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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 Palæontology--
  Reasons for choosing Ammocoetes rather than Amphioxus for the
  investigation of this