invertebrates in the topo-
graphical arrangement of the important organs. The mouth is situated ventrally.
and leads by means of the oesophagus into an alimentary canal which is situated
dorsally to the central nervous system. Thus the oesophagus pierces the central
nervous system and divides it into two parts, the supra-oesophageal ganglia
and the infra-cesophageal gangdia. This is an 'almost universal plan among
invertebrates, but apparently does not hold for vertebrates, for in them the
central nervous system is always situated dorsally and the alimentary canal
ventrally, and there is no piercing of the central nervous system by an oesophagus.

Yet a remarkable resemblance exists between the central nervous system of
the vertebrate and that of the higher invertebrates, of so striking - a character as
to compel one school of anatomists to attempt the derivation of vertebrates
from annelids. Now, the central nervous system of vertebrates forms a hollow
tube, and a diverticulum of this hollow tube, known as the infundibulum, passes
to the ventral surface of the brain in the very position where the oesophagus
would have been if that brain had belonged to an annelid or an arthropod.
This school of anatomists therefore concluded that this infundibular tube
rejn'esented the original invertebrate oesophagus which had become closed and
no longer opened into the alimentary canal owing to the formation of a new
niouth in the vertebrate. As, however, the alimentary canal of the vertebrate
is ventral to the central nervous system, and not dorsal, as in the invertebrate,
it follows that the remains of the original invertebrate mouth into which the
oesophagus (in the vertebrate the infundibular tube) must have opened must be
searched for on the dorsal side of the vertebrate ; and so the theory was put
forward that the vertebrate had arisen from the annelid by the reversal of
surfaces, the back of the one animal becoming the front of the other.

The difficulties in the way of accepting such reversal of surfaces have proved
insuperable, and another school has arisen which suggests that evolution has
throughout proceeded on two lines, the one forming - g - roups of animals in which
the central nervous system is pierced by the food-channel and the gut therefore
lies dorsally to it, the other in which the central nervous system always lies
dorsally to the alimentary canal and is not pierced by it. In both cases the
highest products of the evolution have become markedly segmented animals, in
the former, annelids and arthropods ; in the latter, vertebrates. The only
evidence on which such theory is based is the existence of low forms of animals,
known as the Enteropneusta, the best known example of which is called
BalauiHjlossiis ; they are looked upon as aberrant annelid forms by many
observers.

This theoiy does not attempt to explain the peculiarities of the tube of the
vertebrate central nervous system, or to account for the extraordinary resemblance
between the structure and arrangement of the central nervoiis systems of
vertebrates and of the highest invertebrate group.

Neither of these theories is satisfactory or has secured universal acceptance.
The problem must be considered entirely anew. What are the g - uiding principles
in this investigation ?

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 65

The evolution of animal life on this earth can clearly, on the whole, he
described as a process of upward progress culminating in the highest form —
man ; but it must always be remembered that whole groups of animals such as the
Tunicata have been able to survive owing to a reverse process of degeneration.

If there is one organ more than another which increases in complexity as
evolution proceeds, which is the most essential organ for upward progress, surely
it is the central nervous system, especially that portion of it called the brain.
This consideration points directly to the origin of vertebrates from the most
highly organized invertebrate group — the Arthropoda — for among all the groups
of animals living on the earth in the present day they alone possess a central
nervous system closely comparable with that of vertebrates. Not only has an
upward progress taken place in animals as a whole, but also in the tissues them-
selves a similar evolution is apparent, and the evidence shows that the vertebrate
tissues resemble more closely those of the arthropod than of any other inverte-
brate group.

The evi deuce of geology points to the same conclusion, for the evidence of
the rocks shows that before the highest mammal — man— appeared, the dominant
race was the mammalian quadruped, from whom the highest mammal of all —
man — sprung ; then comes, in Mesozoic times, the age of reptiles which were
dominant when the mammal arose from them. Preceding this era we find in
Carboniferous times that the amphibian was dominant, and from them the next
higher group — the reptiles — arose. Below the Carboniferous come the Devonian
strata with their evidence of the dominance of the fish, from whom the
amphibian was directly evolved. The evidence is so clear that each succeeding
higher form of vertebrate arose from the highest stage reached at the time,
as to compel one to the conclusion that the fishes arose from the race which
was dominant at the time when the fishes first appeared. This brings us to the
Silurian age, in which the evidence of the rocks points unmistakably to the sea-
scorpions, king-crabs, and trilobites as being the dominant race. It was preceded
by the great trilobite age, and the whole period, from the first appearance of the
trilobite to the time of dwindling away of the sea-scorpions, may be designated
the Pakeostracan age, using the term Palaeostraca to include both trilobites and
the higher scorpion and king-crab forms evolved from them. The evidence of
geology then points directly and strongly to the origin of vertebrates from the
Palaeostraca — arthropod forms which were not crustacean and not arachnid,
but gave origin both to the modern-day crustaceans and arachnids. The
history of the rocks further shows that these ancient fishes, when they first
appeared, resembled in a remarkable manner members of the palaeostracan group,
so that again and again paleontologists have found great difficulty in determin-
ing whether a fossil is a fish or an arthropod. Fortunately, there is still alive
on the earth one member of this remarkable group— the Limulus, or King-
Crab. On the vertebrate side the lowest non-degenerate vertebrate is the
lamprey, or Petromyzon, which spends a large portion of its existence in a
larval stage, known as the Ammoccetes stage of the lamprey, because it was
formerly considered to be a separate species and received the name of
Ammoccetes. The larval stages of any animal are most valuable for the study
of ancestral problems, so that it is most fortunate for the solution of the ancestry
of vertebrates that Limulus on the one side and Ammoccetes on the other are

F

66 THE ORIGIN OF VERTEBRATES

available for thorough investigation and comparison. There are no trilobites
still alive, but in Branchipus and Apus we possess the nearest approach to the
trilobite organization among living crustaceans.

So strongly do all these different lines of argument point to the origin of
vertebrates from arthropods as to make it imperative to reconsider the position
of that school of anatomists who derived vertebrates from annelids by reversing
the back and front of the animal. Let us not turn the animal over, but
re-consider the position, the infundibular tube of the vertebrate still representing
the oesophagus of the invertebrate, the cerebral hemispheres and basal ganglia
the supra-oesophageal ganglia, the crura cerebri the oesophageal commissures,
and the infra-infundibular part of the brain the infra-oesophageal ganglia. It
is immediately apparent that just as the invertebrate oesophagus leads into the
large cephalic stomach, so the infundibular tube leads into the large cavity of
the brain known as the third ventricle, which, together with the other ventricles,
forms in the embryo a large anterior dilated part of the neural tube. In the
arthropod this cephalic stomach leads into the straight narrow intestine ; in the
vertebrate the fourth ventricle leads into the straight narrow canal of the spinal
cord. In the arthropod the intestine terminates in the anus ; in the vertebrate
embryo the canal of the spinal cord terminates in the anus by way of the
neurenteric canal. Keep the animal unreversed, and immediately the whole
mystery of the tubular nature of the central nervous system is revealed, for it
is seen that the nervous matter, which corresponds bit by bit with that of the
arthropod, has surrounded to a greater or less extent and amalgamated with
the tube of the arthropod alimentary canal, and thus formed the so-called
central nervous system of the vertebrate.

The manner in which the nervous material has invaded the walls of the tube
is clearly shown both by the study of the comparative anatomy Of the central
nervous system in the vertebrate and also by its development in the embryo.

This theory implies that the vertebrate alimentary canal is a new formation
necessitated by the urgency of the case, and, indeed, there was cause for urgency,
for the general plan of the evolution of the invertebrate from the ccelenterate
involved the piercing of the anterior portion of the central nervous system by the
oesophagus, while, at the same time, upward progress meant brain-development ;
brain-development meant concentration of nervous matter at the anterior end
of the animal, with the result that in the highest scorpion and spider-like
animals the brain-mass has so grown round and compressed the food-tube that
nothing but fluid pabulum can pass through into the stomach ; the whole group
have become blood-suckers. These kinds of animals — the sea-scorpions — were
the dominant race when the vertebrates first appeared : here in the natural com-
petition among members of the dominant race the difficulty must have become
acute. Further upward evolution demanded a larger and larger brain with the
ensuing consequence of a greater and greater difficulty of food-supply. Nature's
mistake was rectified and further evolution secured, not by degeneration in the
brain-region, for that means degradation not upward progress, but by the
formation of a new food-channel, in consequence of which the brain was free
to develop to its fullest extent. Thus the great and mighty kingdom of the
Vertebrata was evolved with its culminating organism — man — whose massive
brain with all its possibilities could never have been evolved if he had still been

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 J

compelled to pass the whole of his food through the narrow oesophageal tube,
still existent in him as the infundibular tube. This, then, is the working
hypothesis upon which this book is written. If this view is right, that the
Vertebrate was formed from the Palajostracan without any reversal of surfaces,
but by the amalgamation of the central nervous system and alimentary canal,
then it follows that we have various fixed points of comparison in the central
nervous systems of the two groups of animals from which to search for further
clues. It further follows that from such starting-point every organ of importance
in the body of the arthropod ought to be visible in the corresponding position in
the vertebrate, either as a functional or rudimentary organ. The subsequent
chapters will deal with this detailed comparison of org*ans in the arthropod and
vertebrate respectively.

CHAPTER II

THE EVIDENCE OF THE ORGANS OF VISION

Different kinds of eye. — Simple and compound retinas. —Upright and inverted
retinas. — Median eyes. — Median or pineal eyes of Ammocoetes and their
optic ganglia. — Comparison with other median eyes. — Lateral eyes of verte-
brates compared with lateral eyes of crustaceans. — -Peculiarities of the
lateral eye of the lamprey. — Meaning of the optic diverticula. — Evolution
of vertebrate eyes. — Summary.

The Different Kinds of Eye.

In all animals the, eyes are composed of two parts. 1. A set of
special sensory cells called the retina. 2. A dioptric apparatus for
the purpose of forming an image on the sensory cells. The simplest
eye is formed from a modified patch of the surface-epithelium ; cer-
tain of the hypodermal cells, as they are called, elongate, and their
cuticular surface becomes bulged to form a simple lens. These
elongated cells form the retinal cells, and are connected with the
central nervous system by nerve-fibres which constitute an optic
nerve ; the cells themselves may contain pigment.

The more complicated eyes are modifications of this type for the
purpose of making both tho retina and the dioptric apparatus more
perfect. According to a very prevalent view, these modifications have
been brought about by invaginations of the surface- epithelium.
Thus if ABCD (Fig. 28) represents a portion of the surface-epithelium,
the chitinous cuticle being represented by the dark line, with
the hypodermal cells beneath, and if the part C is modified to form
an optic sense-plate, then an invagination occurring between A and B
will throw the retinal sense-cells with the optic nerve further from
the surface, and the layers B and A between the retina and the source
of light will be available for the formation of the dioptric apparatus.
The lens is now formed from the cuticular surface of A, and the

THE EVIDENCE OF THE ORGANS OF VISION 69

hypodermal cells of A elongate to form the layer known by the name
of corneagen, or vitreogen, the cells of B remaining small and forming
the pre-retinal layer of cells. The large optic nerve end- cells of the
retinal layer, C, take up the position shown in the figure, and their
cuticular surface becomes modified to form rods of varying shape
called rhabdites, which are attached to the retinal cells. Frequently
the rhabdites of neighbouring cells form definite groups, each group
being called a rhabdome. Whatever shape they take it is invariably
found that these little rods (bacilli), or rhabdites, are modifications of
the cuticular surface of the cells which form the retinal layer. Also,
as must necessarily be the case from the method of formation, the
optic nerve arises from the nuclear end of the retinal cells, never from

i-

1

Fig. 28. — Diagram op Formation op an Upright Simple Retina.

the bacillary end. As in the case first mentioned, so in this case, the
light strikes direct upon the bacillary end of the retinal cells ; such
eyes, therefore, are eyes with an upright retina.

It may happen that the part invaginated is the optic sense-plate
itself, as would be the case if in the former figure, instead of C, the
part B was modified to form a sense-plate. This will give rise to
an eye of a character different from the former (Fig. 29). The optic
nerve- fibres now lie between the source of light and the retinal end-
cells, the layer A as before forms the cuticular lens, and its hypo-
dermal cells elongate to form the corneagen ; there is no pre-retinal
layer, but, on the contrary, a post-retinal layer, C, called the tapetum,
and, as is seen, the light passes through the retinal layer to the

'O

THE ORIGIN OF VERTEBRATES

tapetum. The cuticular surface of the retinal cells forming the rods
or bacilli is directed towards the tapetal layer away from the source of
light, and the nuclei of the retinal cells are pre-bacillary in position,
in contradistinction to the upright eye, where they are post-bacillary.
The retinal end-cells are devoid of pigment, the pigment being in the
tapetal layer.

Such an eye, in contradistinction to the former type, is an eye
with an inverted retina ,• but still the same law holds as in the former
case — the optic nerve-fibres enter at the nuclear ends of the cells,
and the rods are formed from the cuticular surface.

In these eyes the pigmented tapetal layer is believed to act as a
looking-glass ; the dioptric apparatus throws the image on to its

I

I

Fig. 29. — Diagram op Formation of an Inverted Simple Retina.

The arrow shows the direction of the source of light in this as in the preceding figure.
In both figures the cuticular rhabdites are represented by thick black lines.

shiny surface, from whence it is reflected directly on to the rods,
which are in close contact with the tapetum. A similar process has
been suggested in the case of the mammalian lateral eye, with its
inverted retina. Johnson describes the post-retinal pigmented layer
as being frequently coloured and shiny, and imagines that it reflects
the image directly back on to the rods.

Thus we see that eyes can be placed in different categories, e.g.
those with an upright retina and those with an inverted retina ; also,
according to the presence or absence of a tapetum, eyes have been
grouped as tapetal or non-tapetal. All the eyes considered so far
are called simple eyes, or ocelli ; and a number of ocelli may be

THE EVIDEXCE OF THE ORGANS OF VISION 7 1

contiguous though separate, as in the lateral eyes of the scorpion.
They may, however, come into close contact and form one single,
large, compound eye. Such ocelli, in a very large number of cases,
retain each its own dioptric apparatus, and therefore the external
appearance of the compound eye represents not a single lens, but a
large number of facets, as is seen in the eyes of insects. Owing to
these differences, eyes have been divided into simple and compound,
and into facetted and non-facetted.

Yet another complication occurs in the formation of eyes, which
is, perhaps, the most important of all : the retinal portion of the eye,
instead of consisting of simple retinal cells, with their accompanying
rhabdites, may include within itself a portion of the central nervous
system.

The rationale of such a formation is as follows : The external
covering of the body is formed by a layer of external epithelial cells
— the ectodermal cell-layer — and an underlying neural layer, of which
the latter gives origin to the central nervous system. As development
proceeds, this central nervous system sinks inwards, leaving as its
connection with the ectoderm the sensory nerves of the skin. That
part of the neural layer which underlies the optic plate forms the
optic ganglion, and when the central nervous system leaves the
surface to take up its deeper position, the strand of nerve-fibres
known as the optic nerve, is left connecting it with the retinal cells
as seen in Figs. 28, 29. It may, however, happen that part of the
optic ganglion remains at the surface, in close connection with the
retinal end-cells, when the rest of the central nervous system sinks
inwards. The retina of such an eye is composed of the combined
optic ganglion and retinal end-cells ; the strand of nerve-fibres which
is left as the connection between it and the rest of the brain, which
is also called the optic nerve, is not a true peripheral nerve, as in
the first case, but rather a tract of fibres connecting two parts of the
brain, of which one has remained at the periphery. Such a retina,
in contradistinction to the first kind, may be called a compound retina.

The optic ganglion, as seen in eyes with a simple retina, consists
of a cortical layer of small, round nerve-cells, and an internal medulla
of fine nerve-fibres, which form a thick network known as 'Punct-
substanz,' or in modern terminology, 'Neuropil.' Fibres which pass
into this 'neuropil' from other parts of the brain connect them
with the optic ganglion.

72

THE ORIGIN OF VERTEBRATES

At the present time, owing to the researches of Golgi, Ramon y
Cajal, and others, the nervous system is considered to be composed
of a number of separate nerve-units, called neurones, each neurone
consisting of a nerve-cell with its various processes; one of these
— the neuraxon— constitutes the nerve-fibre belonging to that nerve-
cell, the other processes— the dendrites — establish communication
with other neurones. The place where these processes come together
is called a synapse, and the tangle of fine fibres formed at a number
of synapses forms the ' neuropil.'

When, therefore, a compound retina is formed by the amalgama-
tion of the ectodermal part— the retinal cells proper — with the
neurodermic part— to which the name 'retinal ganglion' may be

Pig. 30. — Diagram of Formation of an Upright Compound Retina.

ABCD, as in Fig. 28. Op. g. I. and Op. g. II., two optic ganglia which combine

to form the retinal ganglion, Bt. g.

given, — such a retina consists of neuropil substance and nerve-cells,
as well as the retinal end-cells. In all such compound retinas, the
retinal ganglion is not single, but two optic ganglia at least are
included in it, so that there are two sets of nerve-cells and two
synapses are always formed ; one between the retinal end-cells and
the neurones of the first optic ganglion, which may be called the
ganglion of the retina, the other between the first and second
ganglia, which, seeing that the neuraxons of its cells form the
optic nerve, may be called the ganglion of the optic nerve. The
' neuropil ' formed by these synapses forms the molecular layers of the
compound retina, and the cells themselves form the nuclear layers.
Thus an upright compound retina, formed in the same way as the
upright simple retina, would be illustrated by Fig. 30.

THE EVIDENCE OF THE ORGANS OF VISION J 3

Further, in precisely the same way as in the case of the simple
retina, such a compound retina may be upright or inverted. Thus,
in the lateral eyes of crustaceans and insects, a compound retina of
this kind is formed, which is upright ; while in the vertebrates the
compound retina of the lateral eyes is inverted.

The compound retina of vertebrates is usually described as com-
posed of a series of layers, which may be analyzed into their several
components as follows :—

Layer of rods and cones \

External nuclear layer f retina proper j Ectodermic part

External molecular layer -v

Internal nuclear layer > ganglion of retina

Internal molecular layer ) (retinal j neurodermic

Optic nerve-cell layer \ ganglion of optic nerve ) § an g lion ' P art

Layer of optic nerve fibres J

The difference between the development of these two types of
eye — those with a simple retina and those with a compound retina —
has led, in the most natural manner, to the conception that the
retina is developed, in the higher animals, sometimes from the cells of
the peripheral epidermis, sometimes from the tissue of the brain — two
modes of development termed by Balfour 'peripheral' and 'cerebral.'
An historical survey of the question shows most conclusively that all
investigators are agreed in ascribing the origin of the simple retina
to the peripheral method of development, the retina being formed
from the hypodermal cells by a process of invagination, while the
cerebral type of development has been described only in the develop-
ment of the compound retina. The natural conclusion from this fact
is that, in watching the development of the compound retina, it is
more difficult to differentiate the layers formed from the epidermal
retinal cells and those formed from the epidermal optic ganglion-
cells, than in the case of the simple retina, where the latter cells
withdraw entirely from the surface. This is the conclusion to which
Patten has come, and, indeed, judging from the text-book of Kor-
schelt and Heider, it is the generally received opinion of the day
that, as far as the Appendiculata are concerned, the retina, in the
true sense — the retinal end-cells, with their cuticular rods, — is formed,
in all cases, from the peripheral cells of the hypodermal layer, the
cuticular rods being modifications of the general cuticular surface
of the body. The apparent cerebral development of the crustacean

74 THE ORIGIN OF VERTEBRATES

retina, as quoted from Bobretsky by Balfour, is therefore iu reality
the development of the retinal ganglion, and not of the retina proper.
There is, I imagine, a universal belief that the natural mode of
origin of a sense-organ, such as the eye, must always have been from
the cells forming the external surface of the animal, and that direct
origin from the central nervous system is a priori most improbable.
It is, therefore, a matter of satisfaction to find that the evidence for
the latter origin has universally broken down, with the single
exception of the eyes of vertebrates and their degenerated allies ; a
fact which points strongly to the probability that a reconsideration
of the evidence upon which the present teaching of the origin of the
vertebrate eye is based will show that here, too, a confusion has
arisen between that part formed from the epidermal surface and that
from the optic ganglion.

The Median or Pineal Eyes.

Undoubtedly, in recent times, the most important clue to the
ancestry of vertebrates has been given by the discovery that the
so-called pineal gland in the vertebrate brain is all that remains of a
pair of median or pineal eyes, the existence of which is manifest in
the earliest vertebrates ; so that the vertebrate, when it first arose,
possessed a pair of median eyes as well as a pair of lateral eyes.
The ancestor of the vertebrate, therefore, must also have possessed a
pair of median eyes as well as a pair of lateral eyes.

Very instructive, indeed, is the evidence with regard to these
median eyes, for one of the great characteristics of the ancient
palreostracan forms is the invariable presence of a pair of median
eyes as well as a pair of lateral eyes. In the living representative of
such forms — Limulus — the pair of median eyes (Fig. 5) is well
shown, and it is significant that here, according to Lankester and
Bourne, these eyes are already in a condition of degeneration ; so
also in many of the Paheostraca (Fig. 7) the lateral eyes are the large,
well- developed eyes, while the median eyes resemble those of Limulus
in their insignificance.

We see, then, that in the dominant arthropod race at the time
when the fishes first appeared, the type of eyes consisted of a pair of
well-developed lateral eyes and a pair of insignificant, partially
degenerated, median eyes. Further, according to all palaeontologists,

THE EVIDENCE OF THE ORGANS OF VISION 75

in the best-preserved head-shields of the most ancient fishes,
especially well seen in the Osteostraci, in Cephalaspis, Treniataspis,
Auchenaspis, Keraspis, a pair of large, prominent lateral eyes existed,
between which, in the mid-line, are seen a pair of small, insignificant
median eyes.

The evidence of the rocks, therefore, proves that the pair of
median eyes which were originally the principal eyes (Hauptaugen),
had already, in the dominant arthropod group been supplanted by
a pair of lateral eyes, and had, in consequence, become small and
insignificant, at the time when vertebrates first appeared. This dwind-
ling process thus initiated in the arthropod itself has steadily continued
ever since through the whole development of the vertebrates, with the
result that, in the highest vertebrates, these median or pineal eyes
have become converted into the pineal gland with its ' brain-sand.'

In the earliest vertebrate these median eyes may have been
functional ; they certainly were more conspicuous than in later forms.
Alone among living vertebrates the right median eye of Ammoccetes
is so perfect and the skin covering it so transparent that I have
always felt doubtful whether it may not be of use to the animal,
especially when one takes into consideration the undeveloped state
of the lateral eyes in this animal, hidden as they are under the skin.
Thus the one living vertebrate which is comparable with these
extinct fishes is the one in which one of the pineal eyes is most well
defined, most nearly functional.

Before passiDg to the consideration of the structure of the
median eyes of Ammoccetes, it is advisable to see whether these
median eves in other animals, such as arachnids and crustaceans,
belong to any particular type of eyes, for then assuredly the median
eyes of Arnmoccetes ought to belong to the same type if they are
derived from them.

In the specialized crustacean, as in the specialized vertebrate, the
median eyes have disappeared, at all events in the adult, but still
exist in the primitive forms, such as Branchipus, which resemble the
trilobites in some respects. On the other hand, the median eyes
have persisted, and are well developed in the arachnids, both
scorpions and spiders possessing a well- developed pair. The cha-
racteristics of the median eyes must then be especially sought for in
the arachnid group.

Both scorpions and spiders possess many eyes, of which two are

7 6 THE ORIGIN OF VERTEBRATES

always separate and median in position, while the others form lateral
groups ; all these eyes possess a simple retina and a simple corneal
lens. Grenacher was the first to point out that in the spiders two
very distinct types of eye are found. In the one the retina is up-
right ; in the other the retina is inverted, and the eye possesses a
tapetal layer. The distribution of these two types is most suggestive,
for the inverted retina is always found in the lateral eyes, never in
the two median eyes ; these always possess a simple upright retina.

In the crustaceans, the lateral eyes differ also from the median
eyes, but not in the same way as in the arachnids ; for here both
types of eye possess an upright retina, but the retina of the lateral
eyes is compound, while that of the median eyes is simple. In other
words, the median eyes are in all cases eyes with a simple upright
retina and a simple cuticular lens, while the retina of the lateral
eyes is compound or may be inverted, according as the animal
in question possesses crustacean or arachnid affinities. The lateral
eye of the vertebrate, possessing, as it does, an inverted compound
retina, indicates that the vertebrate arose from a stock which was
neither arachnid nor crustacean, but gave rise to both groups — in fact,
was a member of the great palseostracan group. What, then, is the
nature of the median eyes in the vertebrate ?

The Median Eyes of Ammoccetes.

The evidence of Ammoccetes is so conclusive that I, for one, can-
not conceive how it is possible for any zoologist to doubt whether
the parietal organ, as they insist on calling it, had ever been an eye,
or rather a pair of eyes.

Anyone who examines the head of the larval lamprey will see
on the dorsal side, in the median line, first, a somewhat circular orifice
— the unpaired nasal opening ; and then, tailwards to this, a well-
marked circular spot, where the skin is distinctly more transparent
than elsewhere This spot coincides in position with the underlying
dorsal pineal eye, which shines out conspicuously owing to the
glistening w T hiteness of its pigment. Upon opening the brain- case
the appearance as in Fig. 20 is seen, and the mass of the right ganglion
habenulce {G.H.R.), as it has been called, stands out conspicuously as
well as the right or dorsal pineal eye (Pn.). Both eye and ganglion
appear at first sight to be one-sided, but further examination shows
that a left ganglion habenulce is present, though much smaller than on

THE EVIDENCE OE THE ORGANS OE VISION

77

the right side. In connection with this is another eye-like organ — the
left or ventral pineal eye, — much more aborted, much less like an eye
than the dorsal one ; so also there are two bundles of peculiar fibres

Fig. 31. — One op a Series op Horizontal Sections through the Head op

Ammoccetes.

/.;»., upper lip muscles ; m.c, muco-cartilage ; »., nose; na.c, uasal cartilage; pn.,
right pineal eye and nerve; g.h.r., right ganglion habenuhe ; s.m., somatic
muscles; or., membranous wall of cranium; cli., choroid plexus; gl., glandular
substance and pigment filling up brain-case.

called Meynert's bundles, which connect this region with the infra-
infundibular region of the brain ; of these, the right Meynert's bundle

78

THE ORIGIN OF VERTEBRATES

is much larger than the left. This difference between right and left
indicates a greater degeneration on the left side, and points distinctly
to a close relationship between the nerve-masses known as ganglia
habenulcB and the median eyes. In my opinion this ganglion is, in
part, at all events, the optic ganglion of the median eye on each side.
It is built up on the same type as the optic ganglia of invertebrate

Fig. 32. — Eye op Acilius Larva, with Fig. 33. — Pineal Eye op Ammoccetes,
its Optic Ganglion. with its Ganglion Habenula.

On the right side the nerve end-cells On the left side the eye is drawn as it

have been drawn free from pigment. appeared in the section. On the right

side I have removed the pigment from
the nerve end-cells, and drawn the eye
as, in my opinion, it would appear if
it were functional.

simple eyes, with a cortex of small round cells and a medulla of fine
nerve-fibres. Into this ganglion, on the right side, there passes a very
well-defined nerve — the nerve of the dorsal eye. The eye itself with
its nerve, pn. } and its optic ganglion, g.h.r., is beautifully shown by
means of a horizontal section through the head of Ammoccetes
(Fig. 31). Originally, as described by Scott, the eye stood vertically

THE EVIDENCE OF THE ORGANS OF VISION

79

-ghr

above its optic ganglion, and presented an appearance remarkably like
Fig. 32, which represents one of the simple eyes and optic ganglia
of a larva of Acilius as described by Patten ; then, with the forward
growth of the upper lip,
the right pineal eye was
dragged forward and its
nerve pulled horizon-
tally over the ganglion
habenulce. For this
reason the eye, nerve,
and ganglion are better
shown in a nearly hori-
zontal than in a trans-
verse section.

The optic nerve be-
longing to this eye is
most evident and clearly
shown in Fig. 31, and in
the series of consecutive
sections which follow
upon this section ; no
doubt can arise as to
the structure in ques-
tion having been the
nerve of the eye, even
though, as is possible, it
does not contain any
functional nerve-fibres.

The second, ventral
or left, eye, belonging
to the left ganglion
habenuhe is very dif-
ferent in appearance,
being much less evi-
dently an eye. Fig. 34
is one of the same

pn.

Fig. 34.— Horizontal Section through Brain of
Ammoccetes, to show the Left, or Ventral
Pineal Eye.

, left or ventral pineal eye ; pn. u last remnant of
right, or dorsal pineal eye ; g.h.r., right ganglion
habenulce; g.h.l. lt g.h.l. 3 , parts of left ganglion
habenulce ; pi., fold oipia mater which separates
the left ganglion habenulce from the left pineal
eye ; /., strands of nerve-fibres connecting the
left eye with its ganglion, g.h.l. 3 ; V 3 , third

ventricle; Y.aq., ventricle of aquseduct.

series of horizontal sections as Fig. Sl,pn.i being the last remnant
of the right, or dorsal, eye, while pn.% shows the left, or ventral, eye
with its connection with the left ganglion habenulce.

80 THE ORIGIN OF VERTEBRATES

In a series of sections I have followed the nerve of the right pineal
eye to its destination, as described in my paper in the Quarterly
Journal of Microscopical Science, and have found that it enters into
the ganglion habenulce just as the nerve to any simple eye enters
into its optic ganglion. This nerve, as I have shown, is a very dis-
tinct, well-defined nerve, with no admixture of ganglion-cells or of
connective tissue, very different indeed to the connection between
the left pineal eye and its optic ganglion. Here there is no denned
nerve at all ; but the cells of the ganglion habenulce stretch right up to
the remains of the eye itself. Seeing, then, that both the eye and
ganglion on this side have reached a much further grade of degenera-
tion than on the right side, it may be fairly concluded that the
original condition of these two median eyes is more nearly repre-
sented by the right eye, with its well-defined nerve and optic gang-
lion, than by the left eye, or by the eyes in lizards and other animals
which do not show so well-defined a nerve as is possessed by
Ammoccetes. Quite recently Dendy has examined the two median
eyes in the New Zealand lamprey Gcotria australis. In this species
the second eye is much better defined than in the European lamprey,
and its connection with the ganglion habenulce is more nerve-like.
In neither eye, however, is the nerve so clean cut and isolated as is the
nerve of the dorsal, or right, eye in the Ammoccetes stage of Petromy-
zon Planeri; in both, cells resembling those of the cortex of the
ganglion habenulce and connective tissues are mixed up with the
nerve-fibres which pass from each eye to its respective optic ganglion.

The Eight Pineal Eye of Ammoccetes.

The optic fibres of the right median eye of Ammoccetes are con-
nected with a well-defined retina, the limits of which are defined
by the white pigment so characteristic of this eye. This pigment is
apparently calcium phosphate, which still remains as the ' brain-sand '
of the human pineal gland. The cells, which are hidden by this pig-
ment, were described by me in 1890 as the retinal end-cells with large
nuclei. In 1893, Studnicka examined them more closely, and con-
cluded that the retinal cells are of two kinds : the one, nerve end-cells,
the sensory cells proper ; the other, pigmented epithelial cells, which
surround the sense- cells. The sense-cells may contain some of the
white pigment, but not so much as the other cells. Similarly, in the

THE EVIDENCE OF THE ORGANS OF VISION 8 1

median eyes of Limulus, Lankester and Bourne find it difficult to
determine how far the retinal end-cells contain pigment and how far
that pigment really is in the cells surrounding these nerve end-cells.

The interior of the eye presents the appearance of a cavity in
shape like a cornucopia, the stalk of which terminates at the place
where the nerve enters. This cavity is not empty, but the posterior
part of it is filled with the termination of the nerve end-cells of the
retina, as pointed out by me and confirmed by Studnicka. These
terminations are free from pigment, and contain strikingly trans-
lucent bodies, which I have described in my paper in the Quarterly
Journal, and called rhabdites, for they present the same appearance
and are situated in the same position as are many of the rhabdites
on the terminations of the retinal end-cells of arthropod eyes.
Studnicka has also seen these appearances, and figures them in
his second paper on the nerve end-cells of the pineal eye of
Ammoccetes.

Up to this point the following conclusions may be drawn : —

1. Ammoccetes possesses a pair of median eyes, just as was the

case with the most ancient fishes, and with the members of
the contemporary paheostracan group.

2. The retina of one of these eyes is well-defined and upright,

not inverted, and therefore in this respect agrees with that
of all median eyes.

3. The presence of nerve end-cells, with pigment either in them or

in cells around them, to the unpigmented ends of which trans-
lucent bodies resembling rhabdites are attached, is another
proof that this retina agrees with that of the median eyes of
arthropods.

4. The simple nature of the nerve with its termination in an

optic ganglion closely resembling in structure an arthropod

optic ganglion, together with Studnicka' s statement that the

nerve end-cells pass directly into the nerve, points directly

to the conclusion that this retina is a simple, not a compound,

retina, and that it therefore in this respect also agrees with

the retina of all median eyes.

With respect to this last conclusion, neither I myself nor

Studnicka have been able to see any definite groups of cells

between the nerve end-cells and the optic nerve such as a compound

retina necessitates.

G

82 THE ORIGIN OF VERTEBRATES

On the other hand, Dently describes in the New Zealand lamprey,
Gcotria australis, a cavity where the nerve enters into the eye,
which he calls the atrium. This cavity is distinct from the main
cavity of the eye, and is separated from it by a mass of cells similar
in appearance to those of the cortex of the ganglion hahcnulcc. In
these two eyes then, groups of cells, resembling in appearance those
belonging to an optic ganglion, exist in the eyes themselves. This
atrium is evidently that part of the central cavity which I have
called the handle of the cornucopia in the European lamprey, and
the very fact that it is separated from the rest of the central cavity
is evidence that we are dealing here with a later stage in the history
of the pineal eyes than in the case of the Ammoccetes of Petromijzon
Planeri. Taking also into consideration the continuity of the mass
of small ganglion-cells which surround this atrium with the cells of
the ganglion habcnulce by means of the similar cells scattered along
the course of the nerve, and also bearing in mind the fact already
stated that in the more degenerate left eye of Ammocoetes the cells
of the ganglion habenulce extend right up to the eye itself, it seems
more likely than not that these cells do not represent the original
optic ganglion of a compound retina, but rather the subsequent
invasion, by way of the pineal nerve, of ganglion-cells belonging to
a portion of the brain. In the last chapter it has been suggested
that the presence of the trochlear or fourth cranial nerve has given
rise to the formation of the cerebellum by a similar spreading.

There is certainly no appearance in the least resembling a
compound retina such as is seen in the vertebrate or crustacean
lateral eye. In the median eyes of scorpions and of Limulus, cells
are found within the capsule of the eye among the nerve-fibres and
the nerve end-cells. These are especially numerous in the median
eyes of Limulus, as described by Lankester and Bourne, and are
called by them intrusive connective tissue cells. The meaning of
these cells is not, to my mind, yet settled. It is sufficient for my
purpose to point out that the presence of cells interneural in position
among the nerve end-cells of the retina of the median eyes of
Ammoccetes is more probable than not, on the assumption that the
retina of these eyes is built up on the same plan as that of the
median eyes in Limulus and the scorpions.

It is further to be borne in mind that these specimens of Gcotria
worked at by Dendy were in the ' Velasia ' stage of the New Zealand

THE EVIDENCE OF THE ORGANS OF VISION

83

lamprey, and correspond, therefore, more nearly to the l'etromyzon
than to the Ammoccetes stage of the European lamprey.

The Dioptric Apparatus.

Besides the retina, all eyes possess a dioptric apparatus. What
is the evidence as to its nature in these vertebrate median eyes ?
Lankester and Bourne have divided the eyes of scorpions and

I

Til

ret

Fig. 35. — Eye of Acilius Lakv^e. (After Patten.)

I., chitinous lens ; c, corneagen; pr., pre-retinal layer ; rlu, rhabdites ; ret., retinal

end-cells.

Limulus into two kinds, monostichous and diplostichous. In the
first the retinal cells are supposed to give rise to not only rhabdites
but also the cuticular chitinous lens, so that the eye is one-layered ;
in the second the lens is formed by a well-marked hypodermal layer,
in front of the retina, composed of elongated cells, so that these eyes
are two-layered or diplostichous. The lateral eyes, according to
them, are all monostichous, but the median eyes are diplostichous.
This distinction is not considered valid by other observers. Thus,

8 4

THE ORIGIN OF VERTEBRATES

I

as already indicated, Patten looks on all these eyes as three-layered,
and states that in all cases a corneagen or vitreogen layer exists,
which gives origin to the lens. For my own part I agree with

Patten, but we are not con-
cerned here with the lateral
eyes. It is sufficient to note
that all observers are agreed
that the median eyes are
characterized by this well -
marked cell-layer, the so-called
vitreous or corneagen layer of
cells.

This layer (p., Fig. 35) is
composed of much - elongated
cells of the hypodermal layer,
in each of which the large
nucleus is always situated to-
wards the base of the cell.
The space between it and the

Fig. 36. — Eye op Hydrophilus Larva,
with the Pigment over the Retinal
End-cells.

retina contains, according to

I., chitinous lens; c, corneagen; pr., pre-
retinal layer ; rh., rhabdites; ret., retinal

end-cells.

Patten the cells of the pre-
retinal layer (pr.). These may be so few and insignificant as to give
the impression that the vitreous layer is immediately adjacent to the
retina (ret.).

Let us turn now to the right pineal eye of Ammoccetes (Fig. 37)
and see what its further structure is. The anterior part of the eye
is free from pigment, and is composed, as is seen in hsematoxylin or
carmine specimens, of an inner layer of nuclei which are frequently
arranged in a wavy line. From this nucleated layer, strands of tissue,
free from nuclei, pass to the anterior edge of the eye.

In the horizontal longitudinal sections it is seen that these strands
are confined to the middle of the eye. On each side of them the
nuclear layer reaches the periphery, so that if we consider these
strands to represent long cylindrical cells, as described by Beard,
then the anterior wall may be described as consisting of long
cylindrical cells, which are flanked on either side by shorter cells of
a similar kind. The nuclei at the base of these cylindrical cells are
not all alike. We see, in the first place, large nuclei resembling the
large nuclei belonging to the nerve end-cells ; these are the nuclei of

THE EVIDENCE OF THE ORGANS OF VISION

35

the long cylindrical cells. We see also smaller nuclei in among
these larger ones, which look like nuclei of intrusive connective
tissue, or may perhaps form a distinct layer of cells, situated between
the cells of the anterior wall and the terminations of the nerve
end- cells already referred to.

These elongated cells are in exactly the same position and present
the same appearance as the cells of the corneagen layer of any median

eye. Like the latter they are

free from pigment and never
show with osmic staining any
sign of the presence of trans-
lucent rhabdite - like bodies,
such as are seen in the termi-
nation of the retinal cells, and
like the latter their nuclei are
at the base. The resemblance
between this layer and the
corneagen cells of any median
eye is absolute. Between it
and the terminations of the
retinal cells there exists some
ill-defined material certainly
containing cells which may
well correspond to Patten's
pre-retinal layer of cells.

Eetina, corneagen, nerve,
optic ganglion, all are there, all
in their right position, all of
the right structure, what more
is needed to complete the
picture ?

In order to complete the dioptric apparatus a lens is necessary.
Where, then, is the lens in these pineal eyes ? In all the arachnid eyes,
whether median or lateral, the lens is a single corneal lens composed
of the external cuticle, which is thickened over the corneagen cells.
This thickened cuticle is composed of chitin, and is not cellular,
but is dead material formed out of the living underlying corneagen
cells. Such a lens is in marked contrast to the lens of the lateral
vertebrate eye, which is formed by living cells themselves. This

Fig. 37. — Pineal Eye of Ammoccetes,
with its Ganglion Habenulcz.

86 THE ORIGIN OF VERTEBRATES

thickening of the cnticnlar layer to form a lens could only exist as
long as that layer is absolutely external, so that the light strikes
immediately upon it ; for, if from any cause the eye became situated
internally, the place of such a lens must be filled by the structures
situated between it and the surface, and the thickened cuticle would
no longer lie formed.

In all vertebrates these pineal eyes are separated from the
external surface by a greater or less thickness of tissues ; in the
case of Ammoccetes, as is seen in Fig. 31, the eye lies within the
membranous cranial wall, and is attached closely to it. The position,
then, of the cuticular, or corneal lens, as it is often called, on the
supposition that this is a median eye of the arachnid type, is taken
by the membranous cranium, and, as I have described in my
paper in the Quarterly Journal, on carefully lifting the eye in the
fresh condition from the cranial wall, it can be seen under a
dissecting microscope that the cranial wall often forms at this
spot a lens-like bulging, which fits the shallow concavity of the
surface of the eye, and requires some little force to separate it from
the eye.

As will appear in a subsequent chapter, this cranial wall has
been formed by the growth, laterally and dorsally, of a skeletal
structure known by the name of the plastron. The last part of it to
be completed would be that part in the mid-dorsal line, where appa-
rently, in consequence of the insinking of the degenerating eyes, a
dermal and subdermal layer already intervened between the source
of light and the eyes themselves.

When the membranous cranium was completed in the mid-dorsal
region, it was situated here as elsewhere just internally to the sub-
dermal layer, and therefore enclosed the pineal eyes. This, to my
mind, is the reason why the pineal eyes, which, in all other respects,
conform to the type of the median eyes of an arachnid-like animal,
do not possess a cuticular lens. Other observers have attempted to
make a lens out of the elongated cells of the anterior wall of the
eye (my corneagen layer), but without success.

Studnicka, who calls this layer the pellucida, does not look upon
it as the lens, but considers, strangely enough, that the translucent
appearances at the ends of each nerve end-cell represent a lens for
that cell, so that every nerve end-cell has its own lens. Still more
strange is it that, holding this view, he should yet consider these knobs

THE EVIDENCE OF THE ORGANS OF VISION 87

to be joined by filaments to the cells in the anterior wall of the eye,
a conception fatal to the action of such knobs as lenses.

The discovery that the vertebrate possesses, in addition to the
lateral eyes, a pair of median eyes, which are most conspicuous in
the lowest living vertebrate, together with the fact that such eyes
are built up on the same plan as the median eyes of living crus-
taceans or arachnids, not only with respect to the eye itself but also
to its nerve and optic ganglion, constitutes a fact of the very greatest
importance for any theory of the origin of vertebrates ; especially in
view of the further fact, that similar eyes in the same position are
found not only in all the members of the Palaaostraca, but also in all
those ancient forms (classed as fishes) which lived at that time. At
one and the same moment it proves the utter impossibility of
reversing dorsal and ventral surfaces, points in the very strongest
manner to the origin of the vertebrate from some member or other
of the paloeostracan group, and insists that the advocates of the
origin of vertebrates from the Hemichordata, etc., should give an
explanation of the presence of these two median eyes of a more con-
vincing character than that given here.

The Lateral Eyes.

Turning now to the consideration of the lateral eyes, we see that
these eyes in the arachnids often possess an inverted retina, in the
crustaceans always an upright retina. In the arachnids they possess
a simple retina, while in the crustaceans their retina is compound ;
so that in the latter case the so-called optic nerve is in reality a
tract of fibres connecting together the brain-region with a variable
number of optic ganglia, which have been left at the periphery in
close contact with the retinal cells, when the brain sunk away from
the superficial epithelial covering.

There is, then, this difference between the lateral eyes of crus-
taceans and arachnids, that the retina of the former is compound, but
never inverted, while that of the latter may be inverted, but is
always simple.

The retina of the lateral eyes of the vertebrate resembles both of
these, for it is compound, as in the crustacean, and inverted as in
the arachnid.

It must always be borne in mind that in the palreostracan epoch

88 THE ORIGIN OF VERTEBRATES

the dominant race was neither crustacean nor arachnid, but partook
of the characters of both ; also, as is characteristic of dominance,
there was very great variety of form, so that it seems more probable
than not that some of these forms may have combined the arachnid
and crustacean characteristics to the extent of possessing lateral eyes
with an inverted yet compound retina. A certain amount of
evidence points in this direction. As already stated, the compound
retina which characterizes the vertebrate lateral eyes is character-
istic of all facetted eyes, and in the trilobites facetted lateral eyes
are commonly found. From this it may be concluded that many of
the trilobites possessed eyes with a compound retina. There have,
however, been found in certain species, e.g. Harpcs vittatus and
Harpes ungula, lateral eyes which were not facetted, and are believed
by Korschelt and Heider to be of an arachnid nature. They say,
" Palaeontologists have appropriately described them as ocelli,
although, from a zoological point of view, they do not deserve this
name, having most probably arisen in a way similar to that con-
jectured in connection with the lateral eyes of scorpions." If this
conjecture is right, then in these forms the retina may have been
inverted, but because they belonged to the trilobite group, the retina
was most probably compound, so that here we may have had the
combination of the arachnid and crustacean characteristics. On the
other hand, in some forms of Branchipus, and many of the Gamma-
ridse, a single corneal lens is found in conjunction with an eye of the
crustacean type, so that a non-facetted lateral eye, found in a fossil
form, would not necessarily imply the arachnid type of eye with the
possibility of an inverted retina. Whatever may lie the ultimate
decision upon these particular forms, the striking fact remains, that
both in the vertebrate and in the arachnid the median eyes possess
a simple upright retina, while the lateral eyes possess an inverted
retina, and that both in the vertebrate and the crustacean the
median eyes possess a simple upright retina, while the lateral eyes
possess a compound retina.

The resemblance of the retina of the lateral eyes of vertebrates
to that of the lateral eyes of many arthropods, especially crustaceans,
has been pointed out by nearly every one who has worked at these
invertebrate lateral eyes. The foundation of our knowledge of the
compound retina is Berger's well-known paper, the results of which
are summed up by him in the following two main conclusions.

THE EVIDENCE OF THE ORGANS OF VISION

89

1. The optic ganglion of the Arthropoda consists of two parts, of
which the one stands in direct inseparable connection with the
facetted eye, and together with the layer of retinal rods forms the
retina of the facetted eye, while the other part is connected rather
with the brain, and is to be considered as an integral part of the
brain in the narrower sense of the word.

2. In all arthropods examined by him, the retina consists of five
layers, as follows :—

(1) The layer of rods and their nuclei.

(2) The layer of nerve-bundles.

(3) The nuclear layer.

(4) The molecular layer.

(5) The ganglion cell layer.

Berger passes under review the structure and arrangement of
the optic ganglion in a large number of different groups of arthropods,
and concludes that in
all cases one part of
the optic ganglion is
always closely attached
to the visual end-cells,
and this combination
he calls the retina.
On the other hand, the
nerve-fibres which con-
nect the peripheral part
of the optic ganglion
with the brain, the so-
called optic nerve, are
by no means homolo-
gous in the different
groups ; for in some
cases, as in many of
the stalk-eyed crusta-
ceans, the whole optic
ganglion is at the pe-
riphery, while in others, as in the Diptera, only the retinal ganglion
is at the periphery, and the nerve-stalk connects this with the rest
of the optic ganglion, the latter being fused with the main brain-
mass. In the Diptera, in fact, according to Berger, the optic nerve

Fig. 38.— The Retina of Musca. (After Berger.)

Br., brain; O.n., optic nerve; n.l.o.g., nuclear layer of
ganglion of optic nerve; m.L, molecular layer
(Punktsubstanz) ; n.l.r.g.i. and n.l.r.g.o., inner and
outer nuclear layers of tbe ganglion of tbe retina ;
f.lr.r., terminal fibre-layer of retina; r., layer of
retinal end-cells (indicated only).

9 o

THE ORIGIN OF VERTEBRATES

and retina are most nearly comparable to those of the vertebrate.
For this reason I give Berger's picture of the retina of Musca
(Fig. 38), in order to show the arrangement there of the retinal
layers.

In Branchipus and other primitive Crustacea, Berger also finds
the same retinal layers, but is unable to distinguish in the brain the
rest of the optic ganglion. Judging from Berger's description of
Branchipus, and Bellonci's of Spha^roma, it would almost appear
as though the cerebral part of the retina in the higher forms
originated from two ganglionic enlargements, an external and

Sup. Segment Ant I

Ant II
Inf. Segment

Fig. 39. — The Brain op Sphceroma scrratum. (After Bellonci.)

Ant. I. and Ant. II., nerves to 1st and 2nd antennae, f.br.r., terminal fibre-layer of
retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n.,
optic nerve-fibres forming an optic chiasma.

internal enlargement, as Bellonci calls them. The external ganglion
(Op. g. I., Fig. 39) may be called the ganglion of the retina, the cells
of which form the nuclear layer of the higher forms, and the internal
ganglion (Op. g. II., Fig. 39), from which the optic nerve-fibres to the
brain arise, may therefore be called the ganglion of the optic nerve.
Bellonci describes how in this latter ganglion cells are found very
different to the small ones of the external ganglion or ganglion of
the retina. So also in Branchipus, judging from the pictures of
Berger, Claus, and from my own observations (ef. Fig. 46, in which
the double nature of the retinal ganglion is indicated), the peripheral
part of the optic ganglion — i.e. the retinal ganglion — may be spoken

THE EVIDENCE OF THE ORGANS OF VISION

91

f.br.r
b.rn

nl.r.g,

»

immm

$?— ml

of as composed of two ganglia. The external of these is clearly the
ganglion of the retina ; its cells form the nuclear layer, the striking
character of which, and close resemblance to the corresponding layer
in vertebrates, is shown by Claus' picture, which I reproduce (Fig. 40).
The internal ganglion with which the optic nerve is in connection
contains large ganglion cells, which, to-
gether with smaller ones, form the gang-
lionic layer of Berger.

The most recent observations of the
structure of the compound retina of the
crustacean eye are those of Parker, who,
by the use of the methylene blue method,
and Golgi's method of staining, has been
able to follow out the structure of the
compound retina in the arthropod on the
same lines as had already been done for
the vertebrate. These two methods have
led to the conclusion that the arthropod
central nervous system and the verte-
brate central nervous system are built up
in the same manner — viz. by means of a
series of ganglia connected together, each
ganglion being composed of nerve-cells,
nerve-fibres, and a fine reticulated sub-
stance called by Leydig in arthropods
' Punktsubstanz,' and known in verte-
brates and in invertebrates at the present
time as 'neuropil.' A further analysis
resolves the whole system into a combi-
nation of groups of neurones, the cells
and fibres of which form the cells and
fibres of the ganglia, while their dendritic
connections with the terminations of other neurones, together with
the neuroglia-cells form the 'neuropil.' As is natural to expect,
that part of the central nervous system which helps to form the
compound retina is built up in the same manner as the rest of the
central nervous system.

Thus, according to Parker, the mass of nervous tissue which
occupies the central part of the optic stalk in Astacus is composed

Fig. 40. — Bipolar Cells of
Nuclear Layer in Ketina
op Branchipus. (After
Claus.)

f.br.r,, terminal fibre - layer
of retina; n.l.r.g., bipolar
cells of tbe ganglion of the
retina = inner nuclear layer ;
m.l., Punktsubstanz = inner
molecular layer ; b.m., base-
ment membrane formed by
neurilemma round central
nervous system.

92 THE ORIGIN OF VERTEBRATES

of four distinct ganglia; the retina is connected with the first of
these by means of the retinal fibres, and the optic nerve extends
proximally from the fourth ganglion to the brain. Each ganglion con-
sists of ganglion-cells, nerve-fibres, and 'neuropil/ and, in addition,
supporting cells of a neuroglial type. By means of the methylene
blue method and the Golgi method, it is seen that the retinal end-
cells, with their visual rods, are connected with the fibres of the
optic nerve by means of a system of neurones, the synapses of
which take place in and help to form the ' neuropil ' of the various
ganglia. Thus, an impulse in passing from the retina to the brain
would ordinarily travel over five neurones, beginning with one of
the first order and ending with one of the fifth. He makes five
neurones although there are only four ganglia, because he reckons
the retinal cell with its elongated fibre as a neurone of the first
order, such fibre terminating in dendritic processes which form
synapses in the ' neuropil ' of the first ganglion with the neurones of
the second order.

Similarly the neurones of the second order terminate in the
' neuropil ' of the second ganglion, and so on, until we reach the
neurones of the fifth order, which terminate on the one hand in the
' neuropil ' of the fourth ganglion, and on the other pass to the optic
lobes of the brain by their long neuraxons — the fibres of the optic
nerve.

He compares this arrangement with that of Branchipus, Apus,
Estheria, Daphnia, etc., and shows that in the more primitive
crustaceans the peripheral optic apparatus was composed, not of
four but of two optic ganglia, not, therefore, of five but of three
neurones, viz. —

1. The neurone of the first order — i.e. the retinal cell with its
fibre terminating in the ' neuropil ' of the first optic ganglion (ganglion
of the retina).

2. The neurone of the second order, which terminates in the
' neuropil ' of the second ganglion (ganglion of the optic nerve).

3. The neurone of the third order, which terminates in the optic
lobes of the brain by means of its neuraxons (the optic nerve).

We see, then, that the most recent researches agree with the
older ones of Berger, Claus, and Bellonci, in picturing the retina of
the primitive crustacean forms as formed of two ganglia only, and
not of four, as in the specialized crustacean group the Malacostraca.

THE EVIDENCE OF THE ORGANS OF VISION 93

The comparison of the arthropod compound retina with that of
the vertebrate shows, as one would expect upon the theory of the
origin of vertebrates put forward in this book, that the latter retina
is built up of two ganglia, as in the more primitive less specialized
crustacean forms. The modern description of the vertebrate retina,
based upon the Golgi method of staining, is exactly Parker's descrip-
tion of the simpler form of crustacean retina in which the ' neuropil '
of the first ganglion is represented by the external molecular
layer, and that of the second ganglion by the internal molecular
layer ; the three sets of neurones being, according to Parker's
terminology : —

1. The neurones of the first order — viz. the visual cells — the
nuclei of which form the external nuclear layer, and their long
attenuated processes form synapses in the external molecular layer
with

2. The neurones of the second order, the cells of which form the
internal nuclear layer, and their processes form synapses in the
internal molecular layer with

3. The neurones of the third order, the cells of which form the
ganglionic layer and their neuraxons constitute the fibres of the optic
nerve which end in the optic lobes of the brain.

Strictly speaking, of course, the visual cells with their elongated
processes have no right to be called neurones : I only use Parker's
phraseology in order to show how closely the two retinas agree even
to the formation of synapses between the fine drawn-out processes of
the visual cells and the neurones of the ganglion of the retina.

The Eetina of the Lateral Eye of Ammoccetes.

As in the case of all other organs, it follows that if we are dealing
here with a true genetic relationship, then the lower we go in the
vertebrate kingdom the more nearly ought the structure of the retina
to approach the arthropod type. It is therefore a matter of intense
interest to determine the nature of the retina in Ammoccctes in order
to see whether it differs from that of the higher vertebrates, and if
so, whether such differences are explicable by reference to the structure
of the arthropod eye.

Before describing the structure of this retina it is necessary to

CD

clear away a remarkable misconception, shared among others by

94 THE ORIGIN OF VERTEBRATES

Balfour, that this eye is an aborted eye, and that it cannot be
considered as a primitive type. Thus Balfour says : " Considering
the degraded character of the Ammoccete eye, evidence derived from
its structure must be received with caution," and later on, "the most
interesting cases of partial degeneration are those of Myxine and the
Ammoccete. The development of such aborted eyes has as yet been
studied only in the Ammoccete, in which it resembles in most
important features that of other Vertebrata."

Again and again the aborted character of the eye is stated to be
evidence of degeneration in the case of the lamprey. What such a
statement means, why the eye is in any way to be considered as
aborted, is to me a matter of absolute wonderment : it is true that
in the larval form it lies under the skin, but it is equally true that
at transformation it comes to the surface, and is most evidently as
perfect an eye as could be desired. There is not the slightest sign
of any degeneration or abortion, but simply of normal development,
which takes a longer time than usual, lasting as it does throughout
the life-time of the larval form.

Kohl, who has especially studied degenerated vertebrate eyes,
discusses with considerable fulness the question of the Ammocoetes
eye, and concludes that in aborted eyes a retarded development
occurs, and this applies on the whole to Ammocoetes, " but with the
important difference that in this case the period of retarded develop-
ment is not followed by a stoppage, but on the contrary by a period
of very highly intensified progressive development during the meta-
morphosis," with the result that " the adult eye of Petromyzon
Planeri does not diverge from the ordinary type."

Eeferring in his summing up to this retarded development, he
says : " Such reminiscences of embryonic conditions are after all
present here and there in normally developed organs, and by no
means entitle us to speak of abnormal development."

The evidence, then, is quite clear that the eye of Petromyzon,
or, indeed, of the full-grown Ammocoetes, is in no sense an abnormal
eye, but simply that its development is slow during the animoccete
stage. The retina of Petromyzon was figured and described by
Langerhans in 1873. He describes it as composed of the following
layers : —

(1) Membrana limitans interna.

(2) Thick inner molecular layer.

THE EVIDENCE OF THE ORGANS OF VISION

95

(3) Optic fibre layer.

(4) Thick inner nuclear layer.

(5) Peculiar double-layered ganglionic layer.

(6) External molecular layer.

(7) External nuclear layer.

(8) Mcmbrana limitans externa.

(9) Layer of rods.

(10) Pigment-epithelium.
He points out especially the peculiarity of layer (2) (2, Eig. 41), the
inner molecular, in which two rows of nuclei are arranged with great

Fig. 41. — Retina and Optic Nerve of Petromyzon. (After Muller and

Langerhans.)

On the left side the Mullerian fibres and pigment-epithelium are represented alone.
The retina is divided into an epithelial part, C (the layer of visual rod-cells), and
a neurodermal or cerebral part which is formed of, A, the gauglion of the optic
nerve and, B, the ganglion of the retina. 1, int. limiting membrane ; 2, int.
molecular layer with its two layers of cells ; 3, layer of optic nerve fibres ; 4, int.
nuclear layer ; 5, double row of tangential fulcrum cells ; 6, layer of terminal
retinal fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of
rods ; 10, layer of pigment-epithelium. D, axial cell layer (Axenstrang) in optic
nerve. The layer 6 is drawn rather too thick.

regularity, the one row closely touching the mcmbrana limitans
interna, the other at the inner boundary of the middle third of the

96 THE ORIGIN OF VERTEBRATES

molecular layer. Of these two rows of nuclei, he describes the inner-
most as composed almost entirely of large nuclei belonging to ganglion
cells, while the outermost is composed mainly of distinctly smaller
nuclei, which in staining and appearance appear to belong not to
nerve-cells but to the true reticular tissue of the molecular layer.

He also draws special attention to the remarkable layer (5) (5,
Fig. 41), which is not found in the retina of the higher vertebrates,
the cells of which, in his opinion, are of the nature of ganglion-cells.

W. Miiller, in 1874, gave a most careful description of the eye
of Ammoccetes and Petromyzon, and traced the development of the
retina; the subsequent paper of Kohl does not add anything new,
and his drawings are manifestly diagrams, and do not represent the
appearances so accurately as Miiller's illustrations. In the
accompanying figure (Fig. 41) I reproduce on the right-hand side
Miiller's picture of the retina of Petromyzon, but have drawn it, as
in Langerhans' picture, at the place of entry of the optic nerve.

From his comparison of this retina with a large number of other
vertebrate retinas, he comes to the conclusion that the retina of all
vertebrates is divisible into

A. An ectodermal (epithelial) part consisting of the layer of the

visual cells, and

B. A neuroclermal (cerebral) part which forms the rest of the

retina.
Further, Miiller points out that the neuroderm gives origin through-
out the central nervous system to two totally different structures, on
the one hand to the true nervous elements, on the other to a system
of supporting cells and fibres which cannot be classed as connective
tissue, for they do not arise from mesoblast, and are therefore called
by him ' fulcrum-cells.' In the retina he recognizes two distinct
groups of such supporting structures — (1) a system of radial fibres
with well-marked elongated nuclei, which extend between the two
limiting layers, and form at their outer ends a membrane-like
expansion which was originally the outer limit of the retina, but
becomes afterwards co-terminous with the mcmbrana limitans
externa, owing to the piercing through it of the external limbs of the
rods. This system, which is known by the name of the radial
Miillerian fibres (shown on the left-hand side of Fig. 41), has no
connection with (2) the spongioblasts and neurospongium, which
form a framework of neuroglia, in which the terminations of the

THE EVIDENCE OF THE ORGANS OF VISION 97

optic ganglion and of the retinal ganglion ramify to form the mole-
cular layers.

It is evident from Fig. 41 that the retina of Ammoccetes and
Petromyzon differs in a striking manner from the typical vertebrate
retina. The epithelial part (C) remains the same — viz. the visual
rods, the external limiting membrane, and the external nuclear
layer; but the cerebral part, the retinal ganglion (A and B), is
remarkably different. It is true, it consists in the main of the
small-celled mass known as the inner nuclear layer, and of the
reticulated tissue or ' neuropil ' known as the inner molecular layer,
just as in all other compound retinal eyes; but neither the ganglion
cell-layer nor the optic fibre-layer is clearly defined as separate from
this molecular layer ; on the contrary, it is matter of dispute as to
what cells represent the ganglionic layer of higher vertebrates, and
the optic fibres do not form a distinct innermost layer, but pass into
the inner molecular layer at its junction with the inner nuclear
layer. A comparison of this innermost part of the retina (A, Fig.
41), with the corresponding part in Berger's picture of Musca {n.l.o.g.,
Fig. 38), shows a most striking similarity between the two. In both
cases the fibres of the optic nerve (O.n., Fig. 38) which cross at their
entrance pass into the ' neuropil ' of this part of the retinal ganglion,
and are connected probably (though that is not proved in either
case) with the cells of the ganglionic layer. In both cases we find
two well-marked parallel rows of cells in this part of the retina, of
which one, the innermost, is composed in Ammoccetes of large
ganglion-cells, and the other mainly of smaller, deeper staining cells
apparently supporting in function. Similarly, also, in Branchipus, as
I conclude from my own observations as well as from those of Berger
and Claus, the ganglionic layer is composed partly of true ganglion-
cells and partly of supporting cells arranged in a distinct layer. This
part, then, of the retina of Ammoccetes is remarkably like that of a
typical arthropod retina, and forms that part of the retinal ganglion
which may be called the ganglion of the optic nerve.

Next comes the ganglion of the retina (B, Fig. 41) (Parker's first
optic ganglion), the cells of which form the small bipolar granule-
cells of the inner nuclear layer; granule-cells arranged in rows just
as they are shown in Claus' picture of the same layer in the retina
of Branchipus (Fig. 40), just as they are found in the cortical layers
of the optic ganglion of the pineal eye (ganglion habcnulcr), in the

11

9§ THE ORIGIN OF VERTEBRATES

optic lobes and other parts of the Ammoccetes brain, or in the cortical
layers of the optic ganglia of all arthropods.

Between this small-celled nuclear layer (4, Fig. 41) and the layer
of nuclei of the visual rod cells (7, Fig. 41) (the external nuclear
layer), we find in the eye of Ammoccetes and Petromyzon two well-
marked rows of cells of a most striking character — viz. the two
remarkably regular rows of large epithelial-like cells with large
conspicuous nuclei, which give the appearance of two opposing rows
of limiting epithelium (5, Fig. 41), already mentioned in connection
with the researches of Langerhans and W. Miiller. Here, then, is a
striking peculiarity of the retina of the lamprey, and according to
Miiller the obliteration of these two layers can be traced as we pass
upwards in the vertebrate kingdom. Among fishes, they are especially
well seen in the perch ; in the higher vertebrates the whole layer is
only a rudiment represented, he thinks, by the simple layer of round
cells which lies close against the inner surface of the layer of
terminal fibres (Nervenansatze), and is especially evident in birds
and reptiles. In man and the higher mammals they are probably
represented by the horizontal cells of the outer part of the inner
nuclear layer.

Seeing, then, that they are most evident in Ammoccetes, and
become less and less marked in the higher vertebrates, it is clear
that their origin cannot be sought among the animals higher in the
scale than Ammocoetes, but must, therefore, be searched for in the
opposite direction.

Miiller describes them as forming a very conspicuous landmark in
the embryology of the retina, dividing it distinctly into two parts, an
outer thinner, and an inner somewhat thicker part, the zone formed
by them standing out conspicuously on account of the size and regu-
larity of the cells and their lighter appearance when stained. Thus
in his description of the retina of an Ammoccetes 95 mm. in length,
he says, " The layer of pale tangentially elongated cells formed a
double layer and produced the appearance of a pale, very charac-
teristic zone between the outer and inner parts of the retina."

Let us now turn to the retina of the crustacean and see whether
there is any evidence there that the retina is divisible into an outer
and inner part, separated by a zone of characteristically pale staining
cells with conspicuous nuclei. The most elaborate description of
the development of the retina of Astacus is given by Eeichenbach,

THE EVIDENCE OF THE ORGANS OF VISION 99

according to whom the earliest sign of the formation of the retina is an
ectodermic involution (Augen-einstulpung), which soon closes, so that
the retinal area appears as a thickening. In close contiguity to this
thickening, the thickening of the optic ganglion arises, so that that
part of the optic ganglion which will form the retinal ganglion fuses
with the thickened optic plate and forms a single mass of tissue.
Later on a fold (Augen-falte) appears in this mass of tissue, in conse-
quence of which it becomes divided into two parts. The lining walls
of this fold form a double row of cells, the nuclei of which are most
conspicuous because they are larger and lighter in colour than the
surrounding nuclei, so that by this fold the retina is divided into an
outer and an inner wall, the line of demarcation being conspicuous by
reason of these two rows of large, lightly-staining nuclei.

Eeichenbach is unable to say that this secondary fold is coincident
with the primary involution, and that therefore the junction between
the two rows of large pale nuclei is the line of junction between the
retinal ganglion and the retina proper, because all sign of the primary
involution is lost before the secondary fold appears.

Parker compares the appearances in the lobster with Reichenbach's
description in the crayfish, and says that he finds only a thicken-
ing, no primary involution ; at the same time he expressly states
that in the very early stages his material was deficient, and that he
had not grounds sufficient to warrant the statement that no involution
occurs. He also finds that in the lobster the ganglionic tissue which
arises by proliferation is divided into an outer and inner part ; the
separation is effected by a band of large, lightly-staining nuclei, which,
in position and structure, resemble the band figured by Eeichenbach.
According to Parker, then, the line of separation indicated in the
development by Reichenbach's outer and inner walls is not the line
of junction between the retina and the retinal ganglion, as Reichen-
bach was inclined to think, but rather a separation of two rows of
large ganglion-cells belonging to the retinal ganglion.

The similarity between these conspicuous layers of lightly-
staining cells in Ammoccetes and in crustaceans is remarkably close,
and in both cases observers have found the same difficulty in inter-
preting their meaning. In each case one group of observers looks
upon them as ganglion-cells, the other as supporting structures.
Thus in the lamprey, Muller considers them to belong to the support-
ing elements, while Langerhans and Kohl describe them as a double

IOO

THE ORIGIN OF VERTEBRATES

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layer of ganglion-cells. In the crustacean, Berger in Squilla, Gren-

acher in Mysis, and Parker in Astacus, look upon them as supporting

elements, while Viallanes in
Palinurus considers them to be
true ganglionic cells.

Whatever the final interpre-
tation of these cells may prove
to be, we may, it seems to me,
represent an ideal compound
retina of the crustacean type by
combining the investigations of
Berger, Claus, Beichenbach, and
Parker in the following figure.

The comparison of this figure
(Fig. 42) with that of the Pe-
tromyzon retina (Pig. 41) shows
how great is the similarity of
the latter with the arthropod
type, and how the very points
in which it deviates from the
recognized vertebrate type are
explainable by comparison with
that of the arthropod. The
most striking difference between
the retinas in the two figures is
that the layer of terminal nerve
fibres (5, Fig. 42), which, after
all, are only the elongated termi-
nations of the retinal cells be-
longing to Parker's neurones of
the first order, is very much
longer than in Petromyzon or in
any vertebrate, for the external
molecular layer (G, Fig. 41)
(Muller's layer of Nervenan-

satze) is very short and inconspicuous (in Fig. 41 it is drawn too

thick).

Turning from the retina to the fibres of the optic nerve we again

find a remarkable resemblance, for in Ammoccetes, as pointed out by

Fig. 42. — Ideal Diageam of the Layers
in a Crustacean Eye.

The retina is divided into an epithelial
part, C (the layer of retinular cells and
rhabdomes), and a neurodermal or cere-
bral part, which is formed of, A, the
ganglion of the optic nerve, and, B, the
ganglion of the retina. 1, optic nerve
fibres which cross at (their entrance into
the retina ; 2, int. molecular layer with
its two rows of cells ; 3, int. nuclear
layer ; 4, Reichenbach's double row of
large lightly-staining cells ; 5, layer of
terminal retinal fibres ; 6, ext. nuclear
layer ; 7, ext. limiting membrane ; 8,
layer of crystalline cones ; 9, cornea.

THE EVIDENCE OF THE ORGANS OF VISION IOI

Langerkaiis and carefully figured l>y Kohl, a crossing of the fibres of
the optic nerve occurs as the nerve leaves the retina, just as is so uni-
versally the case in all compound retinas. To this crossing Kohl has
given the name chiasma nervi optici, in distinction to the cerebral
chiasma, which he calls chiasma nervorum opticorum. Further, we
find that even this latter chiasma is well represented in the arthro-
pod brain ; thus Bellonci in Sphgeroma, Berger, Dietl, and Krieger in
Astacus, all describe a true optic chiasma, the only difference in
opinion being, whether the crossing of the optic nerves is complete or
not. Especially instructive are Bellonci's figures and description.
He describes the brain of Sphaeroma as composed of three segments
— a superior segment, the cerebrum proper, a middle segment,
and an inferior segment ; the optic fibres, as is seen in Fig. 39,
after crossing, pass direct into the middle segment, in the ganglia of
which they terminate. From this segment also arises the nerve to
the first antenna of that side — i.e. the olfactory nerve. The optic
part, then, of this middle segment is clearly the brain portion of the
optic ganglionic apparatus, and may be called the optic lobes, in
contradistinction to the peripheral part, which is usually called the
optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II.,
as already mentioned. These optic lobes are therefore homologous
with the optic lobes of the vertebrate brain.

The resemblance throughout is so striking as to force one to the
conclusion that the retina of the vertebrate eye is a compound retina,
composed of a retiua and retinal ganglion of the type found in arthro-
pods. From this it follows that the development of the vertebrate
retina ought to show the formation of (1) an optic plate formed
from the peripheral epidermis and not from the brain ; (2) a part of
the brain closely attached to this optic plate forming the retinal
ganglion, which remains at the surface when the rest of the optic
ganglion withdraws ; (3) an optic nerve formed in consequence of
this withdrawal, as the connection between the retinal and cerebral
parts of the optic ganglion.

This appears to me exactly what the developmental process does
show according to Gotte's investigations. He asserts that the retina
arises from an optic plate, being the optical portion of his ' Sinnes-
platte.' At an early stage this is separated by a furrow (Furche)
from the general mass of epidermal cells which ultimately form the
brain. This separation then vanishes, and the retina and brain-mass

102 THE ORIGIN OF VERTEBRATES

become inextricably united into a mass of cells, which are still
situated at the surface. By the closure of the cephalic plate and the
withdrawal of the brain away from the surface, a retinal mass of cells
is left at the surface connected with tjhe tubular central nervous
system by the hollow optic diverticulum or primary optic vesicle.
If we regard only the retinal and nervous elements, and for the
moment pay no attention to the existence of the tube, Gotte's obser-
vation that the true retina has been formed from the optic plate
(Sinnes-platte) to which the retinal portion of the brain (retinal
ganglion) has become firmly fixed, and that then the optic nerve has
been formed by the withdrawal of the rest of the brain (optic lobes),
is word for word applicable to the description of the development of
the compound retina of the arthropod eye, as has been already stated.

The Significance of the Optic Diverticula.

The origin of the retina from an optic epidermal plate in verte-
brates, as in all other animals, brings the cephalic eyes of all animals
into the same category, and leaves the vertebrate eye no longer in an
isolated and unnatural position. In one point the retina of the verte-
brate eye differs from that of a compound retina of an invertebrate ;
in the former, a striking supporting tissue exists, known as Midler's
fibres, which is absent in the latter. This difference of structure is
closely associated with another of the same character as in the central
nervous system, viz. the apparent development of the nervous part from
a tube. We see, in fact, that the retinal and nervous arrangements of
the vertebrate eye are comparable with those of the arthropod eye, in
precisely the same way and to the same extent as the nervous matter
of the brain of the vertebrate is comparable with the brain of the
arthropod. In both cases the nervous matter is, in structure, position,
and function, absolutely homologous ; in both cases there is found in
the vertebrate something extra which is not found in the invertebrate
— viz. a hollow tube, the walls of which, in the case of the brain, are
utilized as supporting tissues for the nerve structures. The explana-
tion of this difference in the case of the brain is the fundamental
idea of my whole theory, namely, that the hollow tube is in reality
the cephalic stomach of the invertebrate, around which the nervous
brain matter was originally grouped in precisely the same manner as
in the invertebrate. What, then, are the optic diverticula ?

THE EVIDENCE OF THE ORGANS OF VISION 103

" The formation of the eye," as taught by Balfour, " commences
with the appearance of a pair of hollow outgrowths from the anterior
cerebral vesicle. These outgrowths, known as the optic vesicles, at
first open freely into the cavity of the anterior cerebral vesicle.
From this they soon, however, become partially constricted, and
form vesicles united to the base of the brain by comparatively
narrow, hollow stalks, the rudiments of the optic nerves."

" After the establishment of the optic nerves, there takes place
(1) the formation of the lens, and (2) the formation of the optic cup
from the walis of the primary optic vesicle."

He then goes on to explain how the formation of the lens forms
the optic cup with its double walls from the primary optic vesicle,
and says —

" Of its double walls, the inner, or anterior, is formed from the
front portion, the outer, or posterior, from the hind portion of the
wall of the primary optic vesicle. The inner, or anterior, which very
speedily becomes thicker than the other, is converted into the retina ;
in the outer, or posterior, which remains thin, pigment is eventually
deposited, and it ultimately becomes the tesselated pigment-layer of
the choroid."

The difficulties in connection with this view of the origin of the
eye are exceedingly great, so great as to have caused Balfour to
discuss seriously Lankester's suggestion that the eye must have been
at one time within the brain, and that the ancestor of the vertebrate
was therefore a transparent animal, so that light might get to the eye
through the outer covering and the brain-mass ; a suggestion, the
unsatisfactory nature of which Balfour himself confessed. Is there
really evidence of any part of either retina or optic nerve being
formed from the epithelial lining of the tube ?

This tube is formed as a direct continuation of the tube of the
central nervous system, and we can therefore apply to it the same
arguments as have been used in the discussion of the meaning of the
latter tube. Now, the striking point in the latter case is the fact
that the lining membrane of the central canal is in so many parts
absolutely free from nervous matter, and so shows, as in the so-called
choroid plexuses, its simple, non-nervous epithelial structure. This
also we find in the optic diverticulum. Where there is no evidence
of any invasion of the tube by nervous elements, there it retains its
simple non-nervous character of a tube composed of a single layer of

104 THE ORIGIN OF VERTEBRATES

epithelial cells — viz. in that part of the tube which, as Balfour says,
remains thin, in which pigment is eventually deposited, and which
ultimately becomes the tesselated pigment-layer of the choroid.
Nobody has ever suggested that this pigment-layer is nervous matter,
or ever was, or ever will be, nervous matter ; it is in precisely the
same category as the membranous roof of the brain in Ammocoetes,
which never was, and never will be, nervous matter. Yet, according
to the old embryology both in the case of the eye and the brain, the
pigment-layer and the so-called choroid plexuses are a part of the
tubular nervous system.

Turning now to the optic nerve, Balfour describes it as derived
from the hollow stalk of the optic vesicle. He says —

" At first the optic nerve is ecrually continuous with both walls
of the optic cup, as must of necessity be the case, since the interval
which primarily exists between the two walls is continuous with the
cavity of the stalk. When the cavity within the optic nerve
vanishes, and the fibres of the optic nerve appear, all connection is
ruptured between the outer wall of the optic cup and the optic
nerve, and the optic nerve simply perforates the outer wall, and
becomes continuous with the inner one."

In this description Balfour, because he derived the optic nerve
fibres from the epithelial wall of the optic stalk, of necessity supposed
that such fibres originally supplied both the outer and inner walls of
the optic cup and, therefore, seeing that when the fibres of the optic
nerve appear they do not supply the outer wall, he supposes that
their original connection with the outer wall is ruptured, because a
discontinuity of the epithelial lining takes place coincidently witli
the appearance of the optic nerve-fibres, and, according to him, the
optic nerve simply perforates the outer wall and becomes continuous
with the inner one. This last statement is very difficult to under-
stand. I presume he meant that some of the fibres of the optic nerve
supplied from the beginning the inner wall of the optic cup, but
that others which originally supplied the outer wall were first ruptured,
then perforated the outer wall, and finally completed the supply to
the inner wall or retina.

This statement of Balfour's is the necessary consequence of his
belief, that the epithelial cells of the optic stalk gave rise to the
fibres of the optic nerve. If, instead of this, we follow Kolliker and
His, who state that the optic nerve-fibres are formed outside the

THE EVIDENCE OF THE ORGANS OF VISION 105

epithelial walls of the optic stalk, and that the cells of the latter
form supporting structures for the nerve-fibres, then the position of
the optic nerve becomes perfectly simple and satisfactory without
any rupturing of its connection with the outer wall and subsequent
perforation, for the optic nerve-fibres from their very first appearance
pass directly to supply the retina — i.e. the inner wall of the optic
cup and nothing else.

They pass, as is well known, without any perforation by way of
the choroidal slit to the inner surface of the inner wall (retina) of
the optic cup; then, when the choroidal
slit becomes closed by the expansion ?

of the optic cup, the optic nerve
naturally becomes situated in the centre
of the base of the cup and spreads over
its inner surface as that surface expands.

A section across the optic cup at an
early stage at the junction of the optic
stalk and optic cup would be repre-
sented by the upper diagram in Fig.
43 ; at a later stage, when the choroidal
slit is closed, by the lower diagram.

The evident truth of this manner
of looking at the origin of the optic
nerve is demonstrated by the appear-
ance of the optic nerve in Amrao-
ccetes and Fetromyzon. In the latter,
although the development is complete,
and the eye, and consequently also the
optic nerve-fibres, are fully functional,
there is still present in the axial core
of the nerve a row of epithelial cells
(Axenstrang) which are altered so as
to form supporting structures, in the
same way as a row of epithelial cells in the retina is altered to form
the system of supporting cells known by the name of the Mtillerian
fibres.

The origin of this axial core of cells is perfectly clear, as has been
pointed out by W. Miiller. He says—

" The development of the optic nerve shows peculiarities in

On

Fig. 43. — Diagram op the RELA-
tion of the optic nerve to
the Optic Cup.

The upper diagram represents a
stage before the formation of the
choroidal slit, the lower one the
stage of closure of the choroidal
slit. R., retina; O.n., optic
nerve ; p., pigment epithelium.

106 THE ORIGIN OF VERTEBRATES

Petromyzon of such a character as to make this animal one of the
most valuable objects for deciding the various controversial questions
connected with the genesis of its elements. The lumen of the stalk
of the primary optic vesicle is obliterated quite early by a prolife-
ration of its lining epithelium. Also the original continuity of this
epithelium with that of the pigment-layer is at an early period
interrupted at the point of attachment of the optic stalk. This
interruption occurs at the time when the fibres of the optic nerve
first become visible."

Further on he says —

" The epithelium of the optic stalk develops entirely into sup-
porting cells, which in Petromyzon fill up the original lumen and so
form an axial core (Axenstrang) to the nerve-fibres which are formed
entirely outside them ; the projections of these supporting cells are
directed towards the periphery, and so separate the bundles of the
optic nerve- fibres. The mesodermal coat of the optic stalk takes no
part in this separation ; it simply forms the connective tissue sheath
of the optic nerve. The development of the optic nerve in the
higher vertebrates also obeys the same law, as I am bound to conclude
from my own observations."

The evidence, then, of Ammococtes is very conclusive. Originally
a tube composed of a single layer of epithelial cells became expanded
at the anterior end to form a bulb. On the outside of this tube or
stalk the fibres of the optic nerve make their appearance, arising from
the ganglion-cell layer of the retina, and, passing over the surface of
the epithelial tube at the choroidal fissure, proceed to the brain by
way of the optic chiasma. Owing to the large number of fibres, their
crossing at the junction of the stalk with the bulb, and the narrow-
ness at this neck, the obliteration of the lumen of the tube which
takes place in the stalk is carried out to a still greater extent at this
narrow part. The result of this is that all continuity of the cell-
layers of the original tube of the optic stalk with those of both the
inner and outer walls of the bulb is interrupted, and all that remains
in this spot of the original continuous line of cells which connected the
tube of the stalk with that of the bulb are possibly some of the groups
of cells which are found scattered among the fibres of the optic nerve
at their entrance into the retina. Such separation of the originally
continuous elements of the epithelial wall of the optic stalk, which
is apparent only at this neck of the nerve in Petromyzon, takes place

THE EVIDENCE OF THE ORGANS OF VISION 107

along the whole of the optic nerve in the higher vertebrates, so that
no continuous axial core of cells exist, but only scattered supporting
cells.

If further proof in support of this view be wanted, it is given by
the evidence of physiology, which shows that the fibres of the optic
nerve are not different from other nerve-fibres of the central nervous
system, but that they degenerate when separated from their nerve-
cell, and that the nerve-cell of which the optic nerve-fibre is a
process is the large ganglion-cell of the ganglionic layer of the retina.
The origin of the ganglionic layer of the retina cannot therefore be
separated from that of the optic nerve-fibres. If the one is outside
the epithelial tube, so is the other, and what holds true of the gan-
glionic layer must hold good of the rest of the retinal ganglion and,
from all that has been said, of the retina itself. We therefore come
to the conclusion that the evidence is distinctly in favour of the
view, that the retina and optic nerve in the true sense are structures
which originally were outside a non-nervous tube, but, just like the
central nervous system as a whole, have amalgamated so closely with
the elements of this tube as to utilize them for supporting structures.
One part of this non-nervous tube, its dorsal wall, like the corre-
sponding part of the brain-tube, still retains its original character,
and by the deposition of pigment has been pressed into the service
of the eye to form the pigmented epithelial layer;

We can, however, go further than this, for we know definitely in
the case of the retina what the fate of the epithelial cells lining
this tube has been. They have become the system of supporting
structures known as Miillerian fibres.

The epithelial layer of the primary optic vesicle can be traced into
direct continuity with the lining epithelium of the brain cavity, as
a single layer of epithelial cells in the core of the optic nerve, form-
ing the optic stalk, which, in consequence of close contact, becomes
the well-known axial layer of supporting cells. This epithelial layer
of the optic stalk then expands to form the optic bulb, the outer or
dorsal wall of which still remains as a single layer of epithelium
and becomes the layer of pigment epithelium. This layer of
epithelium becomes doubled on itself by the approximation of the
inner or ventral wall of the optic cup to the outer or dorsal wall in
consequence of the presence of the lens, and still remaining a single
layer, forms the pars ciliaris retinae ; then suddenly, at the ora

io8

THE ORIGIN OF VERTEBRATES

serrata, the single epithelial layer vanishes, and the layers of the
retina take its place. It has long been known, however, that even
throughout the retina this single epithelial layer still continues, being
known as the fibres of Miiller. This is how the fact is described
in the last edition of Foster's "Text-book of Physiology," p. 1308 —

" Stretching radially from the inner to the outer limiting mem-
brane in all regions of the retina are certain peculiar- shaped bodies
known as the radial fibres of Miiller. Each fibre is the outcome of
the changes undergone by what was at first a simple columnar

epithelial cell. The changes
are, in the main, that the
columnar form is elongated
into that of a more or less
prismatic fibre, the edges of
which become variously
branched, and that while the
nucleus is retained the cell
substance becomes converted
into neuro-keratin. And, in-
deed, at the ora serrata the
fibres of Miiller may be seen
suddenly to lose their peculiar
features and to pass into the
ordinary columnar cells which
form the pars ciliaris retime."
It is then absolutely clear
that the essential parts of the
eye may be considered as
composed of two parts —

. p.c r

- P

- aa.t

Fig. 44. — Diagram representing the
Single-layered Epithelial Tube of
the Vertebrate Eye after removal of
the Nervous and Retinal Elements.

O.n., axial core of cells in optic nerve; 2 } ->
pigment epithelium; p.c.r., pars ciliaris
retina ; m.f., Miillerian fibres; I., lens.

1. A tube or diverticulum
from the tube of the central nervous system, composed throughout
of a single layer of epithelium, which forms the supporting axial
cells in the optic nerve, the pigment epithelium and the Miillerian
fibres of the retina. Such a tube would be represented by the
accompanying Fig. 44, and the left side of Fig. 41.

2. The retina proper with the retinal ganglion and the optic
nerve-fibres as already described. In this part supporting elements
are found, just as in any other compound retina, of the nature of
neuroglia, which are independent of the Miillerian fibres.

THE EVIDENCE OF THE ORGANS OF VIS I OX 1 09

Of these two parts we have already seen that the second is to
all intents and purposes a compound retina of a crustacean eye, and
seeing that the single-layered epithelial tube is continuous with the
single-layered epithelial tube of the central nervous system — i.e. with
the cephalic part of the gut of the arthropod ancestor — it follows with
certainty that the ancestor of the vertebrates must have possessed
two anterior diverticula of the gut, with the wall of which, near the
anterior extremity, the compound retina has amalgamated on either
side, just as the infra-cesophageal ganglia have amalgamated with
the ventral wall of the main gut-tube. In this way, and in this way
alone, does the interpretation of the structure of the vertebrate lateral
eye harmonize in the most perfect manner with the rest of the con-
clusions already arrived at.

The question therefore arises : — Have we any grounds for believing
that the ancient forms of primitive crustaceans and primitive arachnids,
which were so abundant in the time when the Cephalaspids appeared,
possessed two anterior diverticula of the stomach, such as the con-
sideration of the vertebrate eye strongly indicates must have been
the case ?

The beautiful pictures of Blanchard, and his description, show
how, on the arachnid side, paired diverticula of the stomach are
nearly universal in the group. Thus, although they are not present
in the scorpions, still, in the Thelyphonidae, Phrynidas, Solpugidae,
Mygalidse, the most marked characteristic of the stomach-region is
the presence of four pairs of ccecal diverticula, which spread laterally
over the prosomatic region. In the spiders the number of such
diverticula increases, and the whole prosomatic region becomes rilled
up with these tubes. Blanchard considers that they form nutrient
tubes for the direct nutrition of the organs in the prosoma, especially
the important brain-region of the central nervous system. He points
out that these animals are blood-suckers, and that, therefore, their
food is already in a suitable form for purposes of nutrition when it
is taken in by them, so that, as it were, the anterior part of the gut
is transformed into a series of vessels or diverticula conveying blood
directly to the important organs in the prosoma, by means of which
they obtain nourishment in addition to their own blood-supply.

The universality of such diverticula among the arachnids makes
it highly probable that their progenitors did possess an alimentary
canal with one or more pairs of anterior diverticula. In the

I IO

THE ORIGIN OF VERTEBRATES

vertebrate, however, the paired diverticula are associated with a
compound retina, a combination which does not occur among living
arachnids ; we must, therefore, examine the crustacean group for the
desired combination, and naturally the most likely group to examine
is the Phyllopoda, especially such primitive forms as Branchipus and
Artemia, for it is universally acknowledged that these forms are

Al

.— rt.gl

Fig. 45. — Section through one of the two Anterior Diverticula of the Gut
in Artemia and the Retinal Ganglion.

The section is through the extreme anterior end of the diverticulum, thus cutting
through many of the columnar cells at right angles to their axis. AL, gut
diverticulum ; rt. gl., retinal ganglion.

the nearest living representatives of the trilobites. If, therefore, it
be found that the retina and optic nerve in Artemia is in specially
close connection with an anterior diverticulum of the gut on each
side, then it is almost certain that such a combination existed also
in the trilobites.

My friend Mr. W. B. Hardy has especially investigated the
nervous system of Artemia. In the course of his work he cut serial

THE EVIDENCE OF THE ORGANS OF VISION III

le

sections through the whole animal, and, as mentioned in my paper
in the Journal of Anatomy and Physiology, he discovered that the
retinal ganglion of each c.e On

lateral eye is so closely
attached to the end of the
corresponding diverticu-
lum of the gut that the
lining cells of the ventral
part of the diverticulum
form a lining to the reti-
nal ganglion (Fig. 45). In
this animal there are only
two gut-diverticula, which
are situated most ante-
riorly. I have plotted
out this series of sections
by means of a camera
lucida, with the result
that the retina appears as
a bulging attached ventro-
laterally to the extremity of each gut-diverticulum, as is shown in

A!

Fig. 46. — The Brain, Eyes, and Anterior
Termination of the Alimentary Canal of
Artemia, viewed from the Dorsal Aspect.

Br., brain; I.e., lateral eyes; c.e., median eyes; Al.,
alimentary canal.

A B

Fig. 47.— A, The Formation of the Retina of the Eye of Ammoccetes (after
Scott) ;' B, The Formation of the Retina of the Eye of Ammoccetes, on

MY THEORY.

R., retina; l, lens; O.n., optic nerve fibres; Al., cephalic end of invertebrate ali-
mentary canal; V., cavity of ventricles of brain; Aid,, anterior diverticulum
of alimentary canal ; op.d., optic diverticulum.

Fig. 46. It is instructive to compare with this figure Scott's picture
of the developing eye in Ammoccetes, where he figures the retina as

112 THE ORIGIN OF VERTEBRATES

a bulging attached ventrally to the extremity of the narrow tube of
the optic diverticulum. In Fig. 47, A, I reproduce this figure of
Scott, and by the side of it, Fig. 47, B, I have represented the origin
of the vertebrate eye as 1 believe it to have occurred.

We see, then, this very striking fact, that in the most primitive
of the Crustacea, not only are there two anterior diverticula of the
gut, but also the retinal ganglion of the lateral eye is in specially
close connection with the end of the diverticulum on each side. In
fact, we find in the nearest living representative of the trilobites a
retina and retinal ganglion and optic nerve, closely resembling that
of the vertebrate, in close connection with an epithelial tube which
has nothing to do with the organ of sight, but is one of a pair of
anterior gut-diverticula. It is impossible to obtain more decisive
evidence that the trilobites possessed a pair of gut-diverticula sur-
rounded to a greater or less extent by the retina and optic nerve of
each lateral eye.

Such anterior diverticula are commonly found in the lower
Crustacea ; they are usually known by the name of liver-diverticula,
but as they take no part in digestion, and, on the contrary, represent
that part of the gut which is most active in absorption, the term
liver is not appropriate, and it is therefore better to call them simply
the pair of anterior diverticula. Our knowledge of their function in
Daphnia is given in a paper by Hardy and M'Dougall, which does
not appear to be widely known. Hardy succeeded in feeding Daphnia
with yolk of egg in which carmine grains were mixed, and was able
in the living animal to watch the whole process of deglutition,
digestion, and absorption. The food, which is made into a bolus, is
moved down to the middle region of the gut, and there digestion
takes place. Then by an antiperistaltic movement the more fluid
products of the digestion-process are sent right forward into the two
anterior diverticula, where the single layer of columnar cells lining
these diverticula absorbs these products, the cells becoming thickly
studded with fat-drops after a feed of yolk of egg. The carmine
particles, which were driven forward with the proteid- and fat-
particles, are not absorbed, but are at intervals driven back by con-
tractions of the anterior diverticula to the middle region of the gut.

These observations prove most clearly that the anterior diver-
ticula have a special nutrient function, being the main channels by
which new nutrient material is brought into the body, and, as

THE EVIDENCE OF THE ORGANS OF VISION 113

pointed out by the authors, it is a remarkable exception in the
animal kingdom that absorption should occur in that portion of the
gut which is anterior to the part in which digestion occurs. In all
these animals the two anterior diverticula extend forwards over the
brain, and, as we have seen in Artemia, the anterior extremity
of each one is so intimately related to a part of the brain — viz.
the retinal ganglion — as to form a lining membrane to that mass
of nerve-cells. It follows, therefore, that the nutrient fluid absorbed
by the cells of this part of the gut-diverticulum must be primarily
for the service of the retinal ganglion. In fact, the relations of
this anterior portion of the gut to the brain as a whole suggest
strongly that the marked absorptive function of this anterior
portion of the gut exists in order to supply nutrient material
in the first place to the most vital, most important organ in the
animal — the brain and its sense-organs. This conclusion is borne
out by the fact that in these lower crustaceans the circulation of
blood is of a very inefficient character, so that the tissues are mainly
dependent for their nutrition on the fluid immediately surrounding
them. It stands to reason that the establishment of the anterior
portion of the gut as a nutrient tube to the brain would necessitate
a closer and closer application of the brain to that tube, so that the
process of amalgamation of the brain with the single layer of columnar
epithelial cells which constitutes the wall of the gut (which we see
in its initial stage in the retinal ganglion of Artemia), would tend
rapidly to increase as more and more demands were made upon the
brain, until at last both the supra- and infra-cesophageal ganglia, as
well as the retinal ganglia and optic nerves, were in such close
intimate connection with the ventral wall of the anterior portion of
the gut and its diverticula as to form a brain and retina closely
resembling that of Ammoccetes.

Such an origin for the lateral eyes of the vertebrate explains in a
simple and satisfactory manner why the vertebrate retina is a com-
pound retina, and why both retina and optic nerve have an apparent
tubular development.

At the same time one discrepancy still exists which requires
consideration — viz. in no arthropod eye possessing a compound
retina is the retina inverted. All the known cases of inversion
among arthropods occur in eyes, the retina of which is simple, and
are all natural consequences of the process of invagination by which

I

114 THE ORIGIN OF VERTEBRATES

the retina is formed. On the other hand, eyes with an inverted
compound retina are not entirely unknown among invertebrates, for
the eyes of Pecten and of Spondylus possess a retina which is
inverted after the vertebrate fashion and still may be spoken of as
compound rather than simple. It is clear that an invagination, the
effect of which is an inversion of the retinal layer, would lead to
the same result, whether the retinal optic nerves were short or long,
whether, in fact, a retinal ganglion existed or not. Undoubtedly the
presence of the retinal ganglion tends greatly to obscure any process
of invagination, so that, as already mentioned, many observers, with
Parker, consider the retina of the crustacean lateral eye to be
formed by a thickening only, without any invagination, while
Peichenbach says an obscure invagination does take place at a very
early stage. So in the vertebrate eye most observers speak only of
a thickening to form the retina, but Gotte's observation points to an
invagination of the optic plate at an early stage. So also in the eye
of Pecten, Korschelt and Heider consider that the thickening, by
which the retina is formed according to Patten, in reality hides an
invagination process by means of which, as Biitschli suggests, an
optic vesicle is formed in the usual manner. The retina is
formed from the anterior wall of this vesicle, and is therefore
inverted.

The origin of the inverted retina of the vertebrate eye does not
seem to me to present any great difficulty, especially when one
takes into consideration the fact that the retina is inverted in the
arachnid group, only in the lateral eyes. The inversion is
usually regarded as associated with the tubular formation of the
vertebrate retina, and it is possible to suppose that the retina became
inverted in consequence of the involvement of the eye with the gut-
diverticulum. I do not myself think any such explanation is at all
probable, because I cannot conceive such a process taking place with-
out a temporary derangement — to say the least of it— of the power of
vision, and as I do not believe that evolution was brought about by
sudden, startling changes, but by gradual, orderly adaptations, and
as I also believe in the paramount importance of the organs of
vision for the evolution of all the higher types of the animal kingdom,
I must believe that in the evolution from the Arthropod to the
Gephalaspid, the lateral eyes remained throughout functional. I
therefore, for my own part, would say that the inversion of the

THE EVIDENCE OF THE ORGANS OF VISION I 15

retina took place before the complete amalgamation with the gut-
diverticulum, that, in fact, among the proto-crustacean, proto-
arachnid forms there were some sufficiently arachnid to have an
inverted retina, and at the same time sufficiently crustacean to
possess a compound retina, and therefore a compound inverted
retina after the vertebrate fashion existed in combination with the
anterior gut-diverticula. Thus, when the eye and optic nerve sank
into and amalgamated with the gut-diverticulum, neither the dioptric
apparatus nor the nervous arrangements would suffer any alteration,
and the animal throughout the whole process would possess organs
of vision as good as before or after the period of transition.

Further, not only the retina but also the dioptric apparatus of
the vertebrate eye point to its origin from a type that combined
the peculiarities of the arachnids and the crustaceans. In the
former it is difficult to speak of a true lens, the function of a lens
being undertaken by the cuticular surface of the cells of the corneagen
(Mark's ' lentigtn '), while in the latter, in addition to the corneal
covering, a true lens exists in the shape of the crystalline cones.
Further, these crustacean lenses are true lenses in the vertebrate
sense, in that they are formed by modified hypodermal cells, and
not bulgings of the cuticle, as in the arachnid. We see, in fact, that
in the compound crustacean eye an extra layer of hypodermal cells has
become inserted between the cornea and the retina to form a lens.
So also in the vertebrate eye the lens is formed by an extra layer of
the epidermal cells between the cornea and the retina. The fact that
the vertebrate eye possesses a single lens, though its retina is composed
of a number of ommatidia, while the crustacean eye possesses a lens
to each ommatidium, may well be a consequence of the inversion of
the vertebrate retina. It is most probable, as Korschelt and Heider
have pointed out, that the retina of the arachnid eyes is composed
of a number of ommatidia, just as in the crustacean eyes and
in the inverted eyes it is probable that the image is focussed on
to the pigmented tapetal layer, and thence reflected on to the
percipient visual rods. In such a method of vision a single lens is a
necessity, and so it must also be if, as I suppose, eyes existed with
an inverted compound retina. Owing to the crustacean affinities of
such eyes, a lens would be formed and the retina would be compound :
owing to the arachnid affinities, the retina would be inverted and
the hypodermal cells which formed the lens would be massed

Il6 THE ORIGIN OF VERTEBRATES

together to form a single lens, instead of being collected in groups of
four to form a series of crystalline cones.

To sum up : The study of the vertebrate eyes, both median and
lateral, leads to most important conclusions as to the origin of the
vertebrates, for it shows clearly that whereas, as pointed out in this
and subsequent chapters, their ancestors possessed distinct arachnid
characteristics, yet that they cannot have been specialized arachnids,
such as our present-day forms, but rather they were of a primitive
arachnid type, with distinct crustacean characteristics : animals
that were both crustacean and arachnid, but not yet specialized in
either direction : animals, in fact, of precisely the kind which
swarmed in the seas at the time when the vertebrates first made their
appearance. In the opinion of the present day, the ancestral forms
of the Crustacea, which were directly derived from the Annelida,
may be classed as an hypothetical group the Protostraca, the nearest
approach to which is a primitive Phyllopod.

" Starting from the Protostraca," say Korschelt and Heider,
" according to the present condition of our knowledge, we may, as
has been already remarked, assume three great series of development
of the Arthropodan stock, by the side of which a number of smaller
independent branches have been retained. One of these series leads
through the hypothetical primitive Phyllopod to the Crustacea ; the
second through the Pakeostraca (Trilobita, Gigantostraca, Xiphosura)
to the Arachnida ; the third through forms resembling Peripatus to
the Myriapoda and the Insecta. The Pantapoda and the Tardigrada
must probably be regarded as smaller independent branches of the
Arthropodan stock."

To these " three great series of development of the Arthropodan
stock " the evidence of Ammocoetes shows that a fourth must be added,
which, starting also from the Protostraca, and closely connected with
the second, palffiostracan branch, leads through the Cephalaspidae to
the great kingdom of the Vertebrata. Such a direct linking of the
earliest vertebrates with the Annelida through the Protostraca is of
the utmost importance, as will be shown later in the explanation of
the origin of the vertebrate ccelom and urinary apparatus.

THE EVIDENCE OF THE ORGANS OF VISION 1 1 7

Summary.

The most important discovery of recent years which gives a direct clue to
the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland
is all that remains of a pair of median eyes which must have been functional in
the immediate ancestor of the vei'tebrate, seeing 1 how perfect one of them
still is in Ammocoetes. The vertebrate ancestor, then, possessed two pairs of
eyes, one pair situated laterally, the other median. In striking confirmation of
the origin of the vertebrate from Palaeostracans it is universally admitted that
all the Eurypterids and such-like forms resembled Limulus in the possession of
a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient
mailed fishes the Ostracodermata, which are the earliest fishes known, are all said
to show the presence of a pair of median eyes as well as of a pair of lateral eyes.
This evidence 'directly suggests that the structure of both the median and
lateral vertebrate eyes ought to be very similar to that of the median and lateral
arthropod eyes. Such is, indeed, found to be the case.

The retina of the simplest form of eye is formed from a group of the superficial
epidermal cells, and the rods or rhabdites are formed from the cuticular covering
of these cells ; the optic nerve passes from these cells to the deeper-lying brain.
This kind of retina may be called a simple retina, and characterizes the eyes,
both median and lateral, of the scorpion group.

In other cases a portion of the optic ganglion remains at the surface, when
the brain sinks inwards, in close contiguity to the epidermal sense-cells which
form the retina ; a tract of fibres connects this optic ganglion with the under-
lying brain, and is known as the optic nerve. Such a retina may be called
a compound retina and characterizes the lateral eyes of both crustaceans and
vertebrates. Also, owing to the method of formation of the retina by invagina-
tion, the cuticular surface of the retinal sense-cells, from which the rods are
formed, may be directed towards the source of light or away from it. In the
first case the retina may be called upright, in the second inverted.

Such inverted retinas are found in the vertebrate lateral eyes and in the
lateral eyes of the arachnids, but not of the crustaceans.

The evidence shows that all the invertebrate median eyes possess a simple
upright retina, and in structure are remarkably like the right median or pineal
eye of Ammocoetes ; while the lateral eyes possess, as in the crustaceans, an
upright compound retina, or, as in many of the arachnids, a simple inverted
retina. The lateral eyes of the vertebrates alone possess a compound inverted
retina.

This retina, however, is extraordinarily similar in its structure to the
compound crustacean retina, and these similarities are more accentuated in the
retina of the lateral eye of Petromyzon than that of the higher vertebrates.

The evidence afforded by the lateral eye of the vertebrate points unmistakably
to the conclusion that the ancestor of the vertebrate possessed both crustacean
and arachnid characters — belonged, therefore, to a group of animals which gave
rise to both the crustacean and arachnid groups. This is precisely the position
of the Palfeostracan group, which is regarded as the ancestor of both the
crustaceans and arachnids.

Il8 THE ORIGIN OF VERTEBRATES

In two respects the retina of the lateral eyes of vertebrates differs from that
of all arthropods, for it possesses a special supporting - structure, the Mullerian
fibres, which do not exist in the latter, and it is developed in connection with
a tube, the optic diverticulum, which is connected on each side with the main
tube of the central nervous system. These two differences are in reality one
and the same, for the Miillerian fibres are the altered lining cells of the optic
diverticulum, and this tube has the same significance as the rest of the tube of
the nervous system ; it is something which has nothing to do with the nervous
portion of the retina but has become closely amalgamated with it. The explana-
tion is. word for word, the same as for the tubular nervous system, and shows that
the ancestor of the vertebrate possessed two anterior diverticula of its alimentary
canal which were in close relationship to the optic ganglion and nerve of the
lateral eye on each side. It is again a striking coincidence to find that
Ai-temia, which with Branchipus represents a group of living crustaceans most
nearly allied to the trilobites, does possess two anterior diverticula of the gut
which are in extraordinarily close relationship with the optic ganglia of the
retina of the lateral eyes on each side.

The evidence of the optic apparatus of the vertebrate points most remarkably
to the derivation of the Vertebrata from the Palfeostraca.

CHAPTER III

THE EVIDENCE OF THE SKELETON

The bony and cartilaginous skeleton considered, not the notochord. — Nature of
the earliest cartilaginous skeleton. — The mesosomatic skeleton of Amnio-
ccetes ; its topographical arrangement, its structure, its origin in muco-
cartilage. — The prosomatic skeleton of Amnioccetes ; the trabecular and
parachordals, their structure, their origin in white fibrous tissue. — The
mesosomatic skeleton of Linmlus compared with that of Ammoccetes ;
similarity of position, of structure, of origin in muco-cartilage. — The
prosomatic skeleton of Linmlus ; the entosternite or plastron compared with
the trabecular of Ammocoetes; similarity of position, of structure, of origin
in fibrous tissue. — Summary.

The explanation of the two optic diverticula given in the last chapter
accounts in the same harmonious manner for every other part of the
tube around which the central nervous system of the vertebrate has
been grouped. The tube conforms in all respects to the simple epi-
thelial tube which formed the alimentary canal of the ancient type of
marine arthropods such as were dominant in the seas when the verte-
brates first appeared. The whole evidence so far is so uniform and
points so strongly in the direction of the origin of vertebrates from
these ancient arthropods, as to make it an imperative duty to proceed
further and to compare one by one the other parts of the central
nervous system, together with their outgoing nerves in the two groups
of animals.

Before proceeding to do this, it is advisable first to consider
the question of the origin of the vertebrate skeletal tissues, for this
is the second of the great difficulties in the way of deriving verte-
brates from arthropods, the one skeleton being an endo-skeleton
composed of cartilage and bone, and the other an exo-skeleton com-
posed of chitin. Here is a problem of a totally different kind to that
we have just been considering, but of so fundamental a character that
it must, if possible, be solved before passing on to the consideration
of the cranial nerves and the organs they supply.

120 THE ORIGIN OF VERTEBRATES

Is there any evidence which makes it possible to conceive the
method by which the vertebrate skeleton may have arisen from the
skeletal tissues of an arthropod ? By the vertebrate skeleton I mean
the bony and cartilaginous structures which form the backbone, the
cranio-facial skeleton, the pectoral and pelvic girdles, and the bones
of the limbs. I do not include the notochord in these skeletal tissues,
because there is not the slightest evidence that the notochord played
any part in the formation of these structures ; the notochordal tissue
is something mi generis, and never gives rise to cartilage or bone.
The notochord happens to lie in the middle line of the body and is
very conspicuous in the lowest vertebrate ; with the development of
the backbone the notochord becomes obliterated more and more, until
at last it is visible in the higher vertebrates only in the embryo ; but
that obliteration is the result of the encroachment of the growing
bone-masses, not the cause of their growth. Although, then, the
notochord may in a sense be spoken of as the original supporting axial
rod of the vertebrate, it is so different to the rest of the endo-skeleton,
has so little to do with it, that the consideration of its origin is a thing
apart, and must be treated by itself without reference to the origin of
the cartilaginous and bony skeleton.

The Commencement of the Bony Skeleton in the Vertebrate.

What is the teaching of the vertebrate ? What evidence is there
as to the origin of the bony skeleton in the vertebrate phylum
itself ?

The axial bony skeleton of the higher Mammalia consists of two
parts, (1) the vertebral column with its attached bony parts, and
(2) the cranio-facial skeleton. Of these two parts, the bony tissue
of the first arises in the embryo from cartilage, of the second partly
from cartilage, partly from membrane.

In strict accordance with their embryonic origin is their phyloge-
netic origin : as we pass from the higher vertebrates to the lower
these structures can be traced back to a cartilaginous and mem-
branous condition, so that, as Parker has shown, the cranio-facial
bony skeleton of the higher vertebrates can be derived directly from
a non-bony cartilaginous skeleton, such as is seen in Petromyzon
and the cartilaginous fishes.

Balfour, in his " Comparative Embryology," states that the

THE EVIDENCE OF THE SKELETON

121

primitive cartilaginous cranium is always composed of the following
parts : —

1. A pair of cartilaginous plates on each side of the cephalic
section of the notochord known as the parachordals (pa.ch., Fig. 49 ;
iv., Fig. 48). These plates, together with the notochord (ch.) enclosed
between them, form a floor for the hind and mid-braiu.

<S^--Ctr

-au

Fig. 48. — Embryo Pig, two-thirds of an
inch long (from Parker), Elements
of Skull seen from below.

ch., notochord; iv., parachordals; au.,
auditory capsule ; py., pituitary body ; tr.,
trabecula; ctr., trabecular cornu ; pn.,
pre-nasal cartilage ; ppg., palato-pterygoid
tract; mn., mandibular arch; th.h., first
branchial arch ; VII.-XIL, cranial nerves.

Fig. 49. — -Head of Embryo Dog-fish
(from Parker), Basal View of Cranium

FROM ABOVE.

ul., olfactory sacs; au., auditory capsule;
py., pituitary body; pa.ch., parachordal
cartilage; tr., trabecula; inf., infundi-
bulum ; pt.s., pituitary space ; c, eye.

2. A pair of bars forming the floor for the fore-brain, known as
the trabecular (tr). These bars are continued forward from the para-
chordals. They meet posteriorly and embrace the front end of the
notochord, and after separating for some distance bend in again in
such a way as to enclose a space — the pituitary space (pt.s.). In

122 THE ORIGIN OF VERTEBRATES

front of this space they remain in contact, and generally unite. They
extend forward into the nasal region (pn.).

3. The cartilaginous capsules of the sense organs. Of these the
auditory {an.) and the olfactory capsules (ol.) unite more or less inti-
mately with the cranial walls ; while the optic capsules, forming the
usually cartilaginous sclerotics, remain distinct.

The parachordals and notochord form together the basilar plate,
which forms the floor for that section of the brain belonging to
the primitive postoral part of the head, and its extent corresponds
roughly to that of the basioccipital of the adult skull.

The trabecular, so far as their mere anatomical relations are con-
cerned, play the same part in forming the floor for the front cerebral
vesicle as do the parachordals for the mid- and hind-brain. They
differ, however, from the parachordals in one important feature, viz.
that except at their hinder end they do not embrace the notochord.
The notochord always terminates at the infundibulum, and the
trabecular always enclose a pituitary space, in which lies the infun-
dibulum (inf.) and the pituitary body (py.).

In the majority of the lower forms the trabecular arise quite inde-
pendently of the parachordals, though the two sets of elements soon
unite.

The trabecular are usually somewhat lyre-shaped, meeting in
front and behind, and leaving a large pituitary space between their
middle parts. Into this space the whole base of the fore-brain
primitively projects, but the space itself gradually becomes narrowed
until it usually contains only the pituitary body.

The trabecular floor of the brain does not long remain simple.
Its sides grow vertically upwards, forming a lateral wall for the
brain, in which in the higher types, two regions maybe distinguished,
viz. an alisphenoidal region behind, growing out from what is known
as the basisphenoidal region of the primitive trabecular, and an
orbito-sphenoidal region in front, growing out from the presphenoidal
region of the trabecular. These plates form at first a continuous lateral
wall of the cranium. The cartilaginous wails which grow up from the
trabecular floor of the cranium generally extend upwards so as to form
a roof, though almost always an imperfect roof, for the cranial cavity.
The basi-cranial cartilaginous skeleton reduces itself always into
trabecular and parachordals with olfactory and auditory cartilaginous
capsules. ,

THE EVIDENCE OF THE SKELETON

123

An anterior arch known as the
C3 h-v*

In addition, a branchial skeleton exists, which consists of a series
of bars known as the branchial bars, so situated as to afford support
to the successive branchial pouches,
mandibular arch (Fig. 50,
Mn.), placed in front of the
hyo-rnandibular cleft, and
a second arch, known as the
hyoid arch (Hy.), placed in
front of the hyo-branchial
cleft, are developed in all
types; the succeeding arches
are known as the true bran-
chial arches (Br.), and are
only fully developed in the
Ichthyopsida. In all cases
of jaw-bearing (gnathosto-
matous) vertebrates the first
arch has become a support-
ing skeleton for the mouth (Fig. 51), and in the higher vertebrates in
combination with the second or hyoid arch takes part in the formation
of the ear-bones.

The true branchial arches persist, to a certain extent, in the

cr

c r ~

Mn Hu Br i
Hm Na Tr

Fig. 50. — Head op Embryo Dog-fish, eleven
lines long. (From Parker.)

Tr., trabecula ; Mn., mandibular cartilage ; Hy.,
hyoid arch; -Br,., first branchial arch; Na.,
olfactory sac ; E., eye ; An., auditory capsule ;
Hm., hemisphere; C,, C 2 , C z , cerebral vesicles.

Ku'Htj

Fig. 51. — Skull op Adult Dog-pish, Side View. (From Parker.)
cr., cranium; Br., branchial arches; Mn. + Hy., mandibular and hyoid arches.

Amphibia, and become still more degenerated in the Amniota
(reptiles, birds, and mammals) in correlation with the total dis-
appearance of a branchial respiration at all periods of their life.

124 THE ORIGIN OF VERTEBRATES

Their remnants become more or less important parts of the hyoid
bone, and are employed solely in support of the tongue.

In no single animal is there any evidence that the foremost arch,
the mandibular, is a true branchial arch. As low down as the
Elasmobranchs it becomes divided into two elements which form
respectively the upper and lower jaws ; the hyoid arch, on the other
hand, although it has altered its form and acquired the secondary
function of supporting the mandibular arch, still retains its respi-
ratory function.

The evidence afforded by the mode of formation of the skeletal
tissues of vertebrates down to the Elasmobranchs indicates that the
primitive cranial skeleton arose from two paired basal cartilages, the
parachordals and trabecule, to which were attached respectively
cartilaginous cases enclosing the organs of hearing and smell. In
addition, the branchial portion of the cranial region was provided
with cartilaginous bars arranged serially for the support of the
branchiae, with the exception of the foremost, the mandibular bar,
which formed supporting tissues for the mouth — the upper and
lower jaws.

Just as in past times the spinal nerves and the segments they
supplied were supposed to represent the type on which the original
vertebrate was built, so also the spinal vertebrae afforded the type of
the segmented skeleton, and the anatomists of those days strove hard
to resolve the cranio-facial skeleton into a series of modified vertebrae.
Owing especially to the labours of Huxley, who showed that the seg-
mentation in the head-region was essentially a segmentation due to
the presence of branchial bars, this conception was finally laid to rest
and nowadays it is admitted to be hopeless to resolve the cranium
into vertebral segments. Still, however, the vertebrate is a segmented
animal and its segmented nature is visible in the cranial region, so far
as the skeletal tissues are concerned, in the shape of the series of
branchial and visceral bars.

To this segmentation the name of ' branchiomeric ' has been given,
while that due to the presence of vertebrae is called ' mesomeric'

As we have seen, the internal bony skeleton of the vertebrate
commences as a cartilaginous and membranous skeleton. For this
reason the preservation of such skeletons is impossible in the fossil
form, unless the cartilage has become impregnated with lime salts,
so that there is but little hope of ever obtaining traces of such

THE EVIDENCE OF THE SKELETON

125

structures in the fossils of the Silurian age either among the verte-
brate or invertebrate remains. Fortunately for this investigation
there are still living on the earth two representatives of that age ; on
the invertebrate side Limulus, and on the vertebrate side Ammoccetes.

The Elasmobranchs represent the most primitive of the gnatho-
stomatous vertebrates. Below them come the Agnatha, known as the
cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon)
and the hag-fishes (Myxine).

The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial
skeleton composed of a cartilaginous unsegmented cranium, with the
basal trabecule and parachordals and a series of branchial and visceral
cartilaginous bars forming the so-called branchial basket-work ; to
these must be added auditory and nasal capsules. In contradis-
tinction to this elaborate cranio-facial skeleton, the spinal vertebral

na

an

Fig. 52. — Skeleton of Petromyzox. (From Parker.)
na., nasal capsule; an., auditory capsule; nc, uotochord.

skeleton is represented only by segmen tally arranged small pieces of
cartilage formed in the connective tissue dissepiments between
segmented sheets of body-muscles (myotomes).

But Petromyzon is derived from Ammoccetes by a remarkable
process of transformation, and a most important part of that trans-
formation is the formation of new cartilaginous structures. Thus we
see that in Ammoccetes there is no sign of a cartilaginous vertebral
column ; at transformation the rudimentary vertebras of Petromyzon
are formed. In Ammoccetes the brain-case is a simple fibrous mem-
branous covering ; at transformation this becomes cartilaginous. In
Ammoccetes there are no cartilaginous structures corresponding to
the sub-ocular arches ; these are all formed at transformation. It
follows, that we can trace back the bony skeleton of the vertebrate
head to the skeleton of Ammoccetes, and we may therefore conclude

126

THE ORIGIN OF VERTEBRATES

that the primitive cartilaginous skeleton of the vertebrate consisted
of the following structures (Fig. 53, B), viz. the branchial bars

forming a basket-work, the

A

PL

Ent

\

trabecule and parachordals,
the auditory and nasal cap-
sules — a clear proof that the
cranial skeleton is older than
the spinal. Of these struc-
tures the branchial bars are
the only evidently segmented
parts.

The Soft Cartilage of the
Branchial Skeleton of
Ammoccetes.

The study of Ammoccetes
gives yet another clue to the
nature of the earliest skeleton,
for these two marked groups
of cartilage — the branchial and
basi-cranial — are characterized
by a difference in structure as
well as a difference in topo-
graphical position. J. Miiller
was the first to point out that
these two sets of cartilages
differ in appearance and con-
stitution, and he gave to them"
the name of yellow and grey
cartilage. Parker has described
them fully under the terms
soft and hard cartilage, terms
which Schaffer has also used,
and I shall also make use of
them here. The whole of the
branchial cartilaginous skele-
ton is composed of soft cartilage, while the basi-cranial skeleton, con-
sisting of trabecule, parachordals, and auditory capsule, is composed

y

Fig. 53. — Comparison of Cartilaginous
Skeleton of Limulus and Ammoccetes.

A, Diagram of cartilaginons skeleton of
Limulus. Soft cartilage, entapophysial liga-
ments, deep black ; branchial bars simply
hatched; liard cartilage, lateral trabecule
of entosternite, netted ; Ph., position of
pharynx.

B, Diagram of cartilaginous skeleton of
Ammoccetes. Soft cartilage, sub-chordal
cartilaginous bands, deep black ; branchial
basket-work (first formed part), simply
hatched ; hard cartilage, cranio-facial skele-
ton, trabecule, parachordals and auditory
capsules, netted; Inf., position of tube of
infundibulum (old oesophagus).

THE EVIDENCE OF THE SKELETON 1 27

of hard cartilage, the only soft cartilage in this region being that
which forms the nasal capsule, not represented in Fig. 53, B.

These two groups of cartilage arise independently, so that at first
the basi-cranial system is quite separate from the branchial, and only
late in the history of the animal is a junction effected between the
branchial system and the trabecular and parachordals, an initial
separation which is especially striking when we consider that in this
animal all the cartilaginous structures of any one system are con-
tinuous : there is no sign of anything in the nature of joints.

Of these two main groups, the branchial cartilages are formed first
in the embryo, a fact which suggests that they are the most primi-
tive of the vertebrate cartilages, and that, therefore, the first true
formation of cartilage in the invertebrate ancestor may be looked for in
the shape of bars supporting the branchial mechanism. The evidence
of the origin of the cartilaginous structures in Ammoccetes is given
by Shipley in the following words : —

" The branchial bases are the first part of the skeleton to appear.
They arise about the 24th day as straight bars of cartilage, lying
external and slightly posterior to the branchial vessel.

" The first traces of the basi-cranial skeleton appear on the 30th
day as two rods of cartilage — the trabecular."

Our attention must, in the first place, be directed to this branchial
basket-work of Ammoccetes.

Underlying the skin of Ammoccetes in the branchial region is
situated the sheet of longitudinal body-muscles, divided into a series
of segments or myotomes, which forms the somatic muscles so cha-
racteristic of all fishes. This muscular sheet is depicted on the left-
hand side of Fig. 54. It does not extend over the lower lip or over
that part in the middle line where the thyroid gland is situated. In
these parts a sheet of peculiar tissue known by the name of muco-
cartilage lies immediately under the skin, covering over the thyroid
gland and lower lip. The somatic muscular sheet with the super-
jacent skin can be stripped off very easily owing to the vascularity
and looseness of the tissue immediately underlying it. When this is
done the branchial basket-work comes beautifully into view as is
seen on the right-hand side of Fig. 54. It forms a cage within which
the branchiar and their muscles lie entirely concealed.

This is the great characteristic of this most primitive form of the
branchial cartilaginous bars and distinguishes it from the branchial

128

THE ORIGIN OF VERTEBRATES

bars of other higher fishes, in that it forms a system of cartilages
which lie external to the branchia) — an extra-branchial system.

This branchial basket-work is simpler in Ammoccetes than in
Petromyzon, and its actual starting-point consists of a main trans-
verse bar corresponding to each branchial
segment ; from this transverse bar the
system of longitudinal bars by which
the basket-work is formed has sprung.
These transverse bars arise from a
cartilaginous longitudinal rod, situated
close against the notochord on each
side. These rods may be called the
subchordal cartilaginous bands (Fig. 53),
and, according to the observations .of
Schneider and others, each subchordal
band does not form at first a continuous
cartilaginous rod, but the cartilage is
conspicuous only at the places where
the transverse bars arise. In the
youngest Ammoccetes examined by
Schaffer, he could find no absolute dis-
continuity of the cartilage except be-
tween the first two transverse bars, but
he says that the thinning between the
transverse bars was so marked as to
make it highly probable that at an
earlier stage there was discontinuity.
The whole system of branchial bars and
subchordal rods is at first absolutely
disconnected from the cranial system of
trabecule and parachordals, and only
later do the two systems join.

These observations on Ammoccetes
lead most definitely to the conclusion
that the starting-point of the whole cartilaginous skeleton of
the vertebrate consisted of a series of transverse cartilaginous bars,
for the purpose of supporting branchial segments ; these were con-
nected with two axial longitudinal cartilaginous rods, which at first
contained cartilage only near the places of junction of the branchial

Fig. 54. — Ventral View of
Head Region of Ammoccetes.

Th., thyroid gland; M., lower
lip, with its muscles.

THE EVIDENCE OF THE SKELETON 1 29

bars. This system may be called the mesosomatic skeleton, as it is
entirely confined to the branchial or mesosomatic region.

In addition to this primitive cartilaginous framework, which was
formed for the support of the mesosomatic or respiratory segments,
but at a slightly -later period in the phylogenetic history, a separate
cartilaginous system was formed for the support of the prosomatic
segments, viz. the trabecular and parachordals with the auditory cap-
sules : a system which was at first entirely separated from the mesoso-
matic, and, as we shall see, is more advanced in structure than the
branchial system. Later still, the story is completed at the time of
transformation to Petromyzon by the formation of the simple cartila-
ginous skull and the rudimentary vertebrae, the structure of which
is also of a more advanced type.

The Structure of the Soft Branchial Cartilage.

Having considered the topographical position of the primitive
branchial cartilaginous skeleton, we may now inquire, What was
its structure and how was it formed ?

In the higher vertebrates various forms of cartilage are described,
viz. hyaline, fibro-cartilage, elastic cartilage, and parenchymatous
cartilage. Of these, the parenchymatous cartilage is looked upon as
the most primitive form, because it preserves without modification
the characters of embryonic cartilage.

Embryology, then, would lead to the belief that the earliest form
of cartilage in the vertebrate kingdom ought to be of this type, viz.
large cells, each of which is enclosed in a simple capsule, so that the
capsules of the cells form the whole of the matrix, and thus form a
simple homogeneous honeycomb-structure, in the alveoli of w r hich
the cartilage-cells lie singly. If, then, the branchial cartilages of
Ammocoetes are, as has just been argued, the representatives of the
cartilaginous skeleton of the primitive vertebrate, it is reasonable to
suppose that they should resemble in structure this embryonic car-
tilage. Such is undoubtedly the case : all observers who have
described the branchial basket-work of Ammocoetes or Petromyzon
have been struck with the extremely primitive character of the car-
tilage, and the last observer (Schafi'er) describes it as composed of
thin walls of homogeneous material, in which there are no lines of
separation, which form a simple honeycomb-structure, in the alveoli

K

13O THE ORIGIN OF VERTEBRATES

of which the separate cells lie singly. These branchial cartilages are
each surrounded by a layer of perichondrium, and in Fig. 55, A, I
give a picture of a section of a portion of one of the bars.

A B

Fig. 55. — A, Branchial Cartilage of Ammoccetes, stained with Thionin. B,
Branchial Cartilage of Limulus, stained with Thionin.

Hence we see that structurally as well as topographically the
branchial bars of Ammoccetes justify their claim to be considered as
the origin of the vertebrate cartilaginous framework.

On the Structure of the Muco-cartilage in Ammoccetes.

We can, however, go further than this, and ask how this cartilage
itself is formed in Ammoccetes ? The answer is most definite, most
instructive and suggestive, for in all cases this particular kind of car-
tilage is formed from, or at all events in, a peculiar fibrous tissue,
which was called by Schneider " Schleim-Knorpel,''' or muco-cartilage,
a tissue which is distinguishable from other connective tissues, not
only by its structural peculiarities, but also by its strong affinity for
all dyes which differentiate mucoid or chondro-mucoid substances.

This muco-cartilage is thus described by Schneider : — The peri-
chondrium in Ammoccetes is not confined to the true cartilaginous
structures, but extends itself in the form of thin plates in definite
directions. Between these plates of perichondrium a peculiar tissue
(Fig. 56) — the muco-cartilage — exists, consisting of fibrillar, whose
direction is mainly at right angles to the planes of the perichondrial
plates, with star-shaped cells in among them, and with the spaces
between the fibrillse filled up with a semi-fluid mass.

THE EVIDENCE OF THE SKELETON 131

From this tissue all the primitive cartilages which resemble the
branchial bars are formed, either by the invasion of chondroblasts
from the surrounding perichondrium, or by the proliferation and
encapsulation of the cells of the muco- cartilage itself.

This very distinctive tissue — the muco-cartilage — is of very great
importance in all questions of the origin of the skeletal tissues. In
all descriptions of the skeletal tissues it has been practically dis-
regarded until recent years when, besides my own observations, its
distribution has been mapped out by Schaffer. Thus Parker, in his
well-known description of the skeleton of the marsipobranch fishes,
does not even mention its existence. Its importance is shown by its
absolute disappearance at
transformation and its non-
occurrence in any of the

higher vertebrates. It is \PWcH i ' Sv fH\J ■ 4 tot^'l

entirely confined to the head- ^ WU \, ) lli-r ','? f P^m m>-^

ton «■

region, and its distribution r'^'i w<*^f \P s' :

there is most suggestive, for,

scr
later on, it forms a skele

Tmmwm

ton which both in structure ,' V 1 'IfjJnJ Vf

\ S iw» - n^.^ . ., (I ' .W— i ;

)

is will be described fully *>»?$:,*/
Later on, it forms a skele-
ton which both in structure
and position resembles very

closely the head-shields of ~W~JXZ- i *& 3i ~^^' '^"
cephalaspidian fishes. At '"- — ; C " N \ (

the present part of my argu- "~-~- — J v .

ment its more immediate Pig. 56.— Section of Muco-cartilage from
interest lies in the method Dorsal Head-plate of Ammoccetes.

of tracing this tissue. For

this purpose I made use of the micro-chemical reaction of thionin,
a dye which, as shown by Hoyer, stains all mucin-containing sub-
stances a bright purple. Schaffer made use of a corresponding
basophil stain, hsemalum. When stained with thionin, the matrix,
or ground-substance of the branchial cartilages as well as the matrix
or semi-fluid substance in which the fibrils of the muco-cartilaginous
cells are embedded take on a deep purple colour, while the fibrous
material of the cranial walls and other connective tissue strands, such
as the perichondrium, are coloured light blue. Muco-cartilage, then,
may be described as a peculiar form of connective tissue which
differs from other connective tissue not only in its appearance but in

132 THE ORIGIN OF VERTEBRATES

its chemical composition, for unlike white fibrous tissue it contains a
large amount of mucin, and this tissue is the forerunner of the earliest
cartilaginous vertebrate skeleton, the branchial bars of Amnioccetes.

The conclusions to which we are led by the study of the structure,
position, and mode of origin of these primitive cartilages of
Ammoccetes may be thus summed up : — -

1. The immediate ancestor of the vertebrate must have possessed
a peculiar fibrous tissue — the ground-substance of which stained deep
purple with thionin — in which cartilage arose.

2. The cartilage so formed was not like hyaline cartilage, but
resembled in a striking manner parenchymatous cartilage.

3. This cartilage was situated partly in two axial longitudinal
bands, partly as transverse bars, which supported the branchial
apparatus.

The Prosomatic or Basi-cranial Skeleton of Ammoccetes.

Before searching for any evidence of a similar tissue in any
invertebrate group, it is advisable to consider the other portion of
the cartilaginous skeleton of Ammoccetes, which consists of the tra-
becular, parachordals and auditory capsules — the basi-cranial skeleton
— and is composed of hard, not soft cartilage.

This basi-cranial skeleton represented in Fig. 53, B, is confined to
the region of the notochord, the cranial walls being composed entirely
of a white fibrous membrane. It is separated at first entirely from
the sub-chordal portion of the branchial basket-work, and is com-
posed of a foremost part, the trabecular (Tr.), and of a hindermost
part, the parachordals (Pr.ch.), which are characterized by the
attachment on each side of the large auditory capsule {Au.). In
Ammoccetes the trabecular bars are continuous with the parachordals,
the junction being marked by a small lateral projection on each side,
which at transformation is seen to play an important part in the
formation of the sub-ocular arch. The trabecular bar lies close
against the notochord on each side up to its termination ; it then
bends away from the middle line and curves round until it meets
its fellow on the opposite side, thus forming, as it were, the head of
a racquet of which the notochord forms the splice in the handle.
The strings of the racquet are represented by a thin membrane, in
the centre of which the position of the infundibulum {Inf.) of the

THE EVIDENCE OF THE SKELETON

13;

brain can be clearly seen. In an earlier stage of Ammoccetes the
two trabecular horns do not meet, but are separated by connective
tissue, which afterwards becomes cartilaginous.

As far, then, as the topography of this basi-cranial skeleton is
concerned, the striking points are — the shape of the trabecular
portion, diverging as it does around the infundibulum, and the pre-
sence on the parachordal portion of the two large auditory capsules.

These two points indicate, on the hypothesis that infundibulum
and oesophagus are convertible terms, that two supporting structures
of a cartilaginous nature must have existed in the ancestor of the
vertebrate, the first of which surrounded the oesophagus, and the
second was in connection with its auditory apparatus.

Structure of the Hard Cartilages.

The structure of this hard cartilage of the trabecular and auditory
capsules resembles that of the soft, in so far that it consists of large

A

Fig. 57. — A, Cartilage op Trabecule op Ammoccetes, stained with Hema-
toxylin and Picric Acid. B, Nests op Cartilage Cells in Entosternite
of Hypoctonus, stained with Hematoxylin and Picric Acid.

cells with a comparatively small amount of intercellular substance.
Schaffer, who has described it lately, considers that it is a nearer
approach to hyaline cartilage than the soft, but yet cannot be called
hyaline cartilage in the usual sense of the term. Its peculiarities
and its differences from the soft are especially well seen by its
staining reactions. I have myself been particularly struck with
the effect of picrocarmine or combined hseniatoxylin and picric acid

134 THE ORIGIN OF VERTEBRATES

staining (Fig. 57). In the case of the soft cartilage the capsular
substance stains respectively a brilliant red or blue, while that of
the hard cartilage is coloured a deep yellow, so that the junction
between the parachordals and the branchial cartilages is beautifully
marked out. Then, again, with thionin, which gives so marked a
reaction in the case of the soft cartilage, the hard cartilage of the
auditory capsule is not stained at all, and in the trabecule the deep
purple colour is confined to the mucoid cement-substance between
the capsules, just as Schaffer has stated. The same kinds of reactions
have been described by Schaffer: thus by double staining with
hrenialum-eosin the hard cartilage stains red, the soft blue ; and he
points out that even with over-staining by haemalum the auditory
capsule remains colourless, just as I have noticed with thionin. He
infers, precisely as I have done from the thionin reaction, that
chondro-mucoid, which is so marked a constituent of the soft cartilage
and of the muco-cartilage, is absent or present in but slight quantities
in the hard cartilage. Similarly, he points out that double staining
with tropceolin-methyl- violet stains the hard cartilage a bright orange
colour, and the soft cartilage a violet.

The evidence, then, shows clearly that a marked chemical differ-
ence exists between these two cartilages, which may be expressed by
saying that the one contains very largely a basophil substance,
which we may speak of as belonging to the class of chondro-mucoid
substances, while the other contains mainly an oxyphil substance,
probably a chondro-gelatine substance.

We may perhaps go further and attribute this difference of
composition to a difference of origin ; for whereas the soft cartilage
is invariably formed in a special tissue, the muco-cartilage, which
shows by its reaction how largely it is composed of a mucoid sub-
stance, the hard cartilage is certainly, in the case of the cartilage of
the cranium where its origin has been clearly made out, formed in
the membranous tissue of the cranium of Ammoccetes — i.e. in a
tissue which stains light blue with thionin, and contains a gelatinous
rather than a mucoid substratum.

The best opportunity of finding out the mode of origin of the
hard cartilage is afforded at the time of transformation, when so
much of this kind of cartilage is formed anew. Unfortunately, it
is very difficult to obtain the early transformation stages, conse-
cpuently we cannot be said to possess any really exhaustive and

THE EVIDENCE OF THE SKELETON 135

definite account of how the new cartilages are formed. Bujor,
Kaensche, and Schaffer all profess to give a more or less definite
account of their formation, and the one striking impression left on
the mind of the reader is how their descriptions vary. In one
point only are they agreed, and in that I also agree with them, viz.
the manner in which the new cranial walls are formed. Schaffer
describes the process as the invasion of chondroblasts into the
homogeneous fibrous tissue of the cranial walls. Such chondro-
blasts not only form the cartilaginous framework, but also assimilate
the fibrous tissue which they invade, so that finally all that remains
of the original fibrous matrix in which the cartilage was formed are
these lines of cement-substance between the groups of cartilage
cells, which, containing some basophil material, are marked out, as
already mentioned (Fig. 57).

We may therefore conclude, from the investigation of Ammoccetes,
that the front part of the basi-cranial skeleton arose as two trabecular
bars, to which muscles were attached, situated bilaterally with respect
to the central nervous system. These bars were composed of tendinous
material with a gelatinous rather than a mucoid substratum, in which
nests of cartilage- cells were formed, the cartilaginous material formed
by these cells being of the hard variety, not staining with thionin,
and staining yellow with picro-carmine, etc. By the increase of such
nests and the assimilation of the intermediate fibrous material, the
original fibro-cartilage was converted into the close-set semi-hyaline
cartilage of the trabecular and auditory capsules, in which the fibrous
material still marks out by its staining-reaction the limits of the
cell-clusters.

Such I gather to be Schaffer's conclusions, and they are certainly
borne out by my own and Miss Alcock's observations. As far as
we have had an opportunity of observing at present, the first process
at transformation appears to consist of the invasion of the fibrous
tissue of the cranial wall by groups of cells which form nests of cells
between the fibrous strands. These nests of cells form round them-
selves capsular material, and thus form cell-territories of cartilage,
which squeeze out and assimilate the surrounding fibrous tissue, until
at last all that remains of the original fibrous matrix is the lines of
cement-substance which mark out the limits of the various cell-groups.

At present I am inclined to think that both soft and hard cartilage
originate in a very similar manner, viz. by the formation of capsular

136 THE ORIGIN OF VERTEBRATES

material around the invading chondroblasts, and that the difference
in the resulting cartilage is mainly due to the difference in chemical
composition of the matrix of the connective tissue which is invaded.
Thus the difference may be formulated as follows : —

The hard cartilage is formed by the invasion of chondroblasts
into a fibrous tissue, which contains a gelatinous rather than a mucoid
substratum, in contradistinction to the soft cartilage which is formed,
probably also by the invasion of chondroblasts, in a tissue — the
muco-cartilage — which contains a specially mucoid substratum.

Such, then, is the very clearly defined starting-point of the ver-
tebrate skeleton — two distinct formations of different histological
and chemical structure,— the one forming a segmented branchial
skeleton, the other a non-segmented basi-cranial skeleton.

The Cartilaginous Skeleton of Limultjs.

Among the whole of the invertebrates at present living on the
earth, is there any sign of an internal cartilaginous skeleton that
will give a direct clue to the origin of the primitive vertebrate
skeleton ? The answer to this question is most significant : only
one animal among all those at present known possesses a cartilaginous
skeleton, which is directly comparable with that of Ammocoetes, and
here the comparison is very close — only one animal among the
thousands of living invertebrate forms, and that animal is the only
representative still surviving of the palseostracan group, which was
the dominant race when the vertebrate first made its appearance.
The Limulus, or king-crab, possesses a segmented branchial internal
cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage
as the branchial skeleton of Ammocoetes, confined to the mesosomatic
or branchial region, just as in Ammocoetes, forming, as in Ammoccetes,
cartilaginous bars supporting the branchiae, and these bars are situated
externally to the branchiae, as in Ammocoetes. In addition this
animal possesses a basi-cranial internal semi-cartilaginous unseg-
mented plate known as the entosternite or plastron situated, with
respect to the oesophagus, similarly to the position of the trabecular
with respect to the infundibulum in Ammocoetes. Moreover, the
cartilaginous cells in this tissue differ from those in the branchial
region, in precisely the same manner as the hard cartilage differs from
the soft in Ammoccetes.

THE EVIDENCE OF THE SKELETON 1 37

This plastron, it is true, is found in other animals, all of which
are members of the scorpion tribe, except in one instance, and this,
strikingly enough, is the crustacean Apus — a strange primitive form,
which is acknowledged to be the nearest representative of the
Trilobita still living on the earth. None of these forms, however,
possess any sign of an internal cartilaginous branchial skeleton,
such as is possessed by Limulus. Scorpions, Apus, Limulus, are
all surviving types of the stage of organization which had been
reached in the animal world when the vertebrate first appeared.

The Mesosomatic oe Eespiratory Skeleton of Limulus, composed

of Soft Cartilage.

Searching through the literature of the histology of the cartila-
ginous tissues in invertebrate animals, to see whether any cartilage
had been described similar to that seen in the branchial cartilages of
Ammoccetes, and whether such cartilage, if found, arose in a fibrous
tissue resembling muco-cartilage, I was speedily rewarded by finding,
in Ray Lankester's article on the tropho- skeletal tissues of Limulus,
a picture of the cartilage of Limulus, which would have passed muster
for a drawing of the branchial cartilage of Ammoccetes. This clue
I followed out in the manner described in my former paper in the
Journal of Anatomy and Physiology, and mapped out the topography
of this remarkable tissue.

Limulus, like other water-dwelling arthropods, breathes by means
of gills attached to its appendages. These gill-bearing appendages
are confined to the mesosomatic region, as is seen in Fig. 59 ; and these
appendages are very different to the ordinary locomotor appendages,
which are confined to the prosomatic region. Each appendage, as is
seen in Fig. 58, consists mainly of a broad, basal part, which carries
the gill-book on its under surface ; the distal parts of the appendage
have dwindled to mere rudiments and still exist, not for locomotor
purposes, but because they carry on each segment organs of special
importance to the animal (see Chapter XL). As is seen in Fig. 58,
the basal parts of each pair of appendages form a broad, flattened
paddle, by means of which the animal is able to swim in a clumsy
fashion. Very striking and suggestive is the difference between
these gill-bearing mesosomatic appendages and the non-gill-bearing
locomotor appendages of the prosoina.

13^

THE ORIGIN OF VERTEBRATES

At the base of each of these appendages, where it is attached to
the body of the animal, the external chitinous surface is characterized

B

N. E / U LMS.

Fig. 58. — Transverse Section through the Mesosoma op Limulus, to show
the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or
Branchial Appendage.

In each figure the branchial cartilaginous bar, Br.C, has been exposed by dissection
on one side. Ent., entapophysis ; Ent.l., entapophysial ligament cut across;
Br.C, branchial cartilaginous bar, which springs from the entapophysis ; H.,
heart; P., pericardium; Al., alimentary canal; N., nerve cord; L. V.S., longi-
tudinal venous sinus ; Dv., dorso-vencral somatic muscle; Vp., veno-pericardial
muscle.

by a peculiar stumpy, rod-like marking, and upon removing the
chitinous covering, this surface-appearance is seen to correspond to a
well-marked rod of cartilage (Br.C), which extends from the body

THE EVIDENCE OF THE SKELETON 1 39

of the animal well into each appendage. This bar of cartilage arises
on each side from the corresponding entapophysis (Ent.), which is
the name given to a chitinous spur which projects a short distance
(Fig. 58, B) into the animal from the dorsal side, for the purpose of
giving attachment to various segmental muscles. These entapophyses
are formed by an invagination of the chitinous surface on the dorsal
side and are confined to the mesosomatic region, so that the meso-
somatic carapace indicates, by the number of entapophyses, the
number of segments in that region, in contradistinction to the pro-
somatic carapace, which gives no indication on its surface of the
number of its components.

Each entapophysis is hollow and its walls are composed of chitin ;
but from the apex of each spur there stretches from spur to spur
a band of tissue, called by Lankester the entapophysial ligament
(Ent. I.) (Fig. 58), and in this tissue cartilage is formed. Isolated
cartilaginous cells, or rather groups of cells, are found here and there,
but a concentration of such groups always takes place at each enta-
pophysis, forming here a solid mass of cartilage, from which the
massive cartilaginous bar of each branchial appendage arises.

Further, not only is this cartilage exactly similar to parenchy-
matous cartilage, as it occurs in the branchial cartilages of Ammoccetes,
but also its matrix stains a brilliant purple with thionin in striking-
contrast to the exceedingly slight light-blue colour of the surrounding
perichondrium. In its chemical composition it shows, as might be
expected, that it is a cartilage containing a very large amount of
some mucin-body.

The Muco- cartilage of Limulus.

The resemblance between this structure and that of the branchial
bars of Ammoccetes does not end even here, for, as already mentioned,
the cartilage originates in a peculiar connective tissue band, the
entapophysial ligament, and this tissue bears the same relation in
its chemical reactions to the ordinary connective tissue of Limulus,
as muco-cartilage does to the white fibrous tissue of Ammocu'tes.
The white connective tissue of Limulus, as already stated, resembles
that of the vertebrate more than does the connective tissue of any
other invertebrate, and, similarly to that of Ammocn'tes, does not
stain, or gives only a light-blue tinge with thionin. The tissue of

140

THE ORIGIN OF VERTEBRATES

the entapophysial ligament, on the contrary, just like muco-cartilage,
takes on an intense purple colour when stained with thionin. It
possesses a mucoid substratum, just as does muco-cartilage, and in
both cases a perfectly similar soft cartilage is born from it.

One difference, however, exists between the branchial cartilages of
these two animals ; the innermost axial layer of the branchial bar of

Fig. 59. — Diagram of Limulus, to show the Nerves to the Appendages (1-13)

and the Branchial Cartilages.

The branchial cartilages and the entapophysial ligaments are coloured blue, the
branchise red. gl., generative and hepatic glands surrounding the central nervous
system and passing into the base of the flabellum (fl.).

Limulus is very apt to contain a specially hard substance, apparently
chalky in nature, so that it breaks up in sections, and gives the
appearance of a broken-down spongy mass ; if, however, the tissue is
first placed in a solution of hydrochloric acid, it then cuts easily, and
the whole tissue is seen to be of the same structure throughout, the
main difference being that the capsular spaces in the axial region
are much larger and much*more free from cell-protoplasm than are
those of the smaller younger cells near the periphery.

THE EVIDENCE OF THE SKELETON

HI

I have attempted in Fig. 53 to represent this close resemblance
between the segmented branchial skeleton of Limulus and of Ammo-
cojtes, a resemblance so close as to reach even to minute details,
such as the thinning out of the cartilage in the subchordal bands and

Fig. 60. — Diagram of Ammoccetes cut open to show the Lateral System of
Cranial Nerves V., VII., IX., X., and the Branchial Cartilages.

The branchial cartilages and sub-chordal ligaments are coloured blue, the branchhe
red. (jl., glandular substance surrounding the central nervous system and pass-
ing into the auditory capsule with the auditory nerve (VIII).

entapophysial ligaments respectively between the places where the
branchial bars come off.

In Fig. 59 I have shown the prosoma and mesosoma of Limulus,
and indicated the nerves to the appendages together with the meso-
somatic cartilaginous skeleton.

In Fig. 60 I have drawn a corresponding picture of the prosomatic
and mesosomatic region of Aniuioccetes with the corresponding nerves

142 THE ORIGIN OF VERTEBRATES

and cartilages. In this figure the animal is supposed to be slit open
along the ventral mid-line and the central nervous system exposed.

The Prosomatic Skeleton of Limulus, composed of Hard

Cartilage.

The rest of the primitive vertebrate skeleton arose in the proso-
matic region, and formed a support for the base of the brain. This
skeleton was composed of hard cartilage, and arose in white fibrous
tissue containing gelatin rather than mucin.

Is there, then, any peculiar tissue of a cartilaginous nature in
Limulus and its allies, situated in the prosomatic region, which is
entirely separate from the branchial cartilaginous skeleton, which
acts as a supporting internal framework, and contains a gelatinous
rather than a mucoid substratum ?

It is a striking fact, common to the whole of the group of animals
to which our inquiries, deduced from the consideration of the structure
of Ammocoetes, have, in every case, led us in our search for the verte-
brate ancestor, that they do possess a remarkable internal semi-carti-
laginous skeleton in the prosomatic region, called the entosternite or
plastron, which gives support to a large number of the muscles of
that region ; which is entirely independent of the branchial skeleton,
and differs markedly in its chemical reactions from that cartilage, in
that it contains a gelatinous rather than a mucoid substratum.

In Limulus it is a large, tough, median plate, fibrous in character,
in which are situated rows and nests of cartilage-cells. The same
structure is seen in the plastron of Hypoctonus, of Thelyphonus,
and to a certainty in all the members of the scorpion group. Very
different is the behaviour of this tissue to staining from that of the
branchial region. No part of the plastron stains purple with
thionin ; it hardly stains at all, or gives only a very slight blue
colour. In its chemical composition there is a marked preponder-
ance of gelatin with only a slight amount of a mucin-body. In
some cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules
of the cartilage-cells stain a deep yellow with ha^matoxylin and
picric acid, while the fibres between the cell-nests stain a blue-brown
colour, partly from the ha?matoxylin, partly from the picric acid.

All the evidence points to the plastron as resembling the basi-
cranial skeleton of Ammocoetes in its composition and in the origin

THE EVIDENCE OF THE SKELETON

14;

of its cells in a white fibrous tissue. What, then, is its topographical
position ? It is in all cases a median structure lying between the
cephalic stomach and the infra-< esophageal portion of the central
nervous system, and in all cases it possesses two anterior horns which
pass around the cesophagus and the nerve-masses which immediately
enclose the (esophagus (Fig. 61, A). These lateral horns, then,
which lie laterally and slightly ventral to the central nervous
system, and are called by Bay Lankester and Benham the sub-
neural portion of the entosternite, are very nearly in exactly the
position of the raccpuet- shaped head of the trabecuhe in Arnnioccetes.
It is easy to see that, with a more extensive growth of the nervous
material dorsally, such lateral
horns might be caused to take
up a still more ventral posi-
tion. Now, these two lateral
horns of the plastron of Li-
mulus are continued along
its whole length so as to form
two thickened lateral ridges,
which are conspicuous on the
flat surface of the rest of this
median plate. In other cases,
as in the Thelyphonida?, the
plastron consists mainly of
these two lateral ridges or
trabecuhe, as they might be
called, and Schimkewitsch,
who more than any one else has made a comparative study of the
entosternite, describes it as composed in these animals of two lateral
trabecular crossed by three transverse trabecule. I myself can con-
firm his description, and give in Fig. 61, B, the appearance of the
entosternite of Thelyphonus or of Hypoctonus. The supra-cesophageal
ganglia and part of the infra-cesophageal ganglia fill up the space Ph. ;
stretching over the rest of the infra-cesophageal mass is a transverse
trabecula, which is very thin ; then comes a space in which is seen
the rest of the infra-cesophageal mass, and then the posterior part of
the plastron, ventrally to which lies the commencement of the ventral
nerve-cord.

In these forms, in which the central nervous system is more

Fig. 61. — A, Entosternite of Limulus ;
B, Entosternite of Theta'phonus.

Ph., position of pharynx.

144 THE ORIGIN OF VERTEBRATES

concentrated towards the cephalic end than in Liniulus, the whole of
the concentrated brain-mass is separated from the gut only by this thin
transverse band of tissue. Judging, then, from the entosternite of
Thelyphonus, it is not difficult to suppose that a continuation of the
same growth of the brain-region of the central nervous system would
cause the entosternite to be separated into two lateral trabecular,
which would then take up the ventro-lateral position of the two
trabecular of Ammoccetes.

On the other hand, it might be that two lateral trabecular,
similar to those of Thelyphonus and situated on each side of the
central nervous system, were the original form from which, by the
addition of transverse fibres running between the gut and nervous
system, the entosternite of Thelyphonus and of the scorpions, etc.,
was formed. From an extensive consideration of the entosternite in
different animals, Schimkewitsch has come to the conclusion that this
latter explanation is the true one. He points out that the lateral
trabecules can be distinguished from the transverse by their structure,
being much more cellular and less fibrous, and the cell- cavities more
rounded, or, as I should express it, the two lateral trabecular are more
cartilaginous, while the transverse are more fibrous. Schimkewitsch,
from observations of structure and from embryological investi-
gations, comes to the conclusion that the entosternite was originally
composed of two parts —

1. A transverse muscle corresponding to the adductor muscle of
the shell of certain crustaceans, such as Nebalia.

2. A pair of longitudinal mesodermic tendons, which may have
been formed originally out of a number of segmen tally arranged
mesodermic tendons, and are crossed by the fibrils of the transverse
muscular bundles.

These paired tendons of the entosternite he considers to corre-
spond to the intermuscular tendons, situated lengthways, which are
found in the ventral longitudinal muscles of most arthropods.

It is clear from these observations of Schimkewitsch, that the
essential part of the entosternite consists of two lateral trabecular,
which were originally tendinous in nature and have become of the
nature of cartilaginous tissue by the increase of cellular elements in
the matrix of the tissue : these two trabecular function as supports
for the attachment of muscles, which are specially attached at
certain places. At these places transverse fibres belonging to some

THE EVIDENCE OF THE SKELETON 145

of the muscular attachments cross between the two longitudinal
trabecular, and so form the transverse trabecule.

I entirely agree with Schimkewitsch that the nests of cartilage-
cells are much more extensive in, and indeed nearly entirely
confined to, these two lateral trabecular in the entostemite of
Hypoctonus. Kay Lankester describes in the entostemite of Mygale
peculiar cell-nests strongly resembling those of Hypoctonus, and he
also states that they are confined to the lateral portions of the
entostemite.

From this evidence it is easy to see that that portion of the basi-
cranial skeleton known as the trabecular may have originated from
the formation of cartilage in the plastron or entostemite of a pake-
ostracan animal. Such an hypothesis immediately suggests valuable
clues as to the origin of the cranium and of the rest of the basi-
cranial skeleton — the parachordals and the auditory capsules. The
former would naturally be a dorsal extension of the more membranous
portion of the plastron, in which, equally naturally, cartilaginous tissue
would subsequently develop ; and the reason why it is impossible to
reduce the cranium into a series of segments would be self-evident,
for even though, as Schimkewitsch thinks, the plastron may have
been originally segmented, it has long lost all sign of segmentation.
The latter would be derived from a second entostemite of the same
nature as the plastron, but especially connected with the auditory
apparatus of the invertebrate ancestor. The following out of these
two clues will be the subject of a future chapter.

In our search, then, for a clue to the origin of the skeletal tissues
of the vertebrate we see again that we are led directly to the palaros-
tracan stock on the invertebrate side and to the Cyclostomata on that
of the vertebrate ; for in Limulus, the only living representative of
the Palaeostraca, and in Limulus alone, we find a skeleton marvel-
lously similar to the earliest vertebrate skeleton — that found in
Ammocoetes. Later on I shall give reasons for the belief that the
earliest fishes so far found, the Cephalaspidae, etc., were built up on
the same plan as Ammocoetes, so that, in my opinion, in Limulus
and in Ammocoetes we actually possess living examples allied to
the ancient fauna of the Silurian times.

146 THE ORIGIN OF VERTEBRATES

Summary.

The skeleton considered in this chapter is not the notochord, but that
composed of cartilage. The tracing- downwards of the vertebrate bony and
cartilaginous skeleton to its earliest beginnings leads straight to the skeleton of
the larval lamprey (Amnioccetes), in which vertebrae are not yet formed, but the
cranial and branchial skeleton is well marked.

The embryologies! and phylogenetic histories are in complete unison to show
that the cranial skeleton is older than the spinal, and this primitive branchial
skeleton is also in harmony with the laws of evolution, in that its structure, even
in the adult lamprey (Petromyzon). never gets beyond the stage characteristic
of embryonic cartilage in the higher vertebrates.

The simplest and most primitive skeleton is that found in Animoccetes and
consists of two parts : (1) a prosomatic, (2) a mesosomatic skeleton.

The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the
simplest kind — the trabecular and the parachordals with their attached auditory
capsules, just as the embryology of the higher vertebrates teaches us must be
the case. There in the free-living, still-existent Ammoccetes we find the manifest
natural outcome of the embryological history in the shape of simple trabecular
and parachordals, from which the whole complicated basi-cranial skeleton of the
higher vertebrates arose.

The mesosomatic skeleton, which is formed before the prosoniatic, consisted,
in the first instance, of simple branchial bars segmentally arranged, which were
connected together by a longitudinal subchordal bar. situated laterally on each
side of the notochord. These simple branchial bars later on form the branchial
basket-work, which forms an open-work cage within which the branchiae are
situated.

The cartilages which compose these two skeletons respectively are markedly
different in chemical constitution, in that the first (hard cartilag'e) is mainly
composed of chondro-gelatin, the second (soft cartilage) of chondro-mucoid
material.

The same kind of difference is seen in the two kinds of connective tissue
which are the forerunners of these two kinds of cartilage. Thus, the cranial
walls in Ammoccetes are formed of white fibrous tissue, an essentially gelatin-
containing tissue ; at transformation these are invaded by chondro-blasts and
the cartilaginous cranium, formed of hard cartilage, results. On the other hand,
the forerunner of the branchial soft cartilage is a very striking and peculiar
kind of connective tissue loaded with mucoid material, to which the name
muco-cartilage has been given.

The enormous interest of this muco-cartilage consists in the fact that it
forms very well-defined plates of tissue, entirely confined to the head-region,
wliich are not found in any higher vertebrate, not even in the adult form
Petromyzon, for every scrap of the tissue as such disappears at transformation.

It is this evidence of primitive non-vertebrate tissues, which occur in the
larval but not in the adult form, which makes Ammoccetes so valuable for the
investigation of the origin of vertebrates.

The evidence, then, is extraordinarily clear as to the beginnings of the
vertebrate skeletal tissues.

THE EVIDENCE OF THE SKELETON 1 47

In the invertebrate kingdom true cartilage occurs but scantily. There is
a cartilaginous covering of the brain of cepkalopods. It is never found in crabs,
lobsters, bees, wasps, centipedes, butterflies, flies, or any of the great group of
Arthropoda, except, to a slight extent, in some members of the scorpion group,
aud more fully in one single animal, the King-crab or Limulus : a fact significant
of itself, but still more so when the nature of the cartilage and its position in
the animal is taken into consideration, for the identity both in structure and
position of this internal cartilaginous skeleton with that of Anmiocoetes is
extraordinarily g-reat.

Here, in Limulus. just as in Aminoccetes, an internal cartilaginous skeleton
is found, composed of two distinct parts : (1) prosomatic, (2) mesosomatic. As
in Ammocoetes, the latter consists of simple branchial bars, segmentally arranged,
which are connected together on each side by a longitudinal lig'ament contain-
ing cartilage — the entapophysial ligament. This cartilage is identical in
structure and in chemical composition with the soft cartilage of Ammocoetes,
and. as in the latter case, arises in a markedly mucoid connective tissue.
The former, as in Ammocoetes, consists of a non-segmental skeleton, the
plastron, composed of a white fibrous connective tissue matrix, an essentially
gelatin-containing tissue, in which are found nests of cartilage cells of the
hard cartilage variety.

This remarkable discovery of the branchial cartilaginous bars of Limulus,
together with that of the internal prosomatic plastron, causes the original diffi-
culty of deriving an animal such as the vertebrate from an animal resembling"
an arthropod to vanish into thin air, for it shows that in the past ages when the
vertebrates first appeared on the earth, the dominant arthropod race at that time,
the members of which resembled Limulus, had solved the question ; for, in addition
to their external chitinous covering, they had manufactured an internal cartila-
ginous skeleton. Not only so, but that skeleton had arrived, both in structure
and position, exactly at the stage at which the vertebrate skeleton starts.

What the precise steps are by which chitin-f ormation gives place to chondrin-
formation are not yet fully known, but Schmiedeberg has shown that a substance,
glycosamine, is derivable from both these skeletal tissues, and he concludes his
observations in the following words: ''Thus, by means of glycosamine, the
bridge is formed which connects together the chitin of the lower animals with
the cartilage of the more highly organized creations."

The evidence of the origin of the cartilaginous skeleton of the vertebrate
points directly to the origin of the vertebrate from the Palfeostraca, and is
of so' strong a character that, taken alone, it may almost be considered as proof
of such origin.

CHAPTEK IV

THE EVIDENCE OF THE RESPIRATORY APPARATUS

Branchiae considei*ed as internal branchial appendages. — Innervation of branchial
segments. — Cranial region older than spinal. — Three-root system of cranial
nerves, dorsal, lateral, ventral. — Explanation of van Wijhe's segments. —
Lateral mixed root is appendage-nerve of invertebrate. — The branchial
chamber of Ammocoetes. — The branchial unit, not a pouch but an
appendage. — The origin of the branchial musculature. — The branchial
circulation. — The branchial heart of the vertebrate. — Not homologous with
the systemic heart of the arthropod. — Its formation from two longitudinal
venous sinuses. — Summary.

The respiratory apparatus in all the terrestrial vertebrates is of the
same kind — one single pair of lungs. These lungs originate as a
diverticulum of the alimentary canal. On the other hand, the
aquatic vertebrates breathe by means of a series of branchiae, or gills,
which are arranged segmentally, being supported by the segmental
branchial cartilaginous bars, as already mentioned in the last chapter.
The transition from the gill-bearing to the lung-bearing vertebrates
is most interesting, for it has been proved that the lungs are formed
by the modification of the swim-bladder of fishes ; and in a group
of fishes, the Dipnoi, or lung-fishes, of which three representatives
still exist on the earth, the mode of transition from the -fish to the
amphibian is plainly visible, for they possess both lungs and
gills, and yet are not amphibians, but true fishes. But for the
fortunate existence of Ceratodus in Australia, Lepidosiren in South
America, and Protopterus in Africa, it would have been impossible
from the fossil remains to have asserted that any fish had ever
existed which possessed at the same moment of time the two kinds
of respiratory organs, although from our knowledge of the develop-
ment of the amphibian we might have felt sure that such a transitional
stage must have existed. Unfortunately, there is at present no
likelihood of any corresponding transitional stage being discovered

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 49

living on the earth in which both the dorsal arthropod alimentary
canal and the ventral vertebrate one should simultaneously exist in
a functional condition ; still it seems to me that even if Ceratodus,
Lepidosiren, and Protopterus had ceased to exist on the earth, yet
the facts of comparative anatomy, together with our conception of
evolution as portrayed in the theory of natural selection, would have
forced us to conclude rightly that the amphibian stage in the evolu-
tion of the vertebrate phylum was preceded by fishes which possessed
simultaneously lungs and gills.

In the preceding chapter the primitive cartilaginous vertebrate
skeleton, as found in Ammoccetes, was shown to correspond in a
marvellous manner to the cartilaginous skeleton of Limulus. In a
later chapter I will deal with the formation of the cranium from the
prosomatic skeleton ; in this chapter it is the mesosomatic skeleton
which is of interest, and the consideration of the necessary conse-
quences which logically follow upon the supposition that the branchial
cartilaginous bars of Limulus are homologous with the branchial
basket-work of Ammoccetes.

Internal Branchial Appendages.

Seeing that in both cases the cartilaginous bars of Limulus and
Ammoccetes are confined to the branchial region, their homology of
necessity implies an homology of the two branchial regions, and leads
directly to the conclusion that the branchiae of the vertebrate were
derived from the branchiae of the arthropod, a conclusion which,
according to the generally accepted view of the origin of the respira-
tory region in the vertebrate, is extremely difficult to accept ; for the
branchial of Limulus and of the Arthropoda in general are part of
the mesosomatic appendages, while the branchiae of vertebrates are
derived from the anterior part of the alimentary canal. This con-
clusion, therefore, implies that the vertebrate lias utilized in the
formation of the anterior portion of its new alimentary canal the
branchial appendages of the palasostracan ancestor.

Let us consider dispassionately whether such a suggestion is a priori
so impossible as it at first appears. One of the principles of evolution
is that any change which is supposed to have taken place in the
process of formation of one animal or group of animals from a lower
group must be in harmony with changes which are known to have

i5o

THE ORIGIN OF VERTEBRATES

occurred in that lower group. On the assumption, therefore, that
the vertebrate branchiae represent the branchial portion of the
arthropod mesosomatic appendages which have sunk in and so
become internal, we ought to find that in members of this very
group such inclusion of branchial appendages has taken place. This,
indeed, is exactly what we do find, for in all the scorpion tribe, which
is acknowledged to be closely related to Limulus, there are no

external mesosomatic appendages,
but in all cases these appendages
have sunk into the body, have
disappeared as such, and retained
only the vital part of them — the
branchiae. In this way the so-called
lung - books of the scorpion are
formed, which are in all respects
homologous with the branchiae or
gill-books of Limulus. Now, as
already mentioned, the lords of
creation in the palseostracan times
were the sea-scorpions, which, as is
seen in Fig. 62, resembled the land-
scorpions of the present day in the
entire absence of any external ap-
pendages on the segments of the
mesosomatic region. As they lived
in the sea, they must have breathed
with gills, and those branchial ap-
pendages must have been internal,
just as in the land-scorpions of the
present time. Indeed, markings
have been found on the internal
side of the segments 1-5, Fig. 62, which are supposed to indicate
branchiffi, and these segments are therefore supposed to have borne
the branchire. Up to the present time no indication of gill-slits
has been found, and we cannot say with certainty how these
animals breathed. Further, in the Upper Silurian of Lesmahago,
Lanarkshire, a scorpion (Palccoijhonus Hunteri), closely resembling the
modern scorpion, has been found, which, as Lankester states, was in
all probability aquatic, and not terrestrial in its habits. How it

Fig. 62. — Eurypterus.

The segments and appendages on the
right are numbered in correspon-
dence with the cranial system of
lateral nerve-roots as found in verte-
brates. lf.,metastoma. The surface
ornamentation is represented on
the first segment posterior to the
branchial segments. The opercular
appendage is marked out by dots.

THE EVIDENCE OF THE RESPIRATORY APPARATUS 15 I

breathed is unknown ; it shows no signs of stigmata, such as exist in
the scorpion of to-day.

Although we possess as yet no certain knowledge of the position
of the gill-openings in these ancient scorpion-like forms, what we
can say with certainty — and that is the important fact — is, that at
the time when the vertebrates appeared, a very large number of the
dominant arthropod race possessed internally-situated branchife, which
had been directly derived from the branchiae-bearing appendages of
their Limulus-like kinsfolk.

This abolition of the branchiie-bearing appendages as external
organs of locomotion, with the retention of the important branchial
portion of the appendage as internal branchiae, is a very important
suggestion in any discussion of the way vertebrates have arisen from
arthropods; for, if the same principle is of universal application, it
leads directly to the conclusion that whenever an appendage possesses
an organ of vital importance to the animal, that organ will remain,
even though the appendage as such completely vanishes. Thus, as
will be shown later, special sense-organs such as the olfactory remain,
though the animal no longer possesses antennae ; the important ex-
cretory organs, the coxal glands, and important respiratory organs,
the branchiae, are still present in the vertebrate, although the appen-
dages to which they originally belonged have dwindled away, or, at
all events, are no longer recognizable as arthropod appendages.

Innervation of Beanchial Segments.

Passing from a priori considerations to actual facts, it is advisable
to commence with the innervation of the branchial segments ; for,
seeing that the foundation of the whole of this comparative study
of the vertebrate and the arthropod is based upon the similarity of
the two central nervous systems, it follows that we must look in
the first instance to the innervation of any organ or group of organs
in order to find out their relationship in the two groups of animals.

The great characteristic of the vertebrate branchial organs is their
segmental arrangement and their innervation by the vagus group of
nerves, i.e. by the hindermost group of the cranial segmental nerves.
These cranial nerves are divided by Gegenbaur into two great groups
— an anterior group, the trigeminal, which supplies the muscles of
mastication, and a posterior group, the vagus, which is essentially

152 THE ORIGIN OF VERTEBRATES

respiratory in function. Of these two groups, I will consider the
latter group first.

In Limulus the great characteristic of the branchial region is its
oronounced segmental arrangement, each pair of branchial appendages
belonging to a separate segment. This group of segments forms the
mesosoma, and these branchial appendages are the mesosomatic
appendages. Anterior to them are the segments of the prosoma,
which bear the prosomatic or locomotor appendages. The latter are
provided at their base with gnathites or masticating apparatus, so
that the prosomatic group of nerves, like the trigeminal group in the
vertebrate, comprises essentially the nerves subserving the important
function of mastication. As already pointed out, the brain-region
of the vertebrate is comparable to the supra-cesophageal and infra-
cesophageal ganglia of the invertebrate, and it has been shown (p. 54)
how. by a process of concentration and cephalization, the foremost
region of the infra-cesophageal ganglia becomes the prosomatic region,
and is directly comparable to the trigeminal region in the vertebrate ;
while the hindermost region is formed from the concentration of
the mesosomatic ganglia, and is directly comparable to the medulla
oblongata, i.e. to the vagus region of the vertebrate brain.

As far, then, as concerns the centres of origin of these two groups
of nerves and their exits from the central nervous system, they are
markedly homologous in the two groups of animals.

Comparison of the Cranial and Spinal Segmental Nerves.

It has often been held that the arrangements of the vertebrate
nervous system differ from those of other segmented animals in one
important particular. The characteristic of the vertebrate is the
origin of every segmental nerve from two roots, of which one con-
tains the efferent fibres, while the other possesses a sensory ganglion,
and contains only afferent fibres. This arrangement, which is found
along the whole spinal cord of all vertebrates, is not found in the
segmental nerves of the invertebrates ; and as it is supposed that the
simpler arrangement of the spinal cord was the primitive arrange-
ment from which the vertebrate central nervous system was built up,
it is often concluded that the animal from which the vertebrate arose
must have possessed a series of nerve-segments, from each of which
there arose bilaterally ventral (efferent) and dorsal (afferent) roots.

THE EVIDENCE OE THE RESPIRATORY APPARATUS 1 53

Now, the striking fact of the vertebrate segmental nerves consists
in this, that, as far as their structure and the tissues which they
innervate are concerned, the cranial segmental nerves are built up on
the same plan as the spinal ; but as far as concerns their exit from
the central nervous system they are markedly different. A large
amount of ingenuity, it is true, has been spent in the endeavour to
force the cranial nerves into a series of segmental nerves, which
arise in the same way as the spinal by two roots, of which the ven-
tral series ought to be efferent and the dorsal series afferent, but
without success. We must, therefore, consider the arrangement of
the cranial segmental nerves by itself, separately from that of the
spinal nerves, and the problem of the origin of the vertebrate seg-
mental nerves admits of two solutions — either the cranial arrange-
ment has arisen from a modification of the spinal, or the spinal from
a simplification of the cranial. The first solution implies that the
spinal cord arrangement is older than the cranial, the second that
the cranial is the oldest.

In my opinion, the evidence of the greater antiquity of the cranial
region is overwhelming.

The evidence of embryology points directly to the greater phylo-
genetic antiquity of the cranial region, for we see how, quite early in
the development, the head is folded off, and the organs in that
region thereby completed at a time when the spinal region is only at
an early stage of development. We see how the first of the trunk
somites is formed just posteriorly to the head region, and then more
and more somites are formed by the addition of fresh segments poste-
riorly to the one first formed. We see how, in Ammoccetes, the first
formed parts of the skeleton are the branchial bars and the basi-
cranial system, while the rudiments of the vertebra? do not appear
until the Petromyzon stage. We see how, with the elongation of the
animal by the later addition of more and more spinal segments,
organs, such as the heart, which were originally in the head, travel
down, and the vagus and lateral-line nerves reach their ultimate
destination. Again, we see that, whereas the cranial nerves, viz. the
ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and
vagus nerves, are wonderfully fixed and constant in all vertebrates,
the only shifting being in the spino-occipital region, in fact, at the
junction of the cranial and spinal region, the spinal nerves, on the
other hand, are not only remarkably variable in number in different

154 THE ORIGIN OF VERTEBRATES

groups of animals, but that even in the same animal great variations
are found, especially in the manner of formation of the limb-plexuses.
Such marked meristic variation in the spinal nerves, in contrast to
the fixed character of the cranial nerves, certainly points to a more
recent formation of the former nerves.

Also the observations of Assheton on the primitive streak of the
rabbit, and on the growth in length of the frog embryo, have led
him to the conclusion that, as in the rabbit so in the frog, there
is evidence to show that the embryo is derived from two definite
centres of growth : the first, phylogenetically the oldest, being a
protoplasmic activity, which gives rise to the anterior end of the
embryo ; the second, one which gives rise to the growth in length of
the embryo. This secondary area of proliferation coincides with the
area of the primitive streak, and he has shown, in a subsequent
paper, by means of the insertion of sable hairs into the unincubated
blastoderm of the chick, that a hair inserted into the centre of the
blastoderm appears at the anterior end of the primitive streak, and
subsequently is found at the level of the most anterior pair of somites.

He then goes on to say —

"From these specimens it seems clear that all those parts in
front of the first pair of mesoblastic somites — that is to say, the
heart, the brain and medulla oblongata, the olfactory, optic, auditory
organs and foregut — are developed from that portion of the un-
incubated blastoderm which lies anterior to the centre of the blasto-
derm, and that all the rest of the embryo is formed by the activity
of the primitive streak area."

In other words, the secondary area of growth, i.e. the primitive
streak area, includes the whole of the spinal cord region, while the
older primary centre of growth is coincident with the cranial region.

In searching, then, for the origin of the segmental nerves, we
must consider the type on which the cranial nerves are arranged
rather than that of the spinal nerves.

The first striking fact occurs at the spino-occipital region, where
the spinal cord merges into the medulla oblongata, for here in the
cervical region we find each spinal segment gives origin to three dis-
tinct roots, not two — a dorsal root, a ventral root, and a lateral root.
This third root gives origin to the spinal accessory nerve, and in the
region of the medulla oblongata these lateral roots merge directly
into the roots of the vagus nerve; more anteriorly the same system

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 55

continues as the roots of the glossopharyngeal nerve, as the roots of
the facial nerve, and as a portion, especially the motor portion, of
the trigeminal nerve. Now, all these nerves belong to a well-defined
system of nerves, as Charles Bell 1 pointed out in 1830, a system of
nerves concerned with respiration and allied mechanisms, such as
laughing, sneezing, mastication, deglutition, etc., nerves innervating a
set of muscles of very different kind from the ordinary body-muscles
concerned with locomotion and equilibration. Also the centres from
which these motor nerves arise are well defined, and form cell-masses
in the central nervous system, quite separate from those which give
origin to somatic muscles.

This original idea of Charles Bell, after having been ignored for so
long a time, is now seen to be a very right one, and it is an extra-
ordinary thing that his enunciation of the dual nature of the spinal
roots, which was, to his mind, of subordinate importance, should so
entirely have overshadowed his suggestion, that in addition to the
dorsal and ventral roots, a lateral system of nerves existed, which
were not exclusively sensory or exclusively motor, but formed a
separate system of respiratory nerves.

Further, anatomists divide the striated muscles of the body into
two great natural groups, characterized by a difference of origin and
largely by a difference of appearance. The one set is concerned
with the movements of internal organs, and is called visceral, the
other is derived from the longitudinal sheet of musculature which
forms the myotomes of the fish, and has been called parietal or
somatic. The motor nerves of these two sets of muscles correspond
with the lateral or respiratory and ventral roots respectively.

Finally, it has been shown that the segments of which a verte-
brate is composed are recognizable in the embryo by the segmented
manner in which the musculature is laid down, and van Wijhe has
shown that in the cranial region two sets of muscles are laid down
segmentally, thus forming a dorsal and ventral series of commencing
muscular segments. Of these the anterior segments of the dorsal
series give origin to the striated muscles of the eye which are inner-
vated by the Illrd (oculomotor), IVth (trochlearis), and Vlth (ab-
ducens) nerves, while the posterior segments give origin to the

1 N.B. — In addition to the nerves mentioned, C. Bell included, in his respiratory
system of nerves, the fourth nerve or trochlearis, the phrenic and the external
respiratory of Bell.

i 5 6

THE ORIGIN OF VERTEBRATES

muscles from the cranium to the shoulder-girdle, innervated by the
Xllth (hypoglossal) nerve. The ventral series of segments give
origin to the musculature supplied by the trigeminal, facial, glosso-
pharyngeal, and vagus nerves.

Also, the afferent or sensory nerves of the skin over the whole of
this head-region are supplied by the trigeminal nerve, while the
afferent nerves to the visceral surfaces are supplied by the vagus,
glossopharyngeal and facial nerves.

In van "Wijhe's original paper he arranged the segments belonging
to the cranial nerves in the following table : —

Segment?.

Ventral nerve-roots and muscles
derived from myotomes.

Visceral clefts.

Dorsal nerve-roots and muscles.

1

m.

M. rectus supe-
rior, m. rectus
internus, m.
rectus inferior,
m. obliquus in-

V. N.op-
thalrnicus
profundus

ferior

2

IV.

M. obliquus

V.

Masticating

superior

1st Mandibular

muscles.

3

VI.

M. rectus ex-

VTL,

i Facial muscles
(VIII. is dorsal

4

—

ternum

^{I$£

VII.,

1 branch of VII.)

—

3rd 1st Branchial

IX.

|

6

8

XII.

xn.

j Muscles from j
cranium to I

4th 2nd

5th 3rd „

6th 4th

X.,
X...

x.:

Branchial and
visceral muscles

9

XII.

1 shoulder-girdle |

7th 5th

x. t

1

As is seen in the table, van Wijhe attempts to arrange the cranial
secrmental nerves into dorsal and ventral roots, in accordance with
the arrangement in the spinal region. In order to do this he calls
the Vth. Vllth, IXth, and Xth nerves dorsal roots, although they
are not purely sensory nerves, but contain motor fibres as well.

It is not accidental that he should have picked out for his dorsal
roots the very nerves which form Charles Bell's lateral series of
roots, inasmuch as this system of lateral roots, apart from dorsal and
ventral roots, really is, as Charles Bell thought, an important separate
system, dependent upon a separate segmentation in the embryo of
the musculature supplied by these roots. This segmentation may
receive the name of visceral or splanchnic in contradistinction to
somatic, since all the muscles without exception belong to the visceral
group of striated muscles.

THE EVIDEXCE OF THE RESPIRATORY APPARATUS 1 57

These observations of van Wijhe lead directly to the following
conclusion. In the cranial region there is evidence of a double set
of segments, which mav be called somatic and splanchnic. The
somatic segments, consisting of the outer skin and the body muscu-
lature, are doubly innervated as are those of the spinal cord by a
series of ventral motor roots, the oculomotor or lllrd nerve, the
trochlear or IVth nerve, the abducens or YIth nerve, and the hypo-
glossal or Xllth nerve, and by a series of dorsal sensory roots, the
sensory part of the trigeminal or Yth nerve. But the splanchnic
segments are innervated by single roots, the vagus or Xth nerve,
glossopharyngeal or IXth nerve, facial or Vllth nerve, and trigeminal
or Vth nerve, which are mixed, containing both sensory and motor
fibres, thus differing markedly from the arrangement of the spinal
nerves.

From this sketch it follows that the arrangement seen in the
spinal cord, would result from the cranial arrangement if this third
system of lateral roots were left out. Further, since the cranial
system is the oldest, we must search in the invertebrate ancestor for
a tripartite rather than a dual system of nerve-roots for each segment ;
a system composed of a dorsal root supplying only the sensory nerves
of the skin-surfaces, a lateral mixed root supplying the system con-
nected with respiration with both sensory and motor fibres, and a
ventral root supplying the motor nerves to the body-musculature.

COMPARISON OF THE APPENDAGE NERVES OF LlMULUS AND BrANCHI-

pus to the Lateral Eoot System of the Vertebrate.

If the argument used so far is correct, and this tripartite system
of nerve-roots, as seen in the cranial nerves of the vertebrate, really
represents the original scheme of innervation in the paheostracan
ancestor, then it follows that each segment of Limulus ought to be
supplied by three nerves— (1), a sensory nerve supplying its own
portion of the skin-surface of the prosomatic and mesosumatic
carapaces; (2), a lateral mixed nerve supplying exclusively the
appendage of the segment, for the appendages carry the respiratory
organs ; and (3), a motor nerve supplying the body -muscles of the
segment.

It is a striking fact that Milne-Edwards describes the nerve-roots
in exactly this manner. The great characteristic v£ the nerve-roots

158 THE ORIGIN OF VERTEBRATES

in Limulus as in other arthropods is the largo appendage-nerve,
which is always a mixed nerve; in addition, there is a system of
sensory nerves to the prosomatic and mesosomatic carapaces, called
by him the epimeral nerves, which are purely sensory, and a third
set of roots which are motor to the body-inuseles, and possibly also
sensory to the ventral surface between the appendages.

Moreover, just as in the vertebrate central nervous system the
centres of origin of the motor nerves of the branchial segmentation
are distinct from those of the somatic segmentation, so we find, from
the researches of Hardy, that a similar well-marked separation exists
between the centres of origin of the motor nerves of the appendages
and those of the somatic muscles in the central nervous system of
Branchipus and Astacus.

In the first place, he points out that the nervous system of
Branchipus is of a very primitive arthropod type ; that it is, in fact,
as good an example of an ancient type as we are likely to find in the
present day ; a matter of some importance in connection with my
argument, since the arthropod ancestor of the vertebrate, such as I
am deducing from the study of Ammoccetes, must undoubtedly have
been of an ancient type, more nearly connected with the strange
forms of the trilobite era than with the crabs and spiders of the
present day.

His conclusions with respect to Branchipus may be tabulated as
follows : —

1. Each ganglion of the ventral chain is formed mainly for the
innervation of the appendages.

2. Each ganglion is divided into an anterior and posterior division,
which are connected respectively with the motor and sensory nerves
of the appendages.

3. The motor nerves of the appendages arise as well-defined axis-
cylinder processes of nerve-cells, which are arranged in well-defined
groups in the anterior division of the ganglion.

4. A separate innervation exists for the muscles and sensory
surfaces of the trunk. The trunk-muscles consist of long bundles,
from which slips pass off to the skin in each segment; they are thus
imperfectly segmented. In accordance with this, a diffuse system
of nerve-fibres passes to them from certain cells on the dorsal surface
of each lateral half of the ganglion. These cell-groups are therefore
very distinct from those which give origin to the motor appendage-

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 59

nerves, and, moreover, are not confined to the ganglion, but extend
for some distance into the interganglionic region of the nerve-cords
which connect together the ganglia of the ventral chain.

Hardy's observations, therefore, combined with those of Milne-
Edwards, lead to the conclusion that in such a primitive arthropod
type as my theory postulates, each segment was supplied with
separate sensory and motor somatic nerves, and with a pair of nerves
of mixed function, devoted entirely to the innervation of the pair of
appendages ; that also, in the central nervous system, the motor
nerve-centres were arranged in accordance with a double set of seg-
mented muscles in two separate groups of nerve-cells. These nerve-
cells in the one case were aggregated into well-defined groups, which
formed the centres for the motor nerves of the markedly segmented
muscles of the appendages, and in the other case formed a system of
more diffused cells, less markedly aggregated into distinct groups,
which formed the centres for the imperfectly segmented somatic
muscles.

Such an arrangement suggests that in the ancient arthropod type
a double segmentation existed, viz. a segmentation of the body, and
a segmentation due to the appendages. Undoubtedly, the segments
originally corresponded absolutely as in Branchipus, and every
appendage was attached to a well-defined separate body-segment.
In, however, such an ancient type as Limulus, though the segmen-
tation may be spoken of as twofold, yet the number of segments
in the prosoinatic and mesosomatic regions are much more clearly
marked out by the appendages than by the divisions of the soma ;
for, in the prosoinatic region such a fusion of somatic segments
to form the tergal prosoinatic carapace has taken place that the
segments of which it is composed are visible only in the young con-
dition, while in the mesosomatic region the separate somatic segments,
though fused to form the mesosomatic carapace, are still indicated
by the entapophysial indentations.

Clearly, then, if the mesosomatic branchial appendages of forms
related to Limulus were reduced to the branchial portion of the
appendage, and that branchial portion became internal, just as is
known to be the case in the scorpion group, we should obtain an
animal in which the mesosomatic region would be characterized by
a segmentation predominantly branchial, which might be termed, as
in vertebrates, the oranchiomcric segmentation, but yet would show

160 THE ORIGIN OF VERTEBRATES

indications of a corresponding somatic or mesomeric segmentation.
The nerve supply to these segments would consist of —

1. The epimeral purely sensory nerves to the somatic surface,
equivalent in the vertebrate to the ascending root of the trigeminal.

2. The mixed nerves to the internal branchial segments, equivalent
in the vertebrate to the vagus, glossopharyngeal, and facial.

3. The motor nerves to the somatic muscles, equivalent in the
vertebrate to the original nerve- supply to the somatic muscles
belonging to these segments, i.e. to the muscles derived from van
Wijhe's 4th, 5th, and 6th somites.

Further, the centres of origin of these appendage-nerves would
form centres in the central nervous system separate from the centres
of the motor nerves to the somatic muscles, just as the centres of
origin of the motor parts of the facial, vagus, and glossopharyngeal
nerves form groups of cells quite distinct from the centres for the
hypoglossal, abducens, trochlear, and oculomotor nerves.

In fact, if the vertebrate branchial nerves are looked upon as the
descendants of nerves which originally supplied branchial appendages,
then every question connected with the branchial segmentation, with
the origin and distribution of these nerves, receives a simple and
adequate solution — a solution in exact agreement with the conclusion
that the vertebrate arose from a pakeostracan ancestor.

It would, therefore, be natural to expect that the earliest fishes
breathed by means of branchial appendages situated internally, and
that the evidence for such appendages would be much stronger in
them than in more recent fishes.

Although we know nothing of the nature of the respiratory appa-
ratus in the extinct fishes of Silurian times, we have still living, in
the shape of Ammoccetes, a possible representative of such types.
If, then, we find, as is the case, that the respiratory apparatus of
Ammocoetes differs markedly from that of the rest of the fishes, and,
indeed, from that of the adult form or Petromyzon, and that that
very difference consists in a greater resemblance to internal branchial
appendages in the case of Ammoccetes, then we may feel that the
proof of the origin of the branchial apparatus of the vertebrate from
the internal branchial appendages of the invertebrate has gained
enormously.

THE EVIDENCE OF THE RESPIRATORY APPARATUS l6l

The Eespiratory Chamber of Ammoccetes.

In order to make clear the nature of the branchial segments in
Ammoco?tes, I have divided the head-part of the animal by means of
a longitudinal horizontal section into halves — ventral and dorsal —
as shown in Figs. 63 and 64. These figures are each a combination
of a section and a solid drawing. The animal was slit open by a
longitudinal section in the neighbourhood of the gill-slits, and each
half was slightly flattened out, so as to expose the ventral and dorsal
internal surfaces respectively. The structures in the cut surface were
drawn from one of a series of horizontal longitudinal sections taken
through the head of the animal. These figures show that the head-region
of Ammoccetes consists of two chambers, the contents of which are
different. In front, an oral or stomodseal chamber, which contains the
velum and tentacles, is enclosed by the upper and lower lips, and was
originally separated by a septum from the larger respiratory chamber,
which contains the separate pairs of branchiae. A glance at the two
drawings shows clearly that Eathke's original description of this
chamber is the natural one, for he at that time, looking upon Ammo-
ccetes branchialis as a separate species, described the branchial chamber
as containing a series of paired gills, with the gill-openings between
consecutive gills. His branchial unit or gill, therefore, was repre-
sented by each of the so-called diaphragms, which, as seen in Figs. 63,
64, are all exactly alike, except the first and the last. Any one of
these is represented in section in Fig. 65, and represents a branchial
unit in Eathke's view and in mine. Clearly, it may be described as a
branchial appendage which projects into an open pharyngeal chamber,
so that the series of such appendages divides the chamber into a
series of compartments, each of which communicates with the exterior
by means of a gill-slit, and with each other by means of the open
space between opposing appendages.

Each of these appendages possesses its own cartilaginous bar
(Br. cart.), as explained in Chapter III. ; each possesses its own bran-
chial or visceral muscles (coloured blue in Figs. 63 and 64), separated
absolutely from the longitudinal somatic muscles (coloured dark
red in Figs. 63 and 64) by a space (*S^>.) containing blood and
peculiar fat-cells, etc, Each possesses its own afferent branchial
blood-vessel from the ventral aorta, and its own efferent vessel to
the dorsal aorta (Fig. 65, a. br. and v. br.). Each possesses its own

M

Respiratory Append aqes
$ Nerve Supply

Huoiti

Fig. 63. — Ventral half
of Head-region of Am-
moccetes.

-Pigment

Somatic muscles coloured
red. Branchial and visce-
ral muscles coloured blue.
Tubular constrictor mus-
cles distinguished from
striated constrictor mus-
cles by simple hatching.
Tent., tentacles ; Tent. m.c.,
muco-cartilage of tenta-
cles; Vel. m.c, muco-car-
tilage of the velum ; Hy.
m.c. muco-cartilage of the
hyoid segment; Ps. br.,
pseudo-branchial groove ;
Br. cart., branchial carti-
lages ; Sp., space between
somatic and splanchnic
muscles ; Tit. op., orifice of
thyroid ; //., heart.

Tr.

<Ser.

Fig. 64. — Dorsal
half of head-

REGIOV OF Am-
MOCOiTES.

Inf.

Tr., trabecule;
Pit., pituitary

space ; //;/"., in-
f u n d i b u 1 u m ;
Ser., median ser-
rated flange of
velar folds.

164

THE ORIGIN OF VERTEBRATES

segmental nerve, which supplies its own- branchial muscles and no
others with motor fibres, and sends sensory fibres to the general surface
of each appendage, as also to the special sense-organs in the shape
of the epithelial pits (S., Fig. 65) arranged along the free edges of

m.add

v br.cart

m, :

m.

m.v

Fig. 65. — Section through Branchial Ap-
pendage of Ammoccetes.

br. cart., branchial cartilage; v. br., branchial
vein; a. br., branchial artery; b.s., blood-
spaces ; p., pigment ; 8., sense-organ; c, cili-
ated band; E., I., external and internal
borders ; m. add., adductor muscle ; m.c.s.,
striated constrictor muscle; m.c.t., tubular
constrictor muscle ; m. and m.v., muscles
of valve.

br.cart.

Fig. 66. — Section through Bran-
chial Appendage of Limulus.

br. cart., branchial cartilage ;
v.br., branchial vein ; b.s., blood-
spaces formed by branchial artery ;
P., pigment ; nti, posterior enta-
pophysio-branchial muscle ; m„,
anterior entapophysio-branchial
muscle ; w 3 , external branchial
muscle.

the diaphragms ; each of these nerves possesses its own ganglion —
the epibranchial ganglion.

The work of Miss Alcock has shown that the segmental branchial
nerve supplies solely and absolutely such an appendage or branchial

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 65

segment, and does not supply any portion of the neighbouring branchial
segments. The nerve-supply in Ammoccetes gives no countenance to
the view that the original unit was a branchial pouch, the two sides
of which each nerve supplied, but is strong evidence that the original
unit was a branchial appendage, which was supplied by a single
nerve with both motor and sensory fibres.

Any observer having before him only this picture of the respiratory
chamber of Ammoccetes, upon which to base his view of a vertebrate
respiratory chamber, would naturally look upon the branchial unit of
a vertebrate as a gilled appendage projecting into the open cavity of
the anterior part of the alimentary canal or pharynx. This is not,
however, the usual conception. The branchial unit is ordinarily
described as a gill-pouch, which possesses two openings or slits, an
internal one into the lumen of the alimentary canal, and an external
one into the surrounding medium. This view is based upon embryo-
logical evidence of the following character : —

The alimentary canal of all vertebrates forms a tube stretching
the whole length of the animal ; the anterior part of this tube
becomes pouched on each side at regular intervals, and the walls of
each pouch becoming folded form the respiratory surfaces or gills.
The openings of these separate pouches into the central lumen of the
gut form the internal gill-pouch openings ; the other extremity of
the pouch approaches the external surface of the animal, and finally
breaks through to form a series of external gill-pouch openings.

From the mesoblastic tissue, between each gill-pouch, there is
formed a supporting cartilaginous bar, to which are attached a system
of branchial muscles, with their nerves and blood-vessels. These
cartilaginous bars, in all fishes above the Cyclostomata, form a
supporting framework for the internal gill-slit, so that the gills
are situated externally to them ; the more primitive arrangement is,
as already mentioned, a system of cartilaginous bars, extra-branchial
in position, so that the gills are situated internally to them.

From this description of the mode of formation of the respiratory
apparatus in water-breathing vertebrates the conception has arisen
of the gill-pouch as the branchial unit, a conception which is
absolutely removed from all idea of a branchial unit such as is
found in an arthropod, viz. an appendage.

This conception of spaces as units pervades the whole of embryo-
logy, and is the outcome of the gastrula theory— a theory which

1 66

THE ORIGIN OF VERTEBRATES

teaches that all animals above the Protozoa are derived from a form
which by invagination of its external surface formed an internal
cavity or primitive gut. From pouches of this gut other cavities
were said to be formed, called coelomic cavities, and thus arose the
group of cceloinatous animals. To speak of the developmental history
of animals in terms of spaces ; to speak of the atrophy of a cavity
as though such a thing were possible, is, to my mind, the wrong
way of looking at the facts of anatomy. It resembles the description
of a net as a number of holes tied together with string, which is not
usually considered the best method of description.

There are two ways in which a series of pouches can be formed
from a simple tube without folding, either by a thinning at regular
intervals of the original tissue surrounding the tube, or by the
ingrowth into the tube of the surrounding tissue at regular intervals,
thus —

A

Ep -
Mes-

ODOOOQQQQQDCDC03CG00aQO0Q3

\ it; t 1 1 rr-

OQSDODDaDCDDDaDQOSaOSaDDODBa

'SEGMENT' ;

araQoaQQaaQDaQQbgoQoaaoDOQOSDO

B

Ep-

Mes-

%

1

CDCKBCCCDDDBODCOBGDBDOaDDD.

DDanuuoauauaaDnanaaD

QGCOEBDaCDa00aBQDDBO0DQ3BQQag

Fig. 67.

1 z

-Diagrams to show the two methods of Pouch-formation.

A, by the thinning of the mesoblast at intervals. B, by the ingrowth of rnesoblast at
intervals. Ep., epiblast ; Mes., mesoblast ; Hy., hypoblast.

In the first case (A) the formation of a pouch is the significant
act, and therefore the branchial segments might be expressed in terms
of pouches. In the second ease (B) the formation of a pouch is

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 67

brought about in consequence of the ingrowth of the mesoblastic
tissues at intervals ; here, although the end-result is the same as in
the first case, the pouch-formation is only secondary, the true
branchial unit is the mesoblastic ingrowth.

The evidence all points directly to the second method of forma-
tion. Thus Shipley, in his description of the development of the
lamprey, says —

"The gill-slits appear to me to be the result of the ventral
downgrowth of mesoblast taking place only at certain places, these
forming the gill-bars. Between each downgrowth the hypoblastic
lining of the alimentary canal remains in contact with the epiblast ;
here the gill-opening subsequently appears about the twenty-second
day."

Dohrn describes and gives excellent pictures of the growth of
the diaphragms, as the Aminoccetes grows in size, pictures which
are distinctly reminiscent of the corresponding illustrations given
by Brauer of the growth of the internal gills in the scorpion embryo.

Another piece of evidence confirmatory of the view that the
branchial segments are really of the nature of internal appendages,
as the result of which gill-pouches are formed, is given by the presence
in each of these branchial bars or diaphragms of a separate ccelomic
cavity. From the walls of this cavity the branchial muscles and
cartilaginous bar are formed.

Now, from an embryological point of view, the vertebrate shows
that it is a segmented animal by the formation of somites, which
consist of a series of divisions of the ccelom, of which the walls form
a series of muscular and skeletal segments. In the head-region, as
already mentioned, such ccelomic divisions form two rows — a dorsal
and a ventral set. From the walls of the dorsal set the somatic
musculature is formed. From those of the ventral set the branchial
musculature. From the latter also the branchial cartilaginous bars
are formed. Thus Shipley, in his description of the development
of the lamprey, says: "The mesoblast between the gills arranges
itself into head-cavities, and the walls of these cavities ultimately
form the skeleton of the gill-arches."

Similarly, in the arthropod, the segments in the embryo are
marked out by a series of co?loniic cavities and Kishinouye has
described in Limulus a separate ccelomic cavity for every one of
the mesosomatic or branchial segments, and he states that in Arachnida

1 68 THE ORIGIN OF VERTEBRATES

the segmental ccelomic cavities extend into the limbs. These
cavities both in the vertebrate and in the arthropod disappear
before the adult condition is reached.

The whole evidence thus points strongly to the conclusion that the
true branchial segmental units are the branchial bars or diaphragms,
not the pouches between them.

It is possible to understand why such prominence has been
given to the conception of the branchial unit as a gill-pouch rather
than as a gill-appendage, when the extraordinary change of appear-
ance in the respiratory chamber of the lamprey which occurs at
transformation, is taken into consideration. This change is of a
very far-reaching character, and consists essentially of the formation
of a new alimentary canal in this region, whereby the pharyngeal
chamber of Ammoccetes is cut off posteriorly from the alimentary
canal, and is confined entirely to respiratory purposes, its original
lumen now forming a tube called the bronchus, which opens into the
mouth and into a series of branchial pouches.

In Fig. 68 I give diagrammatic illustrations taken from Nestler's
paper to show the striking change which takes place at transforma-
tion, (A) representing three branchial segments of Ammoco?tes, and (B)
the corresponding three segments of Petromyzon. The corresponding
parts in the two diagrams are shown by the cartilages (br. cart.), the
sense-organs (S), and the branchial veins ( V. br.) ; the corresponding
diaphragms are marked by the figures 1, 2, 3 respectively. As is
clearly seen, it is perfectly possible in the latter case to describe the
respiratory chamber, as Nestler has done, as divided into a series of
separate smaller chambers — the gill-pouches — by means of a series
of diaphragms or branchial bars. The surface of these gill-pouches
is in part thrown into folds for respiratory purposes, and each gill-
pouch opens, on the one hand, into the bronchus (Bro.), and, on the
other, to the exterior by means of the gill-slit. The branchial unit
in Petromyzon is, therefore, according to Nestler and other mor-
phologists, the folded opposed surfaces of two contiguous diaphragms,
and each one of the diaphragms is intersegmental between two gill-
pouches.

Nestler then goes on to describe the arrangement in Ammoccetes
in the same terms, although there is no bronchus or gill-pouch, but
only an open chamber into which these gill-bearing diaphragms
project, which open chamber serves both for the passage of food and

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 69

of the water for respiration. This is manifestly the wrong way to
look at the matter: the adult form is derived from the larval, not vice
verm, and the transformation process shows exactly how the gills,
in Rathke's sense, come together to form the bronchus and so make
the gill-pouches of Petromyzon.

When we bear in mind that almost all observers consider that
the internal branchiae of the scorpion group are directly derived

V.br.

-br.cart^/ \

br.cart.

Bro.

Fig. 68. — Diagram of three Branchial Segments of Ammoccetes (A) compared
with three Branchial Segments after Transformation (B) to show how
the Branchial Appendages of Ammoccstes form the Branchial Pouches
of Petromyzon. (After Nestler.)

In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the
sense-organs (S), are marked out in order to show corresponding points. The
muscles, blood-spaces, branchial arteries, etc., of each branchial segment are
not distinguished, being represented a uniform black colour. Bro., the bronchus
into which each gill-pouch opens.

from branchial appendages of a kind similar to those of Limulus, it
is evident that a branchial appendage such as that of Ammocoates
might also have arisen from such an appendage, because in various
respects it is easier to compare the branchial appendage of Ammo-
ccetes, than that of the scorpion group, with that of Limulus.

In the case of the scorpions, various suggestions have been made
as to the manner in which such a conversion may have taken place.
The most probable explanation is that given by Macleod, in which

170 THE ORIGIN OF VERTEBRATES

each of the branchiae of the scorpion group is directly compared
with the branchial part of the Limulus appendage which has sunk
into and amalgamated with the ventral surface.

According to this view, the modification which has taken place in
transforming the branchial Limulus-appendage into the branchial
scorpion-appendage is a further stage of the process by which the
Limulus branchial appendage itself has been formed, viz. the getting
rid of the free locomotor segments of the original appendage, thus
confining the appendage more and more to the basal branchial
portion. So far has this process been carried in the scorpion that
all the free part of the appendage has disappeared; apparently, also,
the intrinsic muscles of the appendage have vanished, with the
possible exception of the post-stigmatic muscle, so that any direct
comparison between the branchial appendages of Limulus and the
scorpions is limited to the comparison of their branchiae, their nerves,
and their afferent and efferent blood-vessels.

In the case of Ammoccetes the comparison must be made not
with air-breathing but with water-breathing scorpions, such as
existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia,
and with the crowd of trilobite and Limulus-like forms winch were
in past ages so predominant in the sea ; forms in some of which the
branchial appendages had already become internal, but which, from
the very fact of these forms being water-breathers, probably
resembled, in respect of their respiratory apparatus, Limulus rather
than the present-day scorpion.

On the assumption that the branchial appendages of Ammoccetes,
like the branchial appendages of the scorpion group, are to a certain
extent comparable with those of Limulus, it becomes a matter of great
interest to inquire whether the mode in which respiration is effected
in Ammoccetes resembles most that of Limulus or of the scorpion.

The Origin of the Branchial Musculature.

The difference between the movements of respiration in Limulus
and those of the scorpions consists in the fact that, although in both
cases respiration is effected mainly by dorso-ventral muscles, these
muscles are not homologous in the two cases : in the former, the
dorso-ventral appendage-muscles are mainly concerned, in the latter,
the dorso-ventral somatic muscles.

THE EVIDENCE OF THE RESPIRATORY APPARATUS I 7 I

The paper by Benhani gives a full description of the musculature
of Limulus, and according to his arrangement the muscles are
divided into two sets, longitudinal and dorso-ventral. Of the
latter, each mesosomatic segment possesses a pair of dorso-ventral
muscles, attached to the mid-ventral mesosomatic entochondrite, and
to the tergal surface (Fig. 58, Dv.). These muscles are called by
Benham the vertical mesosomatic muscles. I shall call them the
somatic dorso-ventral muscles, in contradistinction to the dorso-
ventral muscles of the branchial appendages. Of the latter, the two
chief are the external branchial (Fig. 66, m 3 ) "and the posterior
entapophysio-branchial (Fig. 66, m{) ; a third muscle is the anterior
entapophysio-branchial (Fig. 66, m 2 ). Of these muscles, the posterior
entapophysio-branchial (mi) is closely attached along the branchial
cartilaginous bar up to its round-headed termination on the anterior
surface of the appendage. The anterior entapophysio-branchial
muscle (m 2 ) is attached to the branchial cartilage near the
entapophysis.

In the case of the scorpion, as described by Miss Beck, the
branchial appendage has become reduced to the branchiae, and the
intrinsic appendage-muscles have entirely disappeared, with the
possible exception of the small post-stigmatic muscle ; on the other
hand, the dorso-ventral somatic muscles, which are clearly homolo-
gous with the corresponding muscles of Limulus, have remained, and
become the essential respiratory muscles.

Of these two possible types of respiratory movement it is quite
conceivable that in the water-breathing scorpions of olden times
and in their allies, the dorso-ventral muscles of their branchial
appendages may have continued their role of respiratory muscles, and
so have given origin to the respiratory muscles of the ancestors of
Ammoccetes.

The respiratory muscles of Ammoccetes are three in number, and
have been described by Nestler and Miss Alcock as the adductor
muscle, the striated constrictor muscle, and the tubular constrictor
muscle (Fig. 65, m. add,, m.c.s., and m.c.t.). Of these, the constrictor
muscle (Fig. 71, m. con. str.) is in close contact with its cartilaginous
bar, while the adductor (Fig. 71, m. add.) is attached to the cartilage
only at its origin and insertion, and the tubular muscles (Fig. 71,
m. con. tub.) have nothing whatever to do with the cartilage at all,
being attached vent-rally to the connective tissue in the neighbourhood

172 THE ORIGIN OF VERTEBRATES

of the ventral aorta (V.A.), and dorsally to the mid-line between the
dorsal aorta (D.A.) and the notochord.

The close relationship of the constrictor muscle to the carti-
laginous branchial bar does not favour the surmise that this muscle
is homologous with the dorso-ventral somatic muscle of the scorpion.
It is, however, directly in accordance with the view that this muscle
is homologous with one of the dorso-ventral appendage-muscles, such
as the posterior entapophysio-branchial muscle (mi, Fig. 66) of the
Limulus appendage, especially when the homology of the Ammoccetes
branchial cartilage with the Limulus branchial cartilage is borne in
mind. I am, therefore, inclined to look upon the constrictor and
adductor muscles of the Ammoccetes branchial segment as more likely
to have been derived from dorso-ventral muscles which belonged
originally to a branchial appendage, such as we see in Limulus, than
from dorso-ventral somatic muscles, such as the vertical mesosomatic
muscles which are found both in Limulus and scorpion. In other
words, I am inclined to hold the view that the somatic dorso-ventral
muscles have disappeared in this region in Ammoc<etes, while dorso-
ventral appendage-muscles have been retained, i.e. the exact reverse
to what has taken place in the air-breathing scorpion.

I am especially inclined to this view because of the manner in
which it fits in with and explains van Wijhe's results. Ever since
Schneider divided the striated muscles of vertebrates into parietal
and visceral, such a division has received general acceptance and, as
far as the head-region is concerned, has received an explanation in
van Wijhe's work; for Schneider's grouping corresponds exactly to the
two segmentations of the head-mesoblast, discovered by van Wijhe,
i.e. to the somatic and splanchnic striated muscles according to my
nomenclature. Of these two groups the splanchnic or visceral
striated musculature, innervated by the Vth, Vllth, IXth, and Xth
nerves, which ought on this theory to be derived from the muscu-
lature of the corresponding appendages, is, speaking generally, dorso-
ventral in direction in Ammoccetes and of the same character through-
out ; the somatic musculature, on the other hand, is clearly divisible,
in the head region, into two sets — a spinal and a cranial set. The
somatic muscles innervated by the spinal set of nerves, including in
this term the spino-occipital or so-called hypoglossal nerves, are in
Ammoca'tes most sharply defined from all the other muscles of
the body. They form the great dorsal and ventral longitudinal

THE EVIDENCE OF THE RESPIRATORY APPARATUS I 73

body-muscles, which extend dorsally as far forward as the nose and
are developed embryologically quite distinctly from the others, being
formed as muscle-plates (Kastchen). On the other hand, the cranial
somatic muscles are the eye-muscles, the formation of which resembles
that of the visceral muscles, and not of the spinal somatic. Their
direction is not longitudinal, but dorso-ventral ; they cannot, in my
opinion, be referred to the somatic trunk- muscles, and must, therefore,
form a separate group to themselves. Thus the striated musculature
of the Ammoccetes must be divided into (1) the visceral muscles ;
(2) the longitudinal somatic muscles ; and (3) the dorso-ventral somatic
muscles. Of these the 1st, on the view just stated, represent the
original appendage-muscles ; the 2nd belong to the spinal region, and
will be considered with that region ; the 3rd represent the original
segmental dorso-ventral somatic muscles, which are so conspicuous
in the musculature of the Limulus and the scorpion group.

The discussion of this last statement will be given when I come
to deal with the prosomatic segments of Ammoccetes. I wish, here,
simply to point out that van Wijhe has shown that the eye-muscles
develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and
therefore represent the somatic muscles belonging to those segments,
while no development of any corresponding muscles takes place in
the 4th, 5th, and 6th segments ; so that if the eye-muscles represent
a group of dorso-ventral somatic muscles, such muscles have been
lost in the 4th, 5th, and 6th segments. The latter segments are,
however, the glossopharyngeal and vagus segments, the branchial
musculature of which is derived from the ventral segments of the
mesoderm. In other words, van Wijhe's observations mean that the
dorso-ventral somatic musculature has been lost in the branchial or
mesosomatic region, while the dorso-ventral appendage musculature
has been retained, and that, therefore, the mode of respiration in
Ammoccetes more closely resembles that of Limulus than of Scorpio.

In addition to these branchial muscles, another and very striking
set of muscles is found in the respiratory region of Ammoccetes — the
so-called tubular muscles. These muscles are of great interest, but
as they are especially connected with the Vllth nerve, their con-
sideration is best postponed to the chapter dealing with that nerve.

Also, in connection with the vagus group of nerves, special sense-
organs are found in the skin covering this mesosomatic region, the
so-called epithelial pit-organs ~(Ep. pit, Fig. 71). They, too, are of

174 THE ORIGIN OF VERTEBRATES

great interest, but their consideration may also better be deferred to
the chapter dealing with those special sense-systems known as the
lateral line and auditory systems.

Comparison of the Branchial Circulation in Ammoccetes and

LlMULUS.

Closely bound up with the respiratory system is the nature of
the circulation of blood through the gills. Before, therefore, proceeding
to the consideration of the segments in front of those which carry
branchire, it is worth while to compare the circulation of the blood
in the gills of Limulus and of Ammoccetes respectively.

In all the higher vertebrates the blood circulates in a closed
system of capillaries, which unite the arterial with the venous systems.
In all the higher invertebrates this capillary system can hardly be
said to exist ; the blood is pumped from the arterial system into blood,
spaces or lacunas, and thus comes into immediate contact with the
tissues. From these it is collected into veins, and so returned to the
heart. There is, in fact, no separate lymph-system in the higher
invertebrates ; the blood-system and lymph-system are not yet
differentiated from each other. This also is the case in Ammoccetes ;
here, too, in many places the blood is poured into a lacunar space,
and collected thence by the venous system ; a capillary system is
only in its commencement and a lymph-system does not yet exist.
In this part of its vascular system Ammoccetes again resembles the
higher invertebrates more than the higher vertebrates.

This resemblance is still more striking when the circulation
in the respiratory organs of the two animals is compared. A
branchial appendage is essentially an appendage whose vascular
system is arranged for the special purpose of aerating blood. In the
.higher vertebrates such a purpose is attained by the pulmonary
capillaries, in Limulus by the division of the posterior surface of the
basal part of the appendage into thin lamellar plates, the interior of
each of which is filled with blood. The two surfaces of each lamella
are kept parallel to each other by means of fibrous or cellular strands
forming little pillars at intervals, called by Macleod " colonettes."
A precisely similar arrangement is found in the scorpion gill-lamella,
as seen in Fig. 69, A, taken from Macleod. In Ammoccetes there are
no well-defined branchial capillaries, but the blood circulates, as in

THE EVIDENCE OE THE RESPIRATORY APPARATUS I 75

the invertebrate gill, in a lamellar space ; here, also, as Nestler has
shown, the opposing walls of the gill-lamella are held in position by
little pillar-like cells, as seen in Fig. 69, B, taken from his paper.

In this representative of the earliest vertebrates the method of
manufacturing an efficient gill out of a lacunar blood-space is pre-
cisely the same as that which existed in Limulus and the scorpion,
and, therefore, as that which existed in the dominant invertebrate
group at the time when vertebrates first appeared. This similarity
indicates a close resemblance between the circulatory systems of the
two groups of animals, and therefore, to the superficial inquirer, would
indicate an homology between the heart of the vertebrate and the
heart of the higher inverte-
brate ; but the former is situ-
ated ventrally to the gut and
the nervous system, while the
latter is composed of a long
vessel which lies in the mid-
dorsal line immediately under
the external dorsal covering.
Indeed, this ventral position of
the heart in the one group of
animals and its dorsal position
in the other, combined with
the corresponding positions of
the central nervous system, is
one of the principal reasons
why all the advocates of the
origin of vertebrates from the

Appendiculata, with the single exception of myself, feel compelled to
reverse the dorsal and ventral surfaces in deriving the vertebrate
from the invertebrate. But there is one most important fact which
ought to make us hesitate before accepting the homology of the
dorsal heart of the arthropod with the ventral heart of the vertebrate
—The heart in all invertebrates is a systemic heart, i.e. drives the
arterial blood to the different organs of the body, and then the veins
carry it back to the respiratory organ, from whence it passes to the

heart.

The only exception to this scheme is found in the vertebrate
where the heart is essentially a branchial heart, the blood being

Fig. 69.— Comparison of Branchial La-
mellae of Limulus and Scorpio with
Branchial Lamellae of Ammoccetes.

A, Branchial lamellae of Scorpio (after
Macleod) ; B, Branchial lamellae of Am-
moccetes (after Nestler).

i 7 6

THE ORIGIN OF VERTEBRATES

driven from the heart to the ventral aorta, from which by the
branchial arteries it is carried to the gills, and then, after aeration, is
collected into the dorsal aorta, whence it is distributed over the
body. The distributing systemic vessel is the dorsal aorta, not the
heart which belongs essentially to the ventral venous system. This
constitutes a very strong reason for believing that the systemic heart
of the invertebrate is not homologous with the heart of the vertebrate.
How, then, did the vertebrate heart arise ?

Let us first see how the blood is supplied to the gills in Limulus.

In Limulus the blood flows into the lamella? from sinuses or
blood-spaces (b.s., Fig. 66) at the base of each of the lamelke, which
sinuses are filled by a vessel which may be called the branchial

Fig. 70. — Longitudinal Diagrammatic Section through the Mesosomatic
Region of Limulus, to show the origin of the Branchial Arteries.
(After Benham.)

L.Y.S., longitudinal venous sinus, or collecting sinus; a.br., branchial arteries-
V.p., veno-pericardial muscles; P., pericardium.

artery, since it is the afferent branchial vessel. On each side of the
middle line of the ventral surface of the body a large longitudinal
venous sinus exists, called by Milne-Edwards the venous collecting
sinus, L. V.S., (Fig. 70 and Fig. 58), which gives off to each of the
branchial appendages on that side a well-defined afferent branchial
vessel — the branchial artery (a. h\). The blood of the branchial artery
flows into the blood-spaces between the anterior and posterior
lamina? of the appendage and thence into the gill-lamella?, from
which it is collected into an efferent vessel or branchial vein, termed
by Milne-Edwards the branchio-cardiac canal, which carries it back
to the dorsal heart. The position of the branchial artery and vein
is shown in Fig. 66, which represents a section through the branchial
appendage of Limulus at right angles to the cartilaginous branchial
bar (br. cart.), just as Fig. 65 represents a section through the

THE EVIDENCE OF THE RESPIRATORY APPARATUS I J J

branchial appendage of Ammoccetes at right angles to the carti-
laginous branchial bar.

Further, the observations of Blanchard, Milne -Ed wards, Kay
Lankester, and Benham concur in showing that in both Limulus and
the scorpion group a striking and most useful connection exists
between the heart and these two collecting venous sinuses, in the
shape of a segmentally arranged series of muscular bands ( V.p., Fig.
70 and Fig. 58), attached, on the one hand, to the pericardium, and
on the other to the venous collecting sinus on each side. These
muscular bands, to which Lankester and Benham have given the
name of ' veno-pericardial muscles,' are so different in appearance
from the rest of the muscular substance, that Milne-Edwards did not
recognize them as muscular, but called them ' brides transparentes.'
Blanchard speaks of them in the scorpion as ' ligaments con-
tractiles,' and considers that they play an important part in assisting
the pulmonary circulation ; for, he says, " en mettant a nu une portion
du cceur, on reinarque que ces battements se font sentir sur les liga-
ments contractiles, et determinent sur les poches pulmonaires une
pression qui fait aussitot refluer et remonter le sang dans les vaisseaux
pneumocardiaques." Lankester, in discussing the veno-pericardial
muscles of Limulus and of the scorpions, says that these muscles
probably contract simultaneously with the heart and are of great
importance in assisting the flow through the pulmonary svstem.
More recently Carlson has investigated the action of these muscles
in the living Limulus and found that they act simultaneously with
the muscles of respiration.

Precisely the same arrangement of veno-pericardial muscles and
of longitudinal venous collecting sinuses occurs in the scorpions. It
is one of the fundamental characters of the group, and we may fairly
assume that a similar arrangement existed in the extinct forms from
which I imagine the vertebrate to have arisen. The further con-
sideration of this group of muscles will be given in Chapter IX.

Passing now to the condition of the branchial blood-vessels of
Ammoccetes, we see that the blood passes into the gill-lamella3 from a
blood-space in the appendage, which can hardly be dignified by the
name of a blood-vessel. This blood-space is supplied by the branchial
artery which arises segmentally from the ventral aorta (V.A.), as seen
in Fig. 71 (taken from Miss Alcock's paper). From the gill-lamellaj
the blood is collected into an efferent or branchial vein (v. br.), which

x

1 7 8

THE ORIGIN OF VERTEBRATES

rims, as seeu in Fig. 65, along the free edge of the diaphragm, and
terminates in the dorsal aorta.

The ventral aorta is a single vessel near the heart, but at the com-
mencement of the thyroid it divides into two, and so forms two ventral
longitudinal vessels, from which the branchial arteries arise segmen tally.

''it.lrfrof-W

'TfT-KT.aai-
^^'M-Con- Cut.

-Jyf. Con- Str.

Fig. 71. — Diageam constructed from a series of Transverse Sections through
a Branchial Segment, showing the arrangement and relative positions
of the Cartilage, Muscles, Nerves, and Blood- Vessels.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured
blue are the internal sensory nerves to the diaphragms and the external sensory
nerves to the sense-organs of the lateral line system. Br. cart., branchial
cartilage; M. con. str., striated constrictor muscles; M. con. tub., tubular
constrictor muscles ; M. add., adductor muscle ; D. A., dorsal aorta ; V.A., ventral
aorta; S., sense-organs on diaphragm; n. Lat., lateral line nerve; X., epibran-
chial ganglia of vagus ; B. br. prof. VII., ramus branchialis profundus of facial ;
J. v., jugular vein; Ep. pit., epithelial pit.

From this description it is clear that the vascular supply of the
branchial segment of Ammoccetes would resemble most closely the
vascular supply of the Limulus branchial appendage, if the ventral
aorta of the former was derived from two longitudinal veins, homo-
logous with the paired longitudinal venous sinuses of the latter.

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 79

A 'priori, such a derivation seems highly improbable ; and yet it
is precisely the manner in which embryology teaches 11s that the
heart and ventral aorta of the vertebrate have arisen.

The Origin of the Invertebrate Heart and the Origin of the

Vertebrate Heart.

Not only does the vertebrate heart differ from that of the inverte-
brate, in that it is branchial while the latter is systemic, but also it
is unique in its mode of formation in the embryo. In the Appen-
diculata the heart is formed as a single organ in the mid-dorsal line
by the growth of the two lateral plates of mesoblast dorsalwards,
the heart being formed where they meet. In Mammalia and Aves,
the heart and ventral aorta commence as a pair of longitudinal veins,
one on each side of the commencing notochord.

If the embryo be removed from the yolk, the surface of the embryo
covering these two venous trunks can be spoken of as the ventral
surface of the embryo at that stage, and indeed we find that in the
present day there is an increasing tendency to speak of this surface
as the ventral surface of the embryo. Thus, Mitsukuri, in his studies
of chelonian embryos, lays great stress on the importance of surface
views and when the embryo has been removed from the yolk,
figures and speaks of its ventral surface. So, also, Locy and ISTeal
find that the best method of seeing the early segments of the embryo
is to remove the embryo from the yolk, and examine what they speak
of as a ventral view. At the period, then, before the formation of the
throat, we may say that on the ventral surface of the embryo a pair
of longitudinal venous sinuses are found, one on each side of the mid-
ventral line, which are in the same position with respect to the mid-
axis of the embryo as are the longitudinal venous sinuses in Limulus.

The next step is the formation of the throat by the extension of
the layers of the embryo laterally to meet in the mid-line and so
form the pharynx, with the consequence that a new ventral surface is
formed ; these two veins, as is well known, travel round also, and,
meeting together in the new mid-ventral line, form the subintestinal
vein, the heart, and the ventral aorta.

What is true of Mammalia and Aves, has been shown by P. Mayer
to be true universally among vertebrates, so that in all cases the heart
and ventral aorta have arisen by the coalescence in the new mid- ventral

i So

THE ORIGIN OF VERTEBRATES

C.N.S

LVS.

line of two longitudinal venous channels, which were originally situ-
ated one on each side of the notochord, in what was then the ventral

surface of this part of the embryo.
This history is especially in-
structive in showing how the
pharyngeal region is formed by
the growing round of the lateral
mesoblast, i.e. the muscular and
other mesoblastic tissues of the
branchial segments, and how the
two longitudinal veins take part
in this process. The phyloge-
netic interpretation of this em-
bryological fact seems to be,
that the new ventral surface of
the vertebrate in this region is
formed, not only by the branchial
appendages, but also by the
growth ventrally of that part
of the original ventral surface
which covered each longitudinal
venous sinus.

The following out of the
consecutive clues, w 7 hich one
after the other arise in har-
monious succession as the neces-
sary sequence of the original
working hypothesis, brings even now into view the manner in which
the respiratory portion of the alimentary canal arose, and gives
strong hints as to the position of that part of the arthropod which
gave origin to the notochord. Here I will say no more at present,
for the origin of the new alimentary canal of the vertebrate and of
the notochord will be more fittingly discussed as a whole, after all
the other organs of the vertebrate have been compared with the
corresponding organs of the arthropod.

The strong evidence that the vertebrate heart was formed from a
pair of longitudinal venous sinuses on the ventral side of the central
canal, carries with it the conclusion that the original single median
dorsal heart of the arthropod is not represented in the vertebrate,

Fig. 72. — Diagram (Upper Half op
Figure) of the Original Position
of Veins (H) which come together

TO FORM THE HEART OF A VERTEBRATE.

C.N.S., central nervous system; 71c,
notochord; m., myotome.

The lower half of figure shows compara-
tive position of the longitudinal venous
sinus (L.V.S.) in Limulus. C.N.S.,
central nervous system ; A I., alimentary
canal; H., heart ; m., body-muscles.

THE EVIDENCE OF THE RESPIRATORY APPARATUS l8l

for the dorsal aorta cannot by any possibility represent that
heart.

Although it is not now functional the original existence of so
important an organ as a dorsal heart may have left traces of its
former presence ; if so, such traces would be most likely to be visible
in the lowest vertebrates, just as the median eyes are much more
evident in them than in the higher forms. In Fig. 58 the position of
the dorsal heart is shown in Limulus, and in Fig. 70 the shape and
extent of this dorsal heart is shown. It extends slightly into the pro-
somatic region, and thins down to a point there, runs along the length
of the animal and finally thins down to a point at the caudal end.

The heart is surrounded by a pericardium, from which at regular
intervals a number of dorso-ventral muscles pass, to be inserted into
the longitudinal venous sinus on each side. These veno-pericardial
muscles are absolutely segmental with the mesosomatic segments,
and are confined to that region, with the exception of two pairs in the
prosomatic region. Their homologies will be discussed later.

Any trace of a heart such as we have just described must be
sought for in Ammocoetes between the central nervous system and
the mid-line dorsally. Now, in this very position a large striking
mass of tissue is found, represented in section in Fig. 73, /. It
forms a column of similar tissue along the whole mid-dorsal region,
except at the two extremities; it tapers away in the caudal region,
and headwards grows thinner and thinner, so that no trace of it is
seen anterior to the commencement of the branchial region. It
resembles in its dorsal position, in its shape, and in its size a dorsal
heart-tube such as is seen in Limulus and elsewhere, but it differs
from such a tube in its extension headwards. The heart-tube of
Limulus ceases at the anterior end of the mesosomatic region, this
fat- column of Ammoccetes at the posterior end. In its structure there
is not the slightest sign of anything of the nature of a heart ; it is
a solid mass of closely compacted cells, and the cells are all very
full of fat, staining intensely black with osmic acid. Nowhere else
in the whole body of Ammocostes is such a column of fat to be found.
It is not skeletogenous tissue with cells of the nature of cartilage-
cells, as Gegenbaur thought and as Balfour has depicted (Vol. II.,
Fig. 315) in his ' Comparative Embryology,' as though this tissue were
a pari of the vertebral column, but is simply fat-cells, such as might
easily have taken the place of some other previously existing organ.

182

THE ORIGIN OF VERTEBRATES

I do not know how to decide the question which thus arises.
Supposing, for the sake of argument, that this column of fat-cells
has really taken the place of the original dorsal heart, what criterion
would there he as to this ? The heart ex hypothesi having ceased to

function, the muscular tissue
would not remain, and the
space would he filled up,
presumably with some form
of connective tissue. As
likely as not, the connective
tissue might take the form
of fatty tissue, the storage
of fat being a physiological
necessity to an animal, while
at the same time no special
organ has been developed
for such a purpose, but fat
is being laid down in all
manner of places in the
body.

This dorsal fat-column,
as it is seen in Ammoccetes,
is not found in the higher
vertebrates, so that it pos-
sesses, at all events, the
significance of being a pecu-
liarity of ancient times
before the vertebrate skele-
tal column was formed.

I mention it here in
connection with my view

Fig. 73. — Section through the Notochord
(nc), the Spinal Canal and the Fat-
column (/.), of Ammoccetes, drawn from
an Osmic Preparation.

sp. c, spinal cord; gl., glandular tissue filling
the spinal canal; sk., Gegenbaur's skeleto-
genous cells ; p., pigment.

as to the origin of verte-

brates, because there it is,
in the very place where the
dorsal heart ought to have been. For my own part, I should not have
expected that a muscular organ such as the heart would leave any
trace of itself if it disappeared, so that its absence in the dorsal region
of the vertebrate does not seem to me in the slightest degree to
invalidate my theory.

THE EVIDENCE OF THE RESPIRATORY APPARATUS 1 83

Summary.

From the close similarity of structure and position between the branchial
skeleton of Limulus and of Arumocoetes, as given in the preceding chapter, it
logically follows that the branchiae of Ammocoetes must be homologous with the
branchiae of Limulus. But the respiratory apparatus of Limulus consists of
branchial appendages. It follows, therefore, that the branchiae of Ammocoetes.
and consequently of the vertebrates, must have been derived from branchial
appendages, and as they are internal, not external, such branchial appendages
must have been of the nature of ; sunk-in ' branchial appendages. Such
internal appendages are characteristic of the scorpion tribe, and of. perhaps,
the majority of the Palaeostraca, for no external respiratory appendages have
been discovered in any of the sea-scorpions.

In the vertebrates — and it is especially well shown in Ammocoetes — a double
segmentation exists in the head-region, a body or somatic segmentation, and
a branchial or splanchnic segmentation, respectively expressed by the terms
mesomeric and branchiomeric segmentations. The nerves which supply the
latter segments form a very well-marked group (Charles Bell's system of lateral
or respiratory nerves) which do not conform to the system of spinal nerves, for
they do not arise from separate motor and sensory roots, but are mixed nerves
from the very beginning.

The system of cranial segmental nerves is older than the spinal system, and
cannot, therefore, be derived from it, but can be arranged as a system supplying
two segments, somatic and splanchnic, which differ in the following way : Each
somatic segment is supplied by two roots, motor and sensory respectively, as in
the spinal cord segments, while each splanchnic segment possesses only one root,
which is mixed in function.

The peculiarities of the grouping of the cranial segmental nerves, which
have hitherto been unexplained, immediately receive a straightforward and
satisfactory explanation if the splanchnic or branchiomeric segments owe their
origin to a system of appendages after the style of those of Limulus.

In Limulus and all the Arthropoda, the segmentation is double, being com-
posed of (1) somatic or body-segments, constituting the mesomeric segmentation ;
(2) appendage-segments, which, seeing that they carry the branchiae, constitute
a branchiomeric segmentation. Similarly to the cranial region of the vertebrate,
the nerves which supply the somatic segments arise from separate sensory and
motor roots, while the single nerve which supplies each appendage contains all
the fibres for the appendage, both motor and sensory.

It follows from this that the branchial segments supplied by the vagus
and glossopharyngeal nerves ought to have arisen from appendages bearing
branchiae.

Although the evidence of such appendages has entirely disappeared in the
higher vertebrates, together with the disappearance of branchiae, and is not
strikingly apparent in the higher gill-bearing fishes, yet in Ammocoetes, so
great is the difference here from all other fishes, it is natural to describe the
pharyngeal or respiratory chamber as a chamber into which a symmetrical series
of respiratory appendages, the so-called diaphragms, are dependent. Each of
these appendages possesses its own mixed nerve, glossopharyngeal or vagus.

184 THE ORIGIN OF VERTEBRATES

its own cartilage, its own set of visceral muscles, its own sense-organs, just as
do the respiratory appendages of Limulus.

The branchial unit in the vertebrate is not the gill-pouch, but the branchial
bar or appendage between the pouches. Embryology shows how each such
appendage grows inwards, how a ccelomic cavity is formed in it, similarly to the
ingrowing of the branchial appendage in scorpions.

We do not know how the palteostracan sea-scorpions breathed ; they resemble
the scorpion of the present day somewhat in form, but they are in many respects
closely allied to Limulus. The present-day scorpion is a land animal, and the
muscles by which he breathes are dorso-ventral somatic muscles, while those of
Limulus are the appendage-muscles.

The old sea-scorpions very probably used their aj>pendage-muscles after the
Limulus fashion, being water-breathers, even although their respiratory appen-
dages were no longer free but sunk in below the surface of the body. The
probability that such was the case is increased after consideration of the method
of breathing in Ammoccetes, for the respiratory muscles of the latter animal are
directly comparable with the muscles of the respiratory appendages of Limulus,
and are not somatic. Even the gills themselves of Ammoccetes are built up in
the same fashion as are those of Limulus and the scorpions. The conception of
the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately
explains the formation of the vertebrate heart, which is so strikingly different
from that of all invertebrate hearts, in that it originates as a branchial and
not as a systemic heart, and is formed by the coalescence of two long-itudinal
veins.

The origin of these two longitudinal veins is immediately apparent if the
vertebrate arose from a palaeostracan, for in Limulus and the whole scorpion
tribe, in which the heart is a systemic heart, the branchife are supplied with
blood from two large longitudinal venous sinuses, situated on each side of the
middle line of the animal in an exactly corresponding position to that of the two
longitudinal veins, which come together to form the heart and ventral aorta of
the vertebrate. The consideration of the respiratory apparatus and of its blood-
supply in the vertebrate still further points to the origin of vertebrates from the
Palasostraca.

CHAPTEE V

THE EVIDENCE OF THE THYROID GLAND

The value of the appendage-unit in non-branchial segments. — The double nature
of the hyoid segment. — Its branchial part. — Its thyroid part. — The double
nature of the opercular appendage. — Its branchial part. — Its genital part.
— Unique character of the thyroid gland of Ammoccetes — Its structure. —
Its openings. — The nature of the thyroid segment. — The uterus of the
scorpion. — Its glands. — Comparison with the thyroid gland of Ammoccetes.
— Cephalic genital glands of Limulus. — Interpretation of glandular tissue
filling up the brain-case of Ammoccetes. — Function of thyroid gland. —
Relation of thyroid gland to sexual functions. — Summary.

I have now given my reasons why I consider that the glosso-
pharyngeal and vagus nerves were originally the nerves belonging to
a series of mesosoinatic branchial appendages, each of which is still
traceable in the respiratory chamber of Ammoco^tes, and gives the
type-form from which to search for other serially homologous,
although it may be specially modified, segments.

As long as the branchial unit consisted of the gill-pouch the
segments of the head-region were always referred to such units,
hence we find Dohrn and Marshall picturing to themselves the
ancestor of vertebrates as possessing a series of branchial pouches
right up to the anterior end of the body. Marshall speaks of
olfactory organs as branchial sense-organs ; Dohrn of the mouth as
formed by the coalescence of gill- slits, of the trigeminal nerve
as supplying modified branchial segments, etc. ; thus a picture of
an animal is formed such as never lived on this earth, or could be
reasonably imagined to have lived on it. Yet Dohrn's conceptions
of the segmentation were sound, his interpretation only was in
fault, because he was obliged to express his segments in terms of
the gill-pouch unit. Once abandon that point of view and take as
the unit a branchial appendage, then immediately we see that in
the region in front of the branchiae we may still have segments

1 86 THE ORIGIN OF VERTEBRATES

homologous to the branchial segments, originally characterized by
the presence of appendages, but that such appendages need never
have carried branchiae. The new mouth may have been formed by
such appendages, which would express Dohrn's suggestion of its
formation by coalesced gill-slits ; the olfactory organ may have been
the sense-organ belonging to an antennal appendage, which would
be what Marshall really meant in calling it a branchial sense-organ.

The Facial Nerve and the Foremost Eespiratory Segment.

This simple alteration of the branchiomeric unit from a gill-pouch
to an appendage, which may or may not bear branchiae, immedi-
ately sheds a flood of light on the segmentation of the head-region,
and brings to harmony the chaos previously existing. Let us, then,
follow out its further teachings. Next anteriorly to the glosso-
pharyngeal and vagus nerves comes the facial nerve ; a nerve which
supplies the hyoid segment, or, rather, according to van Wijhe the two
hyoid segments, for embryologically there is evidence of two segments.
As already mentioned, the facial nerve is usually included in the
trigeminal or pro-otic group of nerves, the opisthotic group being-
confined to the glossopharyngeal and vagus. This inclusion of the
facial nerve into the pro-otic group of nerves forms one of the main
reasons why this group has been supposed to have originally supplied
gill-pouch segments, for the hyoid segment is clearly associated with
branchiae.

When, however, we examine Ammoco^tes (cf. Figs. 63 and 64)
it is clear that the foremost of the segments forming the respiratory
chamber, which must be classed with the rest of the mesosomatic or
opisthotic segments, is that supplied by the facial nerves.

An examination of this respiratory chamber shows clearly that
there are six pairs of branchial appendages or diaphragms, which are
all exactly similar to each other. These are those already considered,
the foremost of which are supplied by the IXth or glossopharyngeal
nerves. Immediately anterior to this glossopharyngeal segment is
seen in the figures the segment supplied by the Vllth or facial
nerves. It is so much like the segments belonging to the glosso-
pharyngeal and vagus nerves as to make it certain that we are dealing
here with a branchial segment, composed of a pair of branchial
appendages similar to those in the other cases, except that the

Respiratory Append ages
$ Nerve Supply

Tertt.
Tent. in. c.

Hyoiti

6- Br

^6

Fig. 74. — Ventral half
of Head-region of Am-

MOCOiTES.

—-"-Pigment

Somatic muscles coloured
red. Branchial and visce-
ral muscles coloured blue.
Tubular constrictor mus-
cles distinguished from
striated constrictor mus-
cles by simple hatching.
Tent., tentacles ; Tent.m.c,
muco-cartilage of tenta-
cles; TV/, m.c, muco-car-
tilage of the velum ; Hy.
m.c, muco-cartilage of the
hyoid segment; Ps. br.,
pseudo-branchial groove ;
Hr. car/., branchial carti-
lages ; Sp., space between
somatic and splanchnic
muscles ; Th. op., orifice of
thyroid ; //., heart.

1 88 THE ORIGIN OF VERTEBRATES

cartilaginous bar is here replaced by a bar of muco-cartilage and
the branchiae are confined to the posterior part of each appendage.
The anterior portion is, as is seen in Fig. 74, largely occupied by
blood-spaces, but in addition carries the ciliated groove (ps. br.) called
by Dohrn ' pseudo-branchiale Einne.' This groove leads directly
into the thyroid gland, which is a large bilateral organ situated in
the middle line, as seen in Fig. 80 and Fig. 85. As shown by Miss
Alcock, the facial nerve supplies this thyroid gland, as well as the
posterior hyoid branchial segment, and, as pointed out by Dohrn,
there is every reason to consider this thyroid gland as indicative of
a separate segment, especially when van Wijhe's statement that the
hyoid segment is in reality double is taken into account.

The evidence, then, of Ammocoetes points directly to this con-
clusion : The facial nerves represent the foremost of the mesoso-
matic group of nerves, and supply two segments, which have amalga-
mated with each other. The most posterior of these, the hyoid
segment, is a branchial segment of the same character as those
supplied by the vagus and glossopharyngeal nerves ; represents,
therefore, the foremost pair of branchial appendages. The anterior or
thyroid segment, on the other hand, differs from the rest in that,
instead of branchiae, it carries the thyroid gland with its two ciliated
grooves. If this segment, which is the foremost of the mesosomatic
segments, also indicates a pair of appendages which carry the thyroid
gland instead of branchiae, then it follows that this pair of appendages
has joined together in the mid-line ventrally and thus formed a
single median organ — the thyroid gland. If, then, we find that the
foremost of the mesosomatic appendages in the Palaeostraca was really
composed of two pairs of appendages, of which the most posterior
carried branchiae, while the anterior pair had amalgamated in the
mid-line ventrally, and carried some special organ instead of
branchiae, then the accumulation of coincidences is becoming so
strong as to amount to proof of the correctness of our line of
investigation.

The First Mesosomatic Segment in Limulus and its Allies.

What, then, is the nature of the foremost pair of mesosomatic
appendages in Limulus. They differ from the rest of the mesosomatic
appendages in that they do not carry branchiae, and instead of being

THE EVIDENCE OF THE THYROID GLAND

189

separate are joined together in the mid-line ventrally to form a single
terminal plate-like appendage known as the operculum. On its
posterior surface the operculum carries the genital duct on each side.

So also in the scorpion group, the operculum is always found
aud always carries the genital ducts.

A survey of the nature of the opercular appendage demonstrates
the existence of three different types —

1. That of Lirnulus, in which the operculum is free, and carries
only the terminations of the genital ducts. In this type the duct on
each side opens to the exterior separately (Fig. 75).

2. The type of Scorpio, Androctonus, Buthus, etc., in which the

Fig. 75.— Operculum op Limulus to
show the two separate genital
Ducts.

Gen. duct

Fig. 76. — Operculum
Scorpion.

Gen. duct.

of Male

17., terminal chamber, or uterus.

operculum is not free, but forms part of the ventral surface of the
body-wall, but, like Limulus, carries only the terminations of
the genital ducts. In this type the duct on each side terminates
in a common chamber (vagina or uterus), which communicates with
the exterior by a single external median opening. This common
chamber, or uterus ( Ut.), extends the whole breadth of the operculum
(as seen in Fig. 76), and is limited to that segment.

3. The type of Thelyphonus, Hypoctonus, Phrynus, and other
members of the Pedipalpi, in which the operculum forms a part
of the ventral surface of the body wall, but no longer covers only
the termination of the genital apparatus. It really consists of two
parts, a median anterior, which covers the terminal genital apparatus,

i go

THE ORIGIN OF VERTEBRATES

Ut. Masc.

Int. Op.

Ext. Op.

and a lateral posterior, which covers the first pair of gills, or lung-
books, as they are called. In this type (Fig. 77) the genital ducts
terminate in a common chamber or uterus, the nature of which will
be further considered.

As has been pointed out by Blanchard, the terminal genital
organs of the scorpions and the Pedipalpi vary considerably in the
different genera, especially the male genital organs. The general
type of structure is the same, and consists in both male and female
of vasa deferentia, which come together to form a common chamber

before the actual opening
to the exterior. This com-
mon chamber has been
called in the female scor-
pion the vagina, or in
Thelyphonus the uterus.
I shall use the latter term,
in accordance with Tar-
nani's work, and the corre-
sponding chamber in the
male will be the uterus
masculinus.

A considerable discus-
sion has taken place about
the method of action of the
external genital organs in
the members of the scorpion
tribe, into which it is hardly
necessary to enter here.
The evidence points to the
conclusion that in all these forms the operculum covers a median
single chamber or uterus, into which the genital ducts open on each
side, the main channels of emission being provided with a massive
chitinous internal framework. We may feel certain that in the old
extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed,
and that therefore in them also the median portion of the operculum
covered a median chamber or uterus composed of the amalgamation
of the terminations of the two genital ducts, which were originally
separate, as in Limulus.

The observations of Schmidt, Zittel, and others show that the

Fig. 77.— Operculum and Following Seg-
ments of Male Thelyphonus.

Opercular segment is marked out by thick black
line. Ut. Masc, uterus masculinus ; Int. Op.,
internal opening of uterus into genital chamber ;
Ext. Op., common external opening to genital
chamber (Gen. Ch.) and pulmonary chamber.

THE EVIDENCE OF THE THYROID GLAND

I 9 I

operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus,
etc., belonged to the type of Thelyphonus, rather than to that of
Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the
ventral aspect of Eurypterus, and by the side of it a picture of the
isolated operculum. Schmidt considers that there were five branchiae-
bearing segments constituting the mesosoma, the foremost of which
formed the operculum. Such operculum is often found isolated, and
is clearly composed of two lateral
appendages fused together in the
middle line, of such a nature as to
form a median elongated tongue,
which lies between and separates
the first three pairs of branchial
segments. This median tongue,
together with the anterior and
median portion of the operculum,
concealed, in all probability, accord-
ing to Schmidt, the terminal parts
of the genital organs, just as the
median part of the operculum in
Phrynus and Thelyphonus conceals
the complicated terminal portions
of the genital organs. The posterior
part of the operculum, like that of
Phrynus and Thelyphonus, carried
the first pair of branchiae, so Schmidt
thinks from the evidence of markings
on some specimens.

Apparently an opercular ap-
pendage of this kind is in reality
the result of a fusion of the genital
operculum with the first branchial appendage in forms such as the
scorpion; for, in order that the tergal plates may correspond in
number with the sternal in Eurypterus, etc., it is necessary to
consider that the operculum is composed of two sternites joined
together. Similarly in Thelyphonus, Phrynus, etc., this numerical
correspondence is only observed if the operculum is looked upon
as double.

A restoration of the mesosomatic region of Eurypterus, viewed

Fig. 78. — Eurypterus.

The segments and appendages on the
right are numbered in correspon-
dence with the cranial system of
lateral nerve-roots as found in verte-
brates. M., metastoma. The sur-
face ornamentation is represented
on the first segment posterior to the
branchial segments. The opercular
appendage is marked out by dots.

192

THE ORIGIN OF VERTEBRATES

from the internal surface, might be represented by Fig. 79, in which
the thick line represents the outline of the opercular segment, and

the fainter lines the succeeding
branchial segments. The middle
and anterior part of the opercular
segment carried the terminations

Gen. dncfc.

TJt. M&sc.

Gen. duct.

Fig. 79. — Diagram to indicate the

PROBABLE NATURE OF THE MeSOSO-

matic Segments of Eurypterus.

The opercular segment is marked out by
the thick black line. The segments
II. -VI. bear branchiae, and segment I.
is supposed in the male to carry the
uterus masculinus (TJt. Masc.) and
the genital ducts.

of the ' genital

organs

these I

have represented, in accordance
with our knowledge of the nature
of these organs in the present-day
scorpions, as a median elongated
uterus, bilaterally formed, from
which the genital ducts passed,
probably as in Limulus, towards
a mass of generative gland in the
cephalic region, and not as in
Scorpio or Thelyphonus, tailwards
to the abdominal region.

It is possible that in Holm's
representation of Eurypterus, Fig.
104, the genital duct on each side
is indicated.

The Thyroid Gland of Ammoccetes.

If we compare this mesosomatic region of Eurypterus with that
of Ammoccetes, the resemblance is most striking, and gives a mean-
ing to the facial nerve which is in absolute accordance with the
interpretation already given of the glossopharyngeal and vagus
nerves. In both cases the foremost respiratory or mesosomatic
segment is double, the posterior lateral part alone bearing the
branchiae, while the median and anterior part bore in the one animal
the uterus and genital ducts, in the other the thyroid gland and
ciliated grooves. We are driven, therefore, to the conclusion that
this extraordinary and unique organ, the so-called thyroid gland of
Ammoccetes, which exists only in the larval condition and is got rid
of as soon as the adult sexual organs are formed, shows the very form
and position of the uterus of this invertebrate ancestor of Ammo-
ccetes. What, then, is the nature of the thyroid gland in Ammoccetes ?

THE EVIDENCE OF THE THYROID GLAXD

193

Throughout the vertebrate kingdom it is possible to compare the
thyroid gland of one group of animals with that of another without
coming across any very marked difference of structure right down
to and including Petromyzon. When, however, we examine Ammo-
coetes, we find that the thyroid has
suddenly become an organ of much
more complicated structure, covering a
much larger space, and bearing no re-
semblance to the thyroid glands of the
higher forms. At transformation the
thyroid of Animoccetes is largely de-
stroyed, and what remains of the gland
in Petromyzon becomes limited to a few
follicles resembling those of other fishes.
The structure and position of this gland
in Ammoccetes is so well known that it
is unnecessary to describe it in detail.
For the purpose, however, of making
my points clear, I give in Fig. 80 the
position and appearance of the thyroid
gland (Th.) when the skin and under-
lying laminated layer has been re-
moved by the action of hypochlorite of
soda. On the one side the ventral
somatic muscles have been removed to
show the branchial cartilaginous basket-
work.

The series of transverse sections in
Pig. 81 represents the nature of the
organ at different levels in front of and
behind the opening into the respiratory
chamber ; and in Fig. 82 I have sketched
the appearance of the whole gland,
viewed so as to show its opening
into the respiratory chamber and its posterior curled-up termi-
nation.

The series of transverse sections (1-6, Fig. 81) show that we are
dealing here with a central glandular chamber, C (Fig. 81 (6) and
Fig. 82), which opens by the thyroid duct (Th. 0.) into the pharyngeal

Fig. 80. — Ventral View op
Head Region of Ammoccetes.

Th., thyroid gland; M., lower
lip, with its muscles.

i 9 4

THE ORIGIN OF VERTEBRATES

\ Tko

4;-

5 6

Fig. 81. — Samples from a Complete Series of Transverse Sections through

the Thyroid Gland of Ammocoztes.

Sections 1 and 2 are anterior to the thyroid opening, Th. o. ; sections 3, 4, and 5 are
■ through the thyroid opening ; and section 6 is posterior to the thyroid opening
before the commencement of the curled portion.

THE EVIDENCE OF THE THYROID GLAND 1 95

chamber, and is curled upon itself in its more posterior part. This
central chamber divides, anteriorly to the thyroid orifice, into two
portions, A, A' (Fig. 82), giving origin to two tubes, B, B', which lie
close alongside of, and extend further back than, the posterior limit
of the curled portion of the central chamber, C. The structure of
the central chamber, C, and, therefore, of the separate coils, is given
in both Schneider's and Dohrn's pictures, and is represented in
Tig. 81 (6), which shows the peculiar arrangement and character of
the glandular cells typical of this organ, and also the nature of the
central cavity, with the arrangement of the ciliated epithelium. The
structure of each of the lateral tubes, B, is different from that of the
central chamber, in that only half the central chamber is present
in them, as is seen by the comparison of the tube B with the tube C
in Fig. 81 (5 and 6), so that we may look upon the central chamber,
C, as formed of two tubes, similar in structure to the tubes B, which
have come together to form a single chamber by the partial absorp-
tion of their walls, the remains of the wall being still visible as the
septum, which partially divides the chamber, Q, into halves.

In the walls of each of these tubes is situated a continuous
glandular line, the structure of the glandular elements being specially
characterized by the length of the cells, by the large spherical nucleus
situated at the very base of each cell, and by the way in which the
cells form a wedge-shaped group, the thin points of all the wedge-
shaped cells coming together so as to form a continuous line along
the chamber wall. This free termination of the cells of the glaud
in the lumen of the chamber constitutes the whole method for the
secretion of the gland ; there is no duct, no alveolus, nothing but this
free termination of the cells.

Moreover, sections through the portion A, A' (Fig. 82) show that
here, as in the central chamber, C, four of these glandular lines open
into a common chamber, but they are not the same four as in the case
of the central chamber, for if we name these glandular lines on the left
side a b, a V (Fig. 81), and on the right side c d, c' d', then the central
chamber has opening into it the glands a b,c d, while the chambers of
A and A' have opening into them respectively a b, a' V, and c d, c' d'.
Further, the same series of sections shows that the glands a and b are
continuous with the glands a' and b' respectively across the apex of A,
and similarly on the other side, so that the two glandular rows a b
are continuous with the two glandular rows a' //, and we see that the

196

THE ORIGIN OF VERTEBRATES

cavity of the portion A or A' is formed by the bending over of the tube
or horn, B or B', with the partial absorption of the septum so formed
between the tube and its bent-over part. If, then, we uncoil the
curled-up part of C, and separate the portion, B, on each side from the
chamber, C, we see that the so-called thyroid of Ammoccetes may be
represented as in Fig. 83, i.e. it consists of a long, common chamber, C,

Ps br!

Th. o .. -v-- : '''

Pit,

•) *

B

Fig. 82. — Diagbammatic Repbesentation of the so-called Thyboid Gland op

Ammoccetes.

C, central chamber; A, A', anterior extremity; B,B', posterior extremity; Tli.o.,
thyroid opening into respiratory chamber; Ps. br., Ps. br'., ciliated grooves,
Dohrn's pseudo-branchial grooves.

Fig. 83.— Thyboid Gland as it would appeab if the Centbal Chambeb were
Uncueled and the Two Hoens, B, B', sepaeated fbom the Centbal
Chambeb.

which, for reasons apparent afterwards, I will call the palceo-hysteron,
which opens, by means of a large orifice, into the respiratory or
pharyngeal chamber. The anterior end of this chamber terminates in
two tubes, or horns, B, B', the structure of which shows that the median
chamber, C, is the result of the amalgamation of two such tubes, and
consequently in this chamber, or palcco-hystcron, the glandular lines
are symmetrically situated on each side.

Any explanation, then, of the thyroid gland of Ammoccetes, must

THE EVIDENCE OF THE THYROID GLAND 1 97

take into account the clear evidence that it is composed of two
tubes, which have in part fused together to form an elongated central
chamber, in part remain as horns to that chamber, and that in its
walls there exist lines of gland-cells of a striking and characteristic
nature.

Further, this central chamber, with its horns, is not a closed
chamber, but is in communication with the pharyngeal or respiratory
chamber by three ways. In the first place, the central chamber, as
is well known, opens into the respiratory chamber by a funnel-shaped
opening — the so-called thyroid duct (Th. 0.). In the second place,
there exist two ciliated grooves (Ps. br., Ps. br'.), the pseudo-branchial
grooves of Dohrn, which have direct communication with the thyroid
chamber. The manner in which these grooves communicate with the
thyroid chamber has never, to my knowledge, been described pre-
viously to my description in the Journal of Physiology and Anatomy ;
it is very instructive, for, as I have there shown, each groove enters
into the corresponding lateral horn, so that, in reality, there are three
openings into the thyroid chamber or paleeo-hysteron — a median
opening into the central chamber, and a separate opening into each
lateral horn.

The system of ciliated grooves on the inner ventral surface of
the respiratory chamber of Ammoccetes was originally described by
Schneider as consisting of a single median groove, which extends
from the opening of the thyroid to the posterior extremity of the
branchial chamber, and a pair of grooves, or semi-canals, which,
starting from the region of the thyroid orifice, run head wards and
diverge from each other, becoming more and more lateral, and more
and more dorsal, till they come together in the mid-dorsal pharyngeal
line below the auditory capsules. The latter are the pseudo-branchial
grooves of Dohrn, of which I have already spoken. Schneider
looked upon the whole of this system as a single system, for he
speaks of " a ciliated groove, which extends from the orifice of the
stomach {i.e. anterior intestine) to the orifice of the thyroid, then
divides into two, and runs forward right and left of the median ridge,
etc." Dohrn rightly separates the median ciliated groove posterior
to the thyroid orifice (seen in Fig. 81 (6)) from the paired pseudo-
branchial grooves ; the former is a shallow depression which opens
into the rim of the thyroid orifice, while the latter has a much more
intimate connection with the thyroid gland itself.

198 THE ORIGIN OF VERTEBRATES

A series of sections, such as is given in Fig. 81, shows the relation
of this pair of ciliated grooves to the thyroid better than any elaborate
description. In the first place, it is clear that they remain separate
up to their termination — they do not join in the middle line to open
into the thyroid duct ; in the second place, they are separate from
the thyroid orifice — they do not terminate at the rim of the orifice,
as is the case with the median groove just mentioned, but continue
on each side on the wall of the thyroid duct (Fig. 81 (2)), gradually
moving further and further away from the actual opening of the duct
into the pharyngeal chamber. During the whole of their course on
the wall of the funnel-shaped duct they retain the character of
grooves, and are therefore open to the lumen of the duct. The direc-
tion of the groove (Ps. br.) shifts as it passes deeper and deeper
towards the thyroid, until at last, as seen in Fig. 81 (3 and 4), it is
continuous with the narrow diverticulum of the turned-down single
part of the thyroid (B), or turned-down horn, as I have called it.
In other words, the median chamber opens into the pharyngeal or
respiratory chamber by a single large, funnel-shaped opening, and, in
addition, the two ciliated grooves terminate in the lateral horns on
each side, and only indirectly into the central chamber, owing to their
being semi-canals, and not complete canals. If they were originally
canals, and not grooves, then the thyroid of Ammoccetes would be
derived from an organ composed of a large, common glandular
chamber, which opened into the respiratory chamber by means of an
extensive median orifice, and possessed anteriorly two horns, from
each of which a canal or duct passed headwards to terminate some-
where in the region of the auditory capsule.

Dohrn has pointed out that a somewhat similar structure and
topographical arrangement is found in Amphioxus and the Tunicata,
the gland-cells being here arranged along the hypobranchial groove
to form the endostyle and not shut off to form a closed organ, as in
the thyroid of Ammoccetes. Dohrn concludes, in my opinion rightly,
that the endostyle in the Tunicata and in Amphioxus represents the
remnants of the more elaborate organ in Ammoccetes, and that,
therefore, in order to explain the meaning of these organs in the
former animals, we must first find out their meaning in Ammoccetes.
Dohrn, however, goes further than this ; for just as he considers
Amphioxus and the Tunicata to have arisen by degeneration from an
Ammoccetes-like form, so he considers Ammoccetes to have arisen

THE EVIDENCE OF THE THYROID GLAND 1 99

from a degenerated Selachian ; therefore, in order to be logical, he
ought to show that the thyroid of Ammoccetes is an intermediate down-
ward step between the thyroid of Selachians and that of Amphioxus
and the Tunicates. Here, it seems to me, his argument utterly breaks
down ; it is so clear that the thyroid of Petromyzon links on to that
of the higher fishes, and that the Ammocoetes thyroid is so immeasur-
ably more complicated and elaborate a structure than is that of
Petromyzon, as to make it impossible to believe that the Ammoccetes
thyroid has been derived by a process of degeneration from that of
the Selachian. On the contrary, the manner in which it is eaten up
at transformation and absolutely disappears in its original form is,
like the other instances mentioned, strong evidence that we are
dealing here with an ancestral organ, which is confined to the larval
form, and disappears when the change to the higher adult condition
takes place. Dohrn's evidence, then, points strongly to the conclu-
sion that the starting-point of the thyroid gland in the vertebrate
series is to be found in the thyroid of Ammoccetes, which has given
rise, on the one hand, to the endostyle of Amphioxus and the Tuni-
cata, and on the other, to the thyroid gland of Petromyzon and the
rest of the Vertebrata.

The evidence which I have just given of the intimate connection
of the two pseudo-branchial grooves with the thyroid chamber shows,
to my mind, clearly that Dohrn is right in supposing that morpho-
logically these two grooves and the thyroid must be considered
together. His explanation is that the whole system represents a
modified pair of branchial segments distinct from those belonging to
the Vllth and IXth nerves. The cavity of the thyroid and the
pseudo-branchial grooves are, therefore, according to him, the remains
of the gill-pouches of this fused pair of branchial segments, which no
longer open to the surface, and the glandular tissue of the thyroid is
derived from the modified gill-epithelium. This view of Dohrn's,
which he has urged most strongly in various papers, is, I think,
right in so far as the separateness of the thyroid segment is con-
cerned, but is not right, and is not proven, iu so far as concerns the
view that the thyroid gland is a modified pair of gills.

We may distinctly, on my view, look upon the thyroid segment,
with its ciliated grooves and its covering plate of muco-cartilage, as
a distinct paired segment, homologous with the branchial segments,
without any necessity of deriving the thyroid gland from a pair of gills,

200

THE ORIGIN OF VERTEBRATES

The evidence that such a median segment has been interpolated
ventrally between the foremost pairs of branchial segments is
remarkably clear, for the limits ventrally of the branchial segments
are marked out on each side by the ventral border of the carti-
laginous basket-work ; and it is well known, as seen in Fig. 80, that
whereas this cartilaginous framework on the two sides meets together
in the middle ventral line in the posterior branchial region, it diverges
in the anterior region so as to form a tongue-shaped space between

-. IX

. X J

X 2
X 3

4-u.LatVII + X

Fig. 84.— Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammo-

C03TES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PlTS ON THE BRAN-
CHIAL Region, and their innervation by VII., the Facial, IX., the
Glossopharyngeal, and X'-X", the Vagus Nerves.

the branchial segments on the two sides. This space is covered over
with a plate of muco-cartilage which bears on its inner surface the
thyroid gland.

In addition to this evidence that we are dealing here with a
ventral tongue-like segment belonging to the facial nerve which is
interpolated between the foremost branchial segments, we find the
most striking fact that at transformation the whole of this muco-
cartilaginous plate disappears, the remarkable thyroid gland of the

THE EVIDENCE OF THE THYROID GLAND

20I

Ammocoetes is eaten up, and nothing is left except a small, totally
different glandular mass ; and now the cartilaginous basket-work
meets together in the middle line in this region as well as in the
more posterior region. In other words, the striking characteristic

\— -v

Ps.br

£&**'

8 X,

9 X 6

Fig. 85. — Facial Segment op Ammoccetes maeked out by Shading.

VII. 1, thyroid part of segment ; VII. 2, hyoid or branchial part ; 3-9, succeeding
branchial segments belonging to IXth and Xth nerves ; V, the velar folds ;
Ps. br., Dohrn's pseudo-branchial groove; Th. o., thyroid opening; C, curled
portion of thyroid.

of transformation here is the destruction of this interpolated seg-
ment, and the resulting necessary drawing together ventrally of
the branchial segments on each side.

Moreover, another most instructive piece of evidence pointing in
the same direction is afforded by the behaviour of the ventral epithelial

202 THE O RIG IX OF VERTEBRATES

pits, as determined by Miss Alcock. Although there is no indication
on the ventral surface of the skin of any difference between the
anterior and posterior portions of the respiratory region, yet when
the ventral rows of the epithelial pits supplied by each branchial
nerve are mapped out, we see how the most anterior ones diverge
more and more from the mid-ventral line, following out exactly the
limits of the underlying muco-cartilaginous thyroid plate (Fig. 84).

The whole evidence strongly leads to the conclusion that the
thyroid portion of the facial segment was inserted as a median tongue
between the foremost branchial segments on each side, and that,
therefore, the whole facial segment, consisting as it does of a thyroid
part and a hyoid or branchial part, may be represented as in Fig.
85, which is obtained by splitting an Ammoccetes longitudinally
along the mid-dorsal line, so as to open out the pharyngeal chamber
and expose the whole internal surface. The facial segment is marked
out by shading lines, the glosso-pharyngeal and vagus segments and
the last of the trigeminal segments being indicated faintly. The
position of the thyroid gland is indicated by oblique lines, C being
the curled portion.

The Uterus of the Scorpion Group.

Seeing how striking is the arrangement and the structure of the
glandular tissue of this thyroid, how large the organ is and how
absolutely it is confined to Ammoccetes, disappearing entirely as
such at transformation, we may feel perfectly certain that a corre-
sponding, probably .very similar, organ existed in the invertebrate
ancestor of the vertebrate ; for the transformation process consists
essentially of the discarding of invertebrate characteristics and the
putting on of more vertebrate characters ; also, so elaborate an organ
cannot possibly have been evolved as a larval adaptation during the
life of Ammoccetes. We may therefore assert with considerable con-
fidence that the thyroid gland was the iKiloco-liysieroiii, and was
derived from the uterus of the ancient pala^ostracan forms. If, then,
it be found that a glandular organ of this very peculiar structure and
arrangement is characteristic of the uterus of any living member of
the scorpion group, then the confidence of this assertion is greatly
increased.

In Limulus, as already stated, the genital ducts open separately

THE EVIDENCE OF THE THYROID GLAND

203

ou each side of the operculum, and do not combine to form a
uterus ; I have examined them and was unable to find any glandular
structure at all resembling that of the thyroid gland of Ammoccetes.
I then turned my attention to the organs of the scorpion, in which
the two ducts have fused to form a single uterus.

I there found that both in the male and in the female the genital

Fig. 86. — Section through the Terminal Chamber or Uterus of the Male

Scorpion.

C, cavity of chamber. A portion of the epithelial lining of the channels of emission
is drawn above the section of the uterus.

ducts on each side terminate in a common chamber or uterus, which
underlies the whole length of the operculum, and opens to the
exterior in the middle line, as shown in Fig. 76. In transverse
section, this uterus has the appearance shown in Fig. 86, i.e. it is
a large tube, evidently expansible, lined with a chitinous layer and
epithelial cells belonging to the chitinogenous layer, except in two
symmetrical places, where the uniformity of the uterine wall is

204

THE ORIGIN OF VERTEBRATES

interrupted by two large, remarkable glandular structures. The
structure of these glands is better shown by means of sagittal sec-
tions. They are composed of very long, wedge-shaped cells, each of
which possesses a large, round nucleus at the basal end of the cell
(Pig. 87). These cells are arranged in bundles of about eight to ten,
which are separated from each other by connective tissue, the apex
of each conical bundle being directed into the cavity of the uterus ;
where this brush -like termination of the cells reaches the surface, the
chitinous layer is absent, so that this layer is, on surface view, seen

Fig. 87. — Longitudinal Sec-
tion THROUGH THREE OF

the Cones op the Uterine
Glands op the Scorpion.

Fig. 88. — Sagittal Section through
the Uterine Gland of Scorpion,
showing the internal chitinous
Surface (b) and the Glandular
Cones (a) cut through at various

DISTANCES FROM THE INTERNAL SUR-
FACE.

(Fig. 88 (b)) to be pitted with round holes over that part of the
internal surface of the uterus where these glands are situated. Each
of these holes represents the termination of one of these cone-shaped
wedges of cells. If the section is cut across at right angles to the
axis of these cones, then its appearance is represented in Fig. 88 («),
and shows well the arrangement of the blocks of cells, separated from
each other by connective tissue. When the section passes through
the basal part of the cones, and only in that case, then the nuclei
of the cells appear, often in considerable numbers in one section, as

THE EVIDENCE OF THE THYROID GLAXD

205

is seen in Yv* 89. In Fig. 88 the section shows at b the holes in
the chitin in which the cones terminate, and then a series of layers
of sections through the cones further and
further away from their apices.

These conical groups of long cells, repre-
sented in Fig. 87, form on each side of the
uterus a gland, which is continuous along
its whole length, and thus forms a line
of secreting surface on each side, just as
in the corresponding arrangement of the
glandular structures in the thyroid of Am-
moccetes. This uterus and glandular ar-
rangement is found in both sexes ; the gland is, however, more
developed in the male than in the female scorpion.

The resemblance between the structure of the thyroid of Ammo-
ccetes and the uterus of the scorpion is most striking, except in two
respects, viz. the nature of the lining of the non-glandular part of
the cavity — in the one case ciliated, in the other chitinous — and the
place of exit of the cavity, the thyroid of Ammoccetes opening into

Fig. 89. — Transverse Sec-
tion THROUGH THE BASAL

Part of the Uterine
Glands op the Scorpion.

AMMOCCETES.

SCORPION.

Muco-cartilage Operculum

Branchial cartilage
Fig. 90.— Section op Central Chamber op Thyroid op Ammoccetes and Section

of Uterus of Scorpion.

the respiratory chamber, while the uterus of Scorpio opens direct to
the exterior.

With respect to the first difference, the same difficulty is met

206 THE ORIGIN OF VERTEBRATES

with in the comparison of the ciliated lining of the tube in the
central nervous system of vertebrates with the chitinous lining
of the intestine in the arthropod. Such a difference does not seem
to me either unlikely or unreasonable, seeing that cilia are found
instead of chitin in the intestine of the primitive arthropod Peri-
patus. Also the worm- like ancestors of the arthropods almost
certainly possessed a ciliated intestine. Finally, the researches of
Hardy and McDougall on the intestine of Daphnia point directly to
the presence of a ciliated rather than a chitinous epithelial lining of
the intestine in this animal — all evidence pointing to the probability
that in the ancient arthropod forms, derived as they were from the
annelids, the intestine was originally ciliated and not chitinous. It
is from such forms that I suppose vertebrates to have sprung, and
not from forms like the living king-crabs, scorpions, Apus, Bran-
chipus, etc. I only use them as illustrations, because they are the
only living representatives of the great archaic group, from which
the Crustacea, Arachnida, and Vertebrata all took origin.

The second difference is more important, and is at first sight
fatal to any comparison between the two organs. How is it possible
to compare the uterus of the scorpion, which opens on the surface by
an external genital opening, with the thyroid of Amnioccetes, which
opens by an internal opening into the respiratory chamber ? However
close may be the histological resemblance of structure in the two
cases, surely such a difference is too great to be accounted for.

It is, however, to be remembered that the operculum of Scorpio
covers only the terminal genital apparatus, and does not, therefore,
resemble the operculum of the presumed ancestor of Ammoccetes,
which, as already argued, must have resembled the operculum of
Thelyphonus with its conjoint branchial and genital apparatus,
rather than that of Scorpio. Before, therefore, making too sure of
the insuperable character of this difficulty, we must examine the
uterus of the Pedipalpi, and see the nature of its opening.

The nature of the terminal genital organs in Thelyphonus has
been described to some extent by Blanchard, and more recently by
Tarnani. The ducts of the generative organs terminate, according to
the latter observer, in the large uterus, which is found both in the
male and female ; he describes the walls of the uterus in the female
as formed of elongated glandular epithelium, with a strongly-
developed porous, chitinized intima. In the male, he says that the

THE EVIDENCE OF THE THYROID GLAND

207

epithelium of the uterus masculiuus and its processes is extraordi-
narily elongated, the chitin covering being thick. In these animals,
then, the common chamber or uterus into which the genital ducts
empty, which, like the corresponding chamber in the scorpion,
occupies the middle region of the operculum, is a large and con-
spicuous organ. Further, and this is a most striking fact, the
uterus masculinus does not open direct to the exterior, but into the
genital cavity, " which lies above the uterus, so that the latter is
situated between the lower wall of the genital cavity and the outer
integument." The opening,
therefore, of the uterus is not
external but internal, into the ^ A

large internal space known
as the genital cavity. The
arrangement is shown in Fig.
91, taken from Tarnani's
paper, which represents a
diagrammatic sagittal section
through the exit of the male
genital duct. Yet another
most striking fact is described
by Tarnani. This genital
cavity is continuous with the

Gen . Ch.

I

Ut.Masc.

— I -II

Ut.Masc.

1 - II

WJ— -Int. Op
--Eart.Op.

.---Ill

Fig. 91. — Sagittal Median Diagrammatic
Section through the Operculum of the
Male Thelyphonus. (From Tarnani.)

pulmonary or gill cavities on The thick line is the operoulum> composed of

each side, SO that instead of a two segments, I. and II. Ut. Masc, uterus
single opening for the genital masculinus ; Gen. Ch., genital chamber ; Int.
L ° Op., internal opening ; Ext. Op., external

products and one on each Side opening common to the genital and respira-
for each gill-pouch, as would tory organs,
be the case if the arrangement

was of the same kind as in the scorpion, there is a single large
chamber, the genital chamber, common to both respiratory and
genital organs.

This genital chamber, according to Tarnani, opens to the exterior
by a single median opening between the operculum and the succeed-
ing segment ; similarly, a communication from side to side exists
between the second pair of gill-pouches. I have been able to
examine Hypoetonm formosus and Thelyphonits caudatus, and in both
cases, in both male and female, the opening to the exterior of the
common chamber for respiration and for the genital products was

208 THE ORIGIN OF VERTEBRATES

not a single opening, as described by Tarnani in Thelyphonus aspe-
ratus, but on each side of the middle line, a round orifice closed by a
lid, like the nest of the trapdoor spider, led into the common genital
chamber (Gen. Ch.) into which both uterus and gills opened. In
Fig. 77 I have endeavoured to represent the arrangement of the
genital and respiratory organs in the male Thelyphonus according to
Tarnani's and my own observations.

If we may take Thelyphonus as a sample of the arrangement in those
scorpions in which the operculum was fused with the first branchial
appendage, among which must be included the old sea-scorpions, then
it is most significant that their uterus should open internally into a
cavity which was continuous with the respiratory cavity. Thus not
only the structure of the gland, but also the arrangement of the internal
opening into the respiratory, or, as it became later, the pharyngeal
cavity, is in accordance with the suggestion that the thyroid of Ammo-
ccetes represents the uterus of the extinct Eurypterus-like ancestor.

Into this uterus the products of the generative organs were poured
by means of the vasa deferentia, so that there was not a single
median opening or duct in connection with it, but also two side
openings, the terminations of the vasa deferentia. These are described
by Tarnani in Thelyphonus as opening into the two horns of the
uterus, which thus shows its bilateral character, although the body
of the organ is median and single ; these ducts then pass within the
body of the animal, dorsal to the uterus, towards the testes or ovaries
as the case may be, organs which are situated in these animals, as in
other scorpions, in the abdomen, so that the direction of the ducts
from the generative glands to the uterus is headwards. If, however,
we examine the condition of affairs in Limulus, we find that the
main mass of the generative material is cephalic, forming with the
liver that dense glandular mass which is packed round the supra -
(esophageal and prosomatic ganglia, and round the stomach and
muscles of the head-region. From this cephalic region the duct
passes out on each side at the junction of the prosomatic and nieso-
somatic carapace to open separately on the posterior surface of the
operculum, near the middle line, as is indicated in Fig. 75.

We have, therefore, two distinct possible positions for the genital
ducts among the group of extinct scorpion-like animals, the one
from the cephalic region to the operculum, and the other from the
abdominal region to the operculum.

THE EVIDENCE OF THE THYROID GLAND 209

The Generative Glands of Limulus and its Allies.

The whole argument, so far, has in every case ended with the
conclusion that the original scorpion-like form with which I have
been comparing Aminoccetes resembled in many respects Limulus
rather than the present-day scorpions, and therefore in the case also
of the generative organs, with which the thyroid gland or palteo-
hysteron was in connection, it is more probable that they were
cephalic in position rather than abdominal. If this were so, then
the duct on each side, starting from the median ventral uterus, would
take a lateral and dorsal course to reach the huge mass of generative
gland lying within the prosomatic carapace, just as I have repre-
sented in the figure of Eurypterus (Fig. 79), a course which would
take much the same direction as the ciliated groove in Ammocuetes.

We ought, therefore, on this supposition, to expect to find the
remains of the invertebrate generative tissue, the ducts of which
terminated in the thyroid, in the head-region, and not in the
abdomen.

Upon removal of the prosomatic carapace of Limulus, a large
brownish glandular-looking mass is seen, in which, if it happens to
be a female, masses of ova are very conspicuous. This mass is com-
posed of two separate glands, the generative glands and the hepatico-
pancreatic glands — the so-called liver — and surrounds closely the
central nervous system and the alimentary canal. From the genera-
tive glands proceed the genital ducts to terminate on the posterior
surface of the operculum. From the liver ducts pass to the pyloric
end of the cephalic stomach, and carry the fluid by means of which
the food is digested, for, in all these animals, the active digesting
juices are formed in the so-called liver, and not in the cells of the
stomach or intestine.

It is a very striking fact that the brain of Ammoccotes is much
too small for the brain-case, and that the space between brain and
brain-case is filled up with a very peculiar glandular-looking tissue,
which is found in Ammocoetes and not elsewhere. Further, it is also
striking that in the brain of Ammocoetes there should still exist the
remains of a tube extending from the IVth ventricle to the surface at
the conns post-eommismralis, which can actually be traced right into
this tissue on the outside of the brain (see Fig. 13, a-e, PI. XXVI.,
in my paper in the Quarterly Journal of Microscopical Science).

2IO

THE ORIGIN OF VERTEBRATES

This, in my opinion, is the last remnant of one of the old liver-ducts
which extended from the original stomach and intestine into the
cephalic liver-mass. This glandular-looking material is shown
surrounding the pineal eye and its nerve, in Fig. 31, also in
Fig. 22, and separately in Fig. 92. It is composed of large cells,
with a badly staining nucleus, closely packed together with lines
of pigment here and there between the cells ; this pigment is
especially congregated at the spot where the so-called liver-duct
loses itself in this tissue. The protoplasm in these large cells does
not stain well, and with osmic acid gives no sign of fat, so that

Ahlborn's description of this tissue as a
peculiar arachnoideal fat -tissue is not
true ; peculiar it certainly is, but fatty
it is not.

/ : .. '"l^^V ^ n * s ki ssue nas Deen largely de-

: 7\ scribed as a peculiar kind of connective

tissue, which is there as packing mate-
rial, for the purpose of steadying a brain
too small for its case. On the face of
it such an explanation is unscientific ;
certainly for all those who really believe
in evolution, it is out of the question
to suppose that a brain-case has been
laid down in the first instance too large for the brain, in order
to provide room for a subsequent increase of brain ; just as it is
out of the question to suppose that the nervous system was laid
down originally as an epithelial tube in order to provide for the
further development of the nervous system by the conversion of
more and more of that tube into nervous matter. Yet this latter
proposition has been seriously put forward by professed believers in
evolution and in natural selection.

This tissue bears no resemblance whatever to any form of con-
nective tissue, either fatty or otherwise. By every test this tissue
tells as plainly as possible that it is a vestige of some former
organ, presumably glandular, which existed in that position ; that
it is not there as packing material because the brain happened
to be too small for its case, but that, on the contrary, the brain
is too small for its case, because the case, when it was formed,
included this organ as well as the brain ; in other words, this tissue

Fig. 92. — Drawing of the
Tissue which surrounds
the Brain op Ammocoetes.

THE EVIDENCE OF THE THYROID GLAND 211

is there because it is the remnant of the great glandular mass which
so closely surrounds the brain and alimentary canal in animals such
as Limulus. In my paper in the Quarterly Journal of Microscopical
Science, in which I was comparing the tube of the vertebrate nervous
system with the alimentary canal of the invertebrate, I spoke of this
tissue as being the remnant of the invertebrate liver. At the same
time the whole point of my argument was that the glandular material
surrounding the brain of Limulus was made up of two glands — liver
and generative gland — so that this tissue might be the remnant of
either one or the other, or both. All I desired, at that time, was
to point out the glandular appearance of this so-called packing tissue,
which surrounded the brain-region of Ammoccetes, in connection with
the fact that the brain and alimentary canal of Limulus were closely
surrounded with a glandular mass composed partly of liver, partly of
the generative gland. At present, I think these large cells found
round the brain in Ammoccetes are much more likely to be the
remnant of the generative gland than of the liver ; the size of the
cells and their arrangement recalls Owen's picture of the generative
gland in Limulus, and seeing how important all generative glands
are in their capacity of internal secreting glands, apart entirely from
the extrusion of the ripe generative products, and how unimportant
is an hepato-pancreas when the alimentary canal is closed, it is much
more likely that of the two glands the former would persist longer
than the latter. It may be that all that is left of the old hepato-
pancreas consists of the pigment so markedly found in between these
cells, especially at the place where the old liver-duct reaches the
surface of the brain ; just as the only remnant of the two pineal eyes
in the higher vertebrates is the remains of the pigment, known as
brain-sand, which still exists in the pineal gland of even the highest
vertebrate. This, however, is a mere speculation of no importance.
What is important is the recognition of this tissue round the brain
as the remnant of the glandular mass round the brain of animals such
as Limulus. Still further confirmation of the truth of this comparison
will be given when the origin of the auditory organ comes up for
discussion.

I conclude, therefore, from the evidence of Ammoccetes, that the
generative glands in the ancestral form were situated largely in the
cephalic region, and suggest that the course and direction of the ciliated
pseudo-branchial grooves on each side indicate the direction of the

2 12 THE ORIGIN OF VERTEBRATES

original opercular ducts by which the generative products were con-
veyed to the uterine chamber, i.e. to the chamber of the thyroid
gland, and thence to the common genital and respiratory cavity, and
so to the exterior.

It is easy to picture the sequence of events. First, the generative
glands, chiefly confined to the cephalic region, communicating with
the exterior by separate ducts on the inner surface of the operculum
as in Limulus. Then, in connection with the viviparous habit, these
two oviducts fused together to form a single chamber, covered by the
operculum, which opened out to the exterior by a single opening as
in Scorpio : or, in forms such as Eurypterus, in which the operculum
had amalgamated with the first branchial appendage and possessed a
long, tongue-like ventral projection, the amalgamated ducts formed
a long uterine chamber which opened internally into the genital
chamber — a chamber which, as in Thelyphonus, was common with
that of the two gill-chambers, while at the same time the genital ducts
from the cephalic generative material opened into two uterine horns
which arose from the anterior part of the uterus, as in Thelyphonus.

Such an arrangement would lead directly to the condition found
in Ammoccetes, if the generative material around the brain lost its
function, owing to a new exit for generative products being formed
in the posterior part of the body. The connection of the genital duct
with this cephalic gland being then closed and cut off by the brain-
case, the position of the oviducts would still be shown by the ciliated
grooves opening into the folded-down thyroid tube, i.e. the folded-
down horns of the uterus ; the uterus itself would remain as the
main body of the thyroid and still open by a conspicuous orifice into
the common respiratory chamber. Next, in the degeneration process,
we may suppose that not only the oviducts opened out to form the
ciliated groove, but that the uterine chamber itself also opened out,
and thus formed the endostyle of Amphioxus and of the Tunicata.

It might seem at first sight improbable that a closed tube should
become an open groove, although the reverse phenomenon is common
enough ; the difficulty, however, is clearly not considered great, for it
is precisely what Dohrn imagines to have taken place in the conversion
of the thyroid of Ammoco^tes into the endostyle of Amphioxus and
the Tunicata ; it is only carrying on the same idea a stage further to
see in the open, ciliated groove of Ammocectes the remains of the
closed genital duct of Limulus and its allies.

THE EVIDENCE OF THE THYROID GLAND 213

Such is the conclusion to which the study of the thyroid gland
in Ammoccetes seems to me to lead, and one cannot help wondering
why such an unused and rudimentary organ should have remained
after its original function had gone. Is it possible to find out its
function in Ammocretes ?

The Function of the Thyroid Gland in Ammoccetes.

The thyroid gland has been supposed to secrete mucus into the
respiratory chamber for the purpose of entangling the particles of food,
and so aiding in digestion. I see no sign of any such function ; neither
by the thionin method, nor by any other test, have Miss Alcock and
myself ever been able to see any trace of mucous secretion in the thy-
roid, and, indeed, the thyroid duct is always remarkably free from any
sign of any secretion whatever. Not only is there no evidence of any
mucous secretion in the thyroid of the fully developed Ammoccetes,
but also no necessity for such secretion from Dohrn's point of view,
for so copious a supply of mucus is poured out by the glands of the
branchiae, along- the whole pharyngeal tract, especially from the cells
of the foremost or hyoid gills, as to mix up with the food as
thoroughly as can possibly be needed. Further, too, the ciliated
pharyngeal bands described by Schneider are amply sufficient to
move this mixed mass along in the way recpiired by Dohrn. Finally,
the evidence given by Miss Alcock is absolutely against the view that
the thyroid takes any part in the process of digestion, while, on the
other hand, her evidence directly favours the view that these
glandular branchial mucus-secreting cells play a most important part
in the digestive process.

In Fig. 93, A is a representation of the respiratory tissue of a
normal gill ; B is the corresponding portion of the first or hyoid gill,
in which, as is seen, the whole of the respiratory epithelium is
converted into gland-tissue of the nature of mucous cells.

To sum up, the evidence is clear and conclusive that the Ammo-
cartes possesses in its pharyngeal chamber mucus-secreting glands,
which take an active part in the digestive process, which do not in
the least resemble either in structure or arrangement the remarkable
cells of the thyroid gland, and that the experimental evidence that
the latter cells either secrete mucus or take any part in digestion
is so far absolutely negative. It is, of course, possible, that they

214

THE ORIGIN OF VERTEBRATES

may contain mucin in the younger developmental stages, and there-
fore possible that they might at that stage secrete it ; they certainly,
however, show no sign of doing so in their more adult condition, and
cannot be compared in the very faintest degree to the glandular cells
of the pharyngeal region. It is also perfectly possible for gland-cells
belonging to a retrograde organ to become mucus-secreting, and so to

give rise to the cells of Am-
phioxus and the Tunicata.

If, then, these cells were
not retained for digestive
purposes, what was their
function ? To answer this
question we must first know
the function of the corre-
sponding gland-cells in the
uterus of the scorpion, which
undoubtedly secreted into
the cavity of the uterus and
took some part in connection
with the generative act, and
certainly not with digestion.
What the function of these
cells is or in what way they
act I am unable at present
to say. I can only suppose
that the reason why the
thyroid gland has persisted
throughout the vertebrate
kingdom, after the genera-
tive tissues had found a new
outlet for their products in the body-cavity of the posterior region,
is because it possessed some important function in addition to that
connected with the exit of the products of the generative organs ; a
function which was essential to the well-being, or even to the life of
the animal. We do not know its function in the scorpion, or the
nature of its secretion in that animal. We know only that physiology
at the present day has demonstrated clearly that the actual external
secretion of a gland may be by no means its most important function ;
in addition, glands possess what is called an internal secretion, viz. a

Fig. 93.— A, Portion op a Gill op Ammo-

C03TES WITH ORDINARY RESPIRATORY EPI-
THELIUM ; B, Corresponding Portion of
the First or Hyoid Gill.

THE EVIDENCE OF THE THYROID GLAND 215

secretion into the blood and lymph, and this latter secretion may be
of the most vital importance. Now, the striking fact forces itself
prominently forward, that the thyroid gland of the higher vertebrates
is the most conspicuous example of the importance of such internal
secretion. Here, although ductless, we have a gland which cannot
be removed without fatal consequences. Here, in the importance of
its internal secretion, we have a reason for the continued existence
of this organ ; an organ which remains much the same throughout
the Yertebrata down to and including Petromyzon, but, as is seen
at transformation, is all that remains of the more elaborate, more
extensive organ of Ammoccetes. Surely we may argue that it is
this second function which has led to the persistence of the thyroid,
and that its original form, without its original function, is seen in
Ammoccetes, because that is a larval form, and not a fully-developed
animal. As soon as the generative organs of Petromyzon are developed
at transformation, all trace of its connection with a genital duct
vanishes, and presumably its internal secretory function alone remains.

Yet, strange to say, a mysterious connection continues to exist
between the thyroid gland and the generative organs, even up to
the highest vertebrate. That the thyroid gland, situated as it is
in the neck, should have any sympathy with sexual functions if it
was originally a gland concerned with digestion is, to say the least
of it, extremely unlikely, but, on the contrary, likely enough if it
originated from a glandular organ in connection with the sexual
organs of the palasostracan ancestor of the vertebrate.

Freund has shown, and shown conclusively, that there is an
intimate connection between the condition of the thyroid gland and
the state of the sexual organs, not only in human beings, but also
in numerous animals, such as dogs, sheep, goats, pigs, and deer. He
points out that the swelling of the gland, which occurs in consequence
of sexual excitement (a fact mentioned both in folk-lore tales and in
poetical literature), and also the swelling at the time of puberty, may
both lead to a true goitrous enlargement ; that most of the permanent
goitres commence during a menstrual period ; that during pregnancy
swelling of the thyroid is almost universal, and may become so ex-
treme as to threaten suffocation, or even cause death ; that the period
of puberty and the climacteric period are the two maximal periods
for the onset of goitre, and that exophthalmic goitre especially is
associated with a special disease connected with the uterus.

2l6 THE ORIGIN OF VERTEBRATES

Summary.

Step by step in the preceding chapters the evidence is accumulating- in
favour of the origin of vertebrates from a member of the palseostracan group.
In a continuously complete and harmonious manner the evidence has throughout
been most convincing when the vertebrate chosen for the purpose of my argu-
ments has been Ammoccetes.

So many fixed points have been firmly established as to enable us to proceed
further with very great confidence, in the full expectation of being able
ultimately to homologize the Vertebrata with the Palfleostraca even to minute
details.

Perhaps the most striking and unexpected result of such a comparison is the
discovery that the thyroid gland is derived from the uterus of the palfeostracan
ancestor. Yet so clear is the evidence that it is difficult to see how the homology
can be denied.

In the one animal (Palasostraca) the foremost pair of mesosomatic appendages
forms the operculum, which always bears the terminal generative organs and
is fused in the middle line. In many forms, essentially in Eurypterus and the
ancient sea-scorpions, the operculum was composed of two segments fused
together : an anterior one which carried the uterus, and a posterior one which
carried the first pair of branchiae.

In the other animal (Ammoccetes) the foremost segments of the mesosomatic
or respiratory region, immediately in front of the glossopharyngeal segments,
are supplied by the facial nerve, and are markedly different from those supplied
by the vagus and glossopharyngeal, for the facial supplies two segments fused
together ; the anterior one, the thyroid segment, carrying the thyroid gland,
the posterior one, the hyoid segment, carrying the first pair of branchiae.

Just as in Eurypterus the fused segment, carrying- the uterus on its internal
surface, forms a long- median tongue which separates the most anterior branchial
segments on each side, so also the fused segment carrying the thyroid forms in
Ammoccetes a long median tongue, which separates the most anterior branchial
segments on each side.

Finally, and this is the most conclusive evidence of all, this thyroid gland
of Ammocoetes is totally unlike that of any of the higher vertebrates, and.
indeed, of the adult form Petromyzon itself, but it forms an elaborate com-
plicated organ, which is directly comparable with the uterus and genital ducts
of animals such as scorpions. Not only is such a comparison valid with respect to
its shape, but also with respect to its structure, which is absolutely unique among-
vertebrates, and very different to that of any other vertebrate gland, but
resembles in a striking- manner a glandular structure found in the uterus, both
of male and female scorpions.

The generative glands in Limulus, together with the liver-glands, form a
large glandular mass, situated in the head-region closely surrounding the central
nervous system, so that the. genital ducts pass from the head-region tailwards
to the operculum. In the scorpion they lie in the abdominal region, so that
their ducts pass headwards to the operculum.

Probably in the Palaeostraca the generative mass was situated in the cephalic
region as in Limulus, and it is probable that the remnant of it still exists in

THE EVIDENCE OF THE THYROID GLAND 21 7

Ammocoetes in the shape of the peculiar large cells packed together, with
pigment masses in between them, which form such a characteristic feature of
the glandular-looking material, which fills up the space between the cranial
walls and the central nervous system.

Finally, the relationship which has been known from time immemorial to
exist between the sexual organs and the thyroid in man and other animals, and
has hitherto been a mystery without any explanation, may possibly be the last
reminiscence of a time when the thyroid glands were the uterine glands of the
palaeostracan ancestor.

The consideration of the facial nerve, and the segments it supplies, still
further points to the origin of the Vertebrata from the Palfeostraca.

CHAPTER VI

THE EVIDENCE OF THE OLFACTORY APPARATUS

Fishes divided into Ainphirhina? and Monorhinfe. — Nasal tube of the lamprey.
— Its termination at the infundihulum. — The olfactory organs of the
scorpion group. — The camerostome. — Its formation as a tube. — Its deriva-
tion from a pair of antenna?. — Its termination at the true mouth. — Com-
parison with the olfactory tube of Ammocoetes. — Origin of the nasal tube
of Ammocoetes from the tube of the hypophysis. — Direct comparison of the
hypophysial tube with the olfactory tube of the scorpion group — Summary.

In the last chapter I finished the evidence given by the consideration
of the mesosomatic or opisthotic nerves, and the segments they
supplied. The evidence is strongly in accordance with that of
previous chapters, and not only confirms the conclusion that verte-
brates arose from some member of the Pakeostraca, but helps still
further to delimit the nature of that member. It is almost startling
to see how the hypothesis put forward in the second chapter, sug-
gested by the consideration of the nature of the vertebrate central
nervous system and of the geological record, has received stronger
and stronger confirmation from the consideration of the vertebrate
optic apparatus, the vertebrate skeleton, the respiratory apparatus,
and, finally, the thyroid gland. All fit naturally into a harmonious
whole, and give a feeling of confidence that a similar harmony will
be found upon consideration of the rest of the vertebrate organs.

Following naturally upon the segments supplied by the opisthotic
(mesosomatic) cranial nerves, we ought to consider now the segments
supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments
belonging to the trigeminal nerve-group in the vertebrate, and in the
invertebrate the segments of the prosoma with their characteristic
appendages. There are, however, in all vertebrates in this foremost
cranial region, in addition to the optic nerves, two other well-marked
nerves of special sense, the olfactory and the auditory. Of these,
the former are in the same class as the optic nerves, for they arise

THE EVIDENCE OF THE OLFACTORY APPARATUS 219

in the vertebrate from the supra-infundibular nerve-mass, and in the
invertebrate from the supra-CESophageal ganglia. The latter arise in
the vertebrate from the infra-infundibular nerve-mass, and, as the
name implies, are situated in the region where the pro-otic nerves
are contiguous to the opisthotic, i.e. at the junction of the prosomatic
and mesosomatic nerve-regions.

The chapter dealing with the evidence given by the olfactory
nerves and the olfactory apparatus ought logically to have followed
immediately upon the one dealing with the optic apparatus, seeing
that both these special sense-nerves belong to the supra-infundibular
segments in the vertebrate and to the supra- oesophageal in the
invertebrate.

I did not deal with them in that logical sequence because it was
necessary for their understanding to introduce first the conception of
modified appendages as important factors in any consideration of
vertebrate segments ; a conception which followed naturally after the
evidence afforded by the skeleton in Chapter III., and by the branchial
segments in Chapter IV. So, too, now, although the discussion of
the prosomatic segmentation ought logically to follow immediately
on that of the mesosomatic segmentation, I have determined to devote
this chapter to the evidence of the olfactory organs, because the
arguments as to the segments belonging to the trigeminal nerve-
group are so much easier to understand if the position of the olfactory
apparatus is first made clear.

In all vertebrates the nose is double and opens into the pharynx,
until we descend to the fishes, where the whole group Pisces has
been divided into two subsidiary groups, MonorhinaB and Ainphirhime,
according as they possess a median unpaired olfactory opening, or a
paired opening. The Monorhinoe include only the Cyclostomata — the
lampreys and hag-fishes.

In the lampreys the single olfactory tube ends blindly, while in
the hag-fishes it opens into the pharynx. In the lamprey, both in
Petromyzon and Ammoccetes, the opening of this nasal tube is a
conspicuous object on the dorsal surface of the head in front of the
transparent spot which indicates the position of the right median
eye. It is especially significant, as showing the primitive nature of
this median olfactory passage, that a perfectly similar opening in the

2 20 THE ORIGIN OF VERTEBRATES

same position is always found in the dorsal head-shields of all the
Cephalaspidse and Tremataspidse, as will be explained more fully in
Chapter X.

All the evidence points to the conclusion that the olfactory
apparatus of the vertebrate originated as a single median tube, con-
taining the special olfactory sense-epithelium, which, although median
and single, was innervated by the olfactory nerve of each side. The
external opening of this tube in the lamprey is dorsal. How does it
terminate ventrally ?

The ventral termination of this tube is most instructive and
suggestive. It terminates blindly at the very spot where the in-
fundibular tube terminates blindly and the notochord ends. After
transformation, when the Ammoccete becomes the Petromyzon, the
tube still ends blindly, and does not open into the pharynx as in
Myxine ; it, however, no longer terminates at the infundibulum, but
extends beyond it towards the pharynx.

This position of the nasal tube suggests that it may originally have
opened into the tube of the central nervous system by way of the
infundibular tube. This suggestion is greatly enhanced in value by
the fact that in the larval Amphioxus the tube of the central nervous
system is open to the exterior, its opening being known as the anterior
neuropore, and this anterior neuropore is situated at the base of a pit,
known as the olfactory pit because it is supposed to represent the
olfactory organ of other fishes.

Following the same lines of argument as in previous chapters,
this suggestion indicates that the special olfactory organs of the
invertebrate ancestor of the vertebrates consisted of a single median
olfactory tube or passage, which led directly into the oesophagus and
was innervated, though single and median, by a pair of olfactory
nerves which arose from the supra-cesophageal ganglia. Let us see
what is the nature of the olfactory organs among arthropods, and
whether such a suggestion possesses any probability.

The Olfactory Organs of the Scorpion Group.

At first sight the answer appears to be distinctly adverse, for it is
well known that in all the Insecta, Crustacea, and the large majority
of Arthropoda, the first pair of antenna 3 , often called the antennules,
are olfactory in function, and these are free-moving, bilaterally

THE EVIDENCE OF THE OLFACTORY APPARATUS 22 1

situated, independent appendages. Still, even here there is the strik-
ing fact that the nerves of these olfactory organs always arise from
the supra-cesophageal ganglia, although those to the second pair of
antenna? arise from the infra-cesophageal ganglia, just as the olfactory
nerves of the vertebrate arise from the supra-infundibular brain-mass.
Not only is there this similarity of position, but also a similarity of
structure in the olfactive lobes of the brain itself of so striking a cha-
racter as to cause Bellonci to sum up his investigations as follows : —

" The structure and connections of the olfactive lobes present the
same fundamental plan in the higher arthropods and in the verte-
brates. In the one, as in the other, the olfactory fibres form, with
the connecting fibres of the olfactory lobes, a fine meshwork, which,
consisting as it does of separate groups, may each one be called an
olfactory glomerulus."

He attributes this remarkable resemblance to a physiological
necessity that similarity of function necessitates similarity of structure,
for he considers it out of the question to suppose any near relationship
between arthropods and vertebrates.

Truly an interesting remark, with the one fallacy that relationship
is out of the question.

The evidence so far has consistently pointed to some member of the
pala?ostracan group as the ancestor of the vertebrates — a group which
had affinities both to the crustaceans and the arachnids ; indeed, many
of its members resembled scorpions much more than they resemble
crustaceans. The olfactory organs of the scorpions and their allies are,
therefore, more likely than any others to give a clue to the position of
the desired olfactory organs. In these animals and their allies paired
olfactory antenna? are not present, either in the living land-forms or
the extinct sea-scorpions, for all the antenna?-like, frequently chelate,
appendages seen in Pterygotus, etc. (Fig. 8), represent the chelicene, and
correspond, therefore, to the second pair of antenna? in the crustaceans.

What, then, represents the olfactory antenna? in the scorpions ? The
answer to this question has been given by Croneberg, and very strik-
ing it is. The two olfactory antenna? of the crustacean have combined
together to form a hollow tube at the base of which the mouth of the
animal is situated, so that the food passes along this olfactory passage
before it reaches the mouth. This organ is often called after Latreille,
the camerostome, sometimes the rostrum ; it is naturally median in
position and appears, therefore, to be an unpaired organ ; its paired

222

THE ORIGIN OF VERTEBRATES

cam

pr.ent

character is, of course, evident enough, for it is innervated by a pair of
nerves, and these nerves, as ought to be the case, arise from the supra-
oesophageal ganglia. In Galeodes it is a conspicuously paired antennae-
like organ (Fig. 94).

Croneberg has also shown that this rostrum, or camerostome, arises
embryologically as a pair of appendages similar to the other append-
ages. This last observation
of Croneberg has been con-
firmed by Brauer in 1894,
who describes the origin of
the upper lip, as he calls it,
in very similar terms, with-
out, however, referring to
Croneberg's paper. Crone-
berg further shows that this
foremost pair of antennas
not only forms the so-called
upper lip or camerostome,
but also a lower lip, for
from the basal part of the
camerostome there projects
on each side of the pharynx
a dependent accessory por-
tion, which in some cases
fuses in the middle line, and
forms, as it were, a lower lip.
The entosclerite belonging
to this dependent portion is
apparently the post - oral
entosclerite of Lankester and
Miss Beck.

At the base of the tubular
passage formed by this modified first pair of antennas the true mouth
is found opening directly into the dilated pharynx, the muscles
of which enable the act of suction to be carried out. The narrow
oesophagus leads out from the pharynx and is completely surrounded
by the supra- and infra-oesophageal nerve masses.

Huxley also describes the mouth of the scorpion in precisely the
same position (cf. o, Fig. 96).

Fig. 94. — Dobsal View of Brain and Came-
rostome op Galeodes.

cam., camerostome; pr. ent., pre-oral entoscle-
rite ; l.l., dependent portion of camerostome ;
ph., pharynx; al., alimentary canal; n. op.,
median optic nerves; pi., plastron; v.c,
ventral nerve chain ; 2, 3, second and third
appendages.

THE EVIDENCE OF THE OLFACTORY APPARATUS

223

In order to convey to my readers the antennae-like character of the
carnerostome in Galeodes (Fig. 101), and its position, I give a figure
(Fig. 94) of the organ from its dorsal aspect, after removal of the
cheliceras and their muscles. A side view of the same organ is given
in Fig. 95 to show the feathered termination of the carnerostome,
and the position of the dependent accessory portion {1.1.) (Crone-
berg's ' untere Anhang ') with its single long antenna-like feather.
In both figures the alimentary canal (al.) is seen issuing from the
conjoined supra- and infra-cesophageal mass.

As is seen in the figures, the bilateral character of the rostrum, as
Croneberg calls it, is apparent not only in its feathered extremity
but also in its chitinous covering, the softer median dorsal part (left

p"er\t

Fig. 95. — Lateral View of Brain and Camerostome of Galeodes.

gl. supr. ces., supra-oesophageal ganglion; gl. infr. ces., infra-cesophageal ganglion.
The rest of the lettering same as in Fig. 94.

white in figure) being bounded by two lateral plates of hard chitin,
which meet in the middle line near the extremity of the organ. In
all the members of the scorpion group, as is clearly shown in Crone-
berg's figures, the rostrum or carnerostome is built up on the same
plan as in Galeodes, though the antenna-like character may not be
so evident.

When we consider that the first pair of antennae in the crustaceans
are olfactory in function, Croneberg's observations amount to this —

In the arachnids and their allies the first pair of antennae form
a pre-oral passage or tube, olfactory in function ; the small mouth,
which opens directly into the pharynx, being situated at the end
of this olfactory passage.

224

THE ORIGIN OF VERTEBRATES

Croneberg's observatiuns and conclusions are distinctly of very
groat importance in bringing the arachnids into line with the crus-
taceans, and it is therefore most surprising that they are absolutely
ignored by Lankester and Miss Beck in their paper published in
1883, in which Latreille only is mentioned with respect to this
organ, and his term " camerostome," or upper lip, is used throughout,
in accordance with the terminology in Lankester's previous paper.
That this organ is not only a movable lip or tongue, but essentially
a sense-organ, almost certainly of smell and taste, as follows from
Croneberg's conclusions, is shown by the series of sections which I
have made through a number of young Thelyphonus (Fig. 102).

pr em

. / Hyp

Olf pass

Fig. 96. — Median Sagittal Section through a Young Thelyphonus.

I give in Fig. 96 a sagittal median section through the head-end
of the animal, which shows clearly the nature of Croneberg's con-
ception. At the front end of the body is seen the median eye (cc),
u is the mouth, Ph. the pharynx, ces. the narrow cesophagus, com-
pressed between the supra-oesophageal (swpr. ces.) and infra-cesopha-
geal (infr. ces.) brain mass, which opens into the large alimentary
canal (A I.) ; Olf. pass, is the olfactory passage to the mouth, lined
with thick- set, very fine hairs, which spring from the hypostome
(Hyp.) as well as from the large conspicuous camerostome (Cam.),
which limits this tube anteriorly. The space between the came-
rostome and the median eye is filled up by the massive chelicerse,
which are not shown in this section, as they begin to appear in the

THE EVIDENCE OF THE OLFACTORY APPARATUS 225

sections on each side of the median one. The muscles of the pharynx
and the muscles of the camerostome are attached to the pre-oral
entosclerite (pr. ent.). The post-oral entosclerite is shown in section
as post. ent. The dorsal blood-vessel, or heart, is indicated at H.

In Tig. 97 I give a transverse section through another specimen
of the same litter, to show the nature of this olfactory tube when cut
across. Both sections show most clearly that we are dealing here with
an elaborate sense-organ, the surface of which is partly covered with
very fine long hairs, partly, as is seen in the figure, is composed of
long, separate, closely-set sense-rods (bat.), w T ell protected by the
long hairs which project on every side in front of them, which recall
to mind Bellonci's figure of the ' batonnets olfactives ' on the antennae
of Sphasroma. Finally, we have the observation of Blanchard quoted
by Huxley, to the effect that this camerostome is innervated by
nerves from the supra-cesophageal ganglia which are clearly bilateral,
seeing that they arise from the ganglion on each side and then unite
to pass into the camerostome ; in other words, paired olfactory nerves
from the supra-cesophageal ganglia.

These facts demonstrate with wonderful clearness that in one
group of the Arthropoda the olfactory autennae have been so modi-
fied as to form an olfactory tube or passage, which leads directly
into the mouth and so to the oesophagus of the animal, and, strikingly
enough, this group, the Arachnida, is the very one to which the
scorpions belong.

If for any cause the mouth in Fig. 96 were to be closed, then
the olfactory tube (olf. i^ciss.) might still remain, owing to its impor-
tance as the organ of smell, and the olfactory tube would terminate
blindly at the very spot where the corresponding tube does terminate
in the vertebrate, according to the theory put forward in this book.

*

The Olfactory Tube of Ammoccetes.

In all cases where there is similarity of topographical position
in the organs of the vertebrate and arthropod we may expect also to
find similarity of structure. At first sight it would appear as though
such similarity fails us here, for a cross-section of the olfactory tube
in Petromyzon represents an elaborate organ such as is shown in Fig.
98, very different in appearance to the section across the olfactory
passage of a young Thelyphonus given in Fig. 97.

Q

26

THE ORIGIN OF VERTEBRATES

1

few

»1&

Fig. 97. — Teansveese Section theough the Olfactoey Passage of a Young

Thelyphonus.

1 and 2, sections of first and second appendages.

--cart.

Fig. 98.— Teansveese Section theough the Olfactoey Passage of Peteomyzon.

cart., nasal cartilage.

THE EVIDENCE OF THE OLFACTORY APPARATUS

227

As is seen, it is difficult to see any connection between these
folds of olfactory epithelium and the simple tube of the scorpion.
But in the nose, as in all other parts of the head-region of the
lamprey, remarkable changes take place at transformation, and
examination of the same tube in Ammococtes demonstrates that the
elaborate structure of the adult olfactory organ is actually derived
from a much simpler form of organ, represented in Fig. 99. Here,
in Ammoccetes, the section is no longer strikingly different from that
of the Thelyphonus organ, but, instead, most strikingly similar to it.
Thus, again, it is shown that this larval form of the lamprey gives

■ cart

Fig. 99. — Transverse Section through the Olfactory Passage op Ammoccetes.

cart., nasal cartilage.

more valuable information as to vertebrate ancestry than all the
rest of the vertebrates put together.

Still, even now the similarity between the two organs is not
complete, for the tube in the lamprey opens on to the exterior on the
dorsal surface of the head, while in the scorpion tribe it is situated
ventrally, being the passage to the mouth and alimentary canal. In
accordance with this there is no sign of any opening on the dorsal
carapace of any of the extinct sea-scorpions or of the living land-
scorpions, such as is so universally found in the cephalaspids, trema-
taspids, and lampreys. Here is a discrepancy of an apparently
serious character, yet so wonderfully does the development of the
individual recapitulate the development of the race, that this very
discrepancy becomes converted into a triumphant vindication of the

2 28 THE ORIGIN OF VERTEBRATES

correctness of the theory advocated in this book, as soon as we turn
our attention to the development of this nasal tube in the lamprey.

We must always remember not only the great importance of a lar-
val stage for the unriddling of problems of ancestry, but also the great
advantage of being able to follow more favourably any clues as to
past history afforded by the development of the larva itself, owing
to the greater slowness in the development of the larva than of the
embryo. Such a clue is especially well marked in the course of
development of Ammoccetes according to Kupffer's researches, for
he finds that when the young Ammocoetes is from 5 to 7 mm. in
length, some time- after it has left the egg, when it is living a free
larval life, a remarkable series of changes takes place with consider-
able rapidity, so that we may regard the transformation which takes
place at this stage, as in some degree comparable with the great trans-
formation which occurs when the Ammoccetes becomes a Petromyzon.

All the evidence emphasizes the fact that the latter transformation
indicates the passage from a lower into a higher form of vertebrate,
and is to be interpreted phylogenetically as an indication of the
passage from the Cephalaspidian towards the Dipnoan style of fish.
If, then, the former transformation is of the same character, it would
indicate the passage from the Paheostracan to the Cephalaspid.

What is the nature of this transformation process as described
by Kupffer ?

It is characterized by two most important events. In the first
place, up to this time the oral chamber has been cut off from the
respiratory chamber by a septum — the velum — so that no food could
pass from the mouth to the alimentary canal. At this stage the
septum is broken through, the oral chamber communicates with the
respiratory chamber, and the velar folds of the more adult Ammocoetes
are left as the remains of the original septum. The other striking
change is the growth of the upper lip, by which the orifice of the nasal
tube is transferred from a ventral to a dorsal position. Fig. 100,
taken from Kupffer's paper, represents a sagittal section through an
Ammoccetes 4 mm. long; l.l. is the lower lip, u.l. the upper lip, and,
as is seen, the short oral chamber is closed by the septum, rel. Open-
ing ventrally is a tube called the tube of the hypophysis, Hy., which
extends close up to the termination of the infundibulum. On the
anterior surface of this tube is the projection called by Kupffer the
olfactory plakode. At this stage the upper lip grows with great

THE EVIDENCE OF THE OLFACTORY APPARATUS 229

rapidity and thickens considerably, thus forcing the opening of the
hypophysial tube more and more dorsalwards, until at last, in the full-
grown Ammoccetes, it becomes the dorsal opening of the nasal tube,
as already described. Here, then, in the hypophysial tube we have
the original position of the olfactory tube of the vertebrate ancestor,
and it is significant, as showing the importance of this organ, to find
that such a hypophysial tube is characteristic of the embryological
development of every vertebrate, whatever may be the ultimate form
of the external nasal orifices.

The single median position of the olfactory organ in the Cyclo-
stomata, in contradistinction to its paired character in the rest of the

v x v\\ IX x

Hu ui Or 11 vel

Fig. 100. — Ganglia of the Cranial Nerves of an Ammoccetes, i mm. in length,

PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)

A-B, the line of epibranchial ganglia; an., auditory capsule; nc, notochord ; Hy.,
tube of hypophysis ; Or., oral cavity; u.l., upper lip ; l.l. lower lip; vel., septum
between oral and respiratory cavities ; V., VII., IX., X., cranial nerves ; x.,
nerve with four epibranchial ganglia.

vertebrates, has always been a stumbling-block in the way of those
who desired to consider the Cyclostomata as degenerated Selachians,
for the origin of the olfactory protuberance, as a single median
plakode, seemed to indicate that the nose arose as a single organ and
not as a paired organ.

On the other hand, the two olfactory nerves of Ammoccetes
compare absolutely with the olfactory nerves of other vertebrates,
and force one to the conclusion that this median organ of Ammo-
ccetes arose from a pair of bilateral organs, which have fused in the
middle line.

The comparison of this olfactory organ with the camerostome

230

THE ORIGIN OF VERTEBRATES

Fig. 101.— Galcodes. (Prom the Royal Natural History.)

THE EVIDENCE OF THE OLFACTORY APPARATUS

2 3 l

gives a satisfactory reason for its appearance in the lowest verte-
brates as an unpaired median organ ; equally so, the history of the
camerostoine itself supplies the reason why the olfactory nerves are
double, why the organ is in reality a paired organ and not a single

Fig. 102. — Thelyphonus. (From the Royal Natural History.)

median one. Thus, in a sense, the grouping of the fishes into Mono-
rhinae and Amphirhinse has not much meaning, seeing that the
olfactory organ is in all cases double.

The evidence of the olfactory organs in the vertebrate not only
confirms, in a most striking manner, the theory of the origin of the

232 THE ORIGIN OF VERTEBRATES

vertebrate from the Palaeostracan, but points indubitably to an origin
from a scorpion-like rather than a crustacean-like stock. To com-
plete the evidence, it ought to be shown that the ancient sea-scorpions
did possess an olfactory passage similar to the modern land-scorpions.
The evidence on this question will come best in the next chapter,
where I propose to deal with the prosomatic appendages of the Palse-
ostracan group.

Summary.

The vertebrate olfactory apparatus commences as a single median tube
which terminates dorsally in the lamprey, and is supplied by the two olfactory
nerves which arise from the supra-infundibular portion of the brain. It is
a long - , tapering- tube which passes ventrally and terminates blindly at the
infundibulum in Ammocoetes. The dorsal position of the nasal opening is not
the original one, but is brought about by the growth of the upper lip. The
nasal tube originally opened ventrally, and was at that period of development
known as the tube of the hypophysis.

The evidence of Ammocoetes thus goes to show that the olfactory
apparatus started as an olfactory tube on the ventral side of the animal, which
led directly up to, and probably into, the oesophagus of the original alimentary
canal of the palaeostracan ancestor.

Strikingly enough, although in the crustaceans the first pair of antenna?
form the olfactory organs, no such free antennas are found in the arachnids,
but they have amalgamated to form a tube or olfactory passage, which leads
directly into the mouth and oesophagus of the animal.

This olfactory passage is very conspicuous in all members of the scorpion
group, and, like the olfactory tube of the vertebrate, is innervated by a pair of
nerves, which resemble those supplying the first pair of antenna? in crustaceans
as to their origin from the supra-cesophageal ganglia.

This nasal passage, or tube of the hypophysis, corresponds in structure and
iu position most closely with the olfactory tube of the scorpion group, the only
difference being- that in the latter case it opens directly into the oesophagus,
while in the former, owing to the closure of the old moutli, it cannot open into
the infundibulum.

The evidence of the olfactory apparatus, combined with that of the optic
apparatus, is most interesting, for, whereas the former points indubitably to an
ancestor having scorpion-like affinities, the structure of the lateral eyes points
distinctly to crustacean, as well as arachnid, affinities.

Taking the two together the evidence is extraordinarily strong that the
vertebrate arose from a member of the palasostracan group with marked
scorpion-like affinities.

CHAPTER VII

THE PROS OM A TIC SEGMENTS OF LIMULUS AND ITS ALLIES

Comparison of the trigeminal with the prosomatic region. — The prosomatic
appendages of the Gigantostraca. — Their number and nature. — Endognaths
and ectognath. — The metastoma.— The coxal glands. — Prosomatic region
of Eurypterus compared with that of Ammocoetes. — Prosomatic segmenta-
tion shown by muscular markings on carapace. — Evidence of coelomic
cavities in Limulus. — Summary.

The derivation of the olfactory organs of the vertebrate from the
olfactory antennae of the arthropod in the last chapter is confirmatory
proof of the soundness of the proposition put forward in Chapter IV.,
that the segmentation in the cranial region of the vertebrate was
derived from that of the prosomatic and mesosomatic regions of the
palseostracan ancestor. Such a segmentation implies a definite series
of body-segments, corresponding to the mesomeric segmentation of
the vertebrate, and a definite series of appendages corresponding to
the splanchnic segmentation of the vertebrate.

Of the foremost segments belonging to the supra-oesophageal
region characterized by the presence of the median eyes, of the lateral
eyes, and of the olfactory organs, a wonderfully exact replica has
been shown to exist in the pineal eyes, the lateral eyes, and the
olfactory organ of the vertebrate, belonging, as they all do, to the
supra-infundibular region.

Of the infra- cesophageal segments belonging to the prosoraa and
mesosoma respectively, the correspondence between the mesosomatic
segments carrying the branchial appendages and the uterus, with
those in the vertebrate carrying the branchiae and the thyroid gland
respectively, has been fully proved in previous chapters.

There remain, then, only the segments of the prosomatic region
to be considered, a region which, both in the vertebrate and inver-
tebrate, is never respiratory in function but always masticatory, such

234 T M£ ORIGIN OF VERTEBRATES

mastication being performed in Limulns and its allies by the muscles
which move the foot-jaws or gnathites, which are portions of the
prosomatic appendages specially modified for that purpose, and in the
vertebrates by the masticatory muscles, which are always innervated
by the trigeminal or Vth cranial nerve. This comparison implies
that the motor part of the trigeminal nerve originally supplied the
prosomatic appendages.

The investigations of van Wijhe and of all observers since the
publication of his paper prove that in this trigeminal region, as in the
vagus region, a double segmentation exists, of which the ventral or
splanchnic segments, corresponding to the appendages in the inver-
tebrate, are supplied by the trigeminal nerves, while the dorsal or
somatic segments, corresponding to the somatic segments in the
invertebrate, are supplied by the Illrd or oculomotor and the IVth
or trochlear nerves — nerves which supply muscles moving the lateral
eyes.

In accordance, then, with the evidence afforded by the nerves of
the branchial segments, it follows that the muscles supplied by the
motor part of the trigeminal ought originally to have moved the ap-
pendages belonging to a series of prosomatic segments. On the other
hand, the eye-muscles ought to have belonged to the body-part of the
prosomatic segments, and must therefore have been grouped origi-
nally in a segmental series corresponding to the prosomatic appendages.

The evidence for and against this conclusion will be the subject
of consideration in this and the succeeding chapters. At the outset
it is evident that any such comparison necessitates an accurate know-
ledge of the number of the prosomatic segments in the Gigantostraca
and of the nature of the corresponding appendages.

In all this group of animals, the evidence as to the number of
segments in either the prosomatic or mesosomatic regions is given
by-

1. The number of appendages.

2. The segmental arrangement of the muscles of the prosoma or
mesosoma respectively.

3. The segmental arrangement of the ccelomic or head-cavities.

4. The divisions of the central nervous system, or neuromeres,
together with their outgoing segmental nerves.

It follows, therefore, that if from any cause the appendages are
not apparent, as is the case in many fossil remains, or have dwindled

PROSOMATIC SEGMENTS OF LIMULUS 235

away and become insignificant, we still have the muscular, ccelomic,
and nervous arrangements left to us as evidence of segmentation in
these animals, just as in vertebrates.

In this prosomatic region, we find in Limulus the same tripartite
division of the nerves as in the mesosomatic region, so that the
nerves to each segment may be classed as (1) appendage-nerve ;
(2) sensory or dorsal somatic nerve, supplying the prosomatic cara-
pace ; (3) motor or ventral somatic nerve, supplying the muscles of
the prosoma, and containing possibly some sensory fibres. The main
difference between these two regions in Limulus consists in the closer
aggregation of the prosomatic nerves, corresponding to the concentra-
tion of the separate ganglia of origin in the prosomatic region of the
brain.

The number of prosomatic segments in Limulus is not evident
by examination of the prosomatic carapace, so that the most reliable
guide to the segmentation of this region is given by the appendages,
of which one pair corresponds to each prosomatic segment.

The number of such segments, according to present opinion, is
seven, viz. : —

(1) The foremost segment, which bears the chelicerae.
(2, 3, 4, 5, 6) The next five segments, which carry the paired
locomotor appendages ; and

(7) The last segment, to which belongs a small abortive pair of
appendages, known by the name of the chilaria, situated between the
last pair of locomotor appendages and the operculum or first pair of
mesosomatic appendages. These appendages are numbered from 1-7
in the accompanying drawing (Fig. 103).

Of these seven pairs of appendages, the significance of the first
and the last has been matter of dispute. With respect to the first
pair, or the chelicerae, the question has arisen whether their nerves
belong to the infra-oesophageal group, or are in reality supra-
cesophageal.

It is instructive to observe the nature and the anterior position of
this pair of appendages in the allied sea-scorpions, especially in Ptery-
gotus, where the only chelate organs are found in these long, antennae-
like chelicerae. In Slimonia and in Stylonurus they are supposed by
Woodward to be represented by the small non-chelate antennas seen
in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case,
then these figures show that a pair of appendages is missing in each

236 THE ORIGIN OF VERTEBRATES

of these forms, for they possess only five free prosomatic appendages
instead of six, as in Limulus and in Pterygotus. Similarly, Wood-
ward only allowed five appendages for Pterygotus, so that his restora-
tions were throughout consistent. Schmidt, in Pterygotus osiliensis
has shown that the true number was six, not five, as seen in his
restoration given in Fig. 8, A (p. 27).

With respect to Eurypterus, Schmidt figures an exceedingly

Fig. 103. — Ventral Surface op Limulus. (Taken from Kishinouye.)

The gnathic bases of the appendages have been separated from those of the other
side to show the promesosternite or endostoma (End.).

minute pair of antennae between the coxal joints of the first pair of
appendages, thus making six pairs of appendages. Gerhard Holm,
however, in his recent beautiful preparations from Schmidt's specimens
and others collected at Bootzikiill, has proved most conclusively that
the chelicera3 of Eurypterus were of the same kind as those of
Limulus. I reproduce his figure (Fig. 104) showing the small chelate
chelicerce (1) overhanging the mouth orifice, just as in Limulus or in
Scorpio.

PROS DMA TIC SEGMENTS OF LIMULUS

?37

So, also, since Woodward's monograph, Laurie has discovered in
Slimonia acuminata a small median pair of chelate appendages
exactly corresponding to the chelicerae of Limulus, or of Eurypterus,
or of Scorpio. We may, therefore, take it for granted that such was
also the case in Stylonurus, and that the foremost pair of proso-
matic appendages in all these extinct sea-scorpions were in the same
position and of the same character as the cheliceral of the scorpions.

In the living scorpion and in Limulus the nerves to this pair of

Fig. 104. — Eurypterus Fischeri. (From Holm.)

appendages undoubtedly arise from the foremost prosomatic ganglia,
and the reason why they appear to beloug to the supra- oesophageal
brain-mass has been made clear by Brauer's investigations on the
embryology of Scorpio ; for he has shown that the cheliceral ganglia
shift from the ventral to the dorsal side of the oesophagus during
development, thus becoming pseudo-supra-cesophageal, though in
reality belonging to the iufra-cesophageal ganglia. This cheliceral
pair of appendages is, in all probability, homologous with the second
pair of antennas in the Crustacea.

2^8 THE ORIGIN OF VERTEBRATES

I conclude, then, that the chelicera? must truly be included in
the pro-somatic group, but that they stand in a somewhat different
category to the rest of the prosomatic appendages, inasmuch as they
take up a very median anterior and somewhat dorsal position, and
their ganglia of origin are also exceptional in position.

Next for consideration come the chilaria (7 in Fig. 103), which
Lankester did not consider to belong to appendages at all, but to
be a peculiar pair of sternites. Yet their very appearance, with
their spinous hairs corresponding to those of the other gnathites and
their separate nerve-supply, all point distinctly to their being a
modified pair of appendages, and, indeed, the matter has been placed
beyond doubt by the observations of Kishinouye, who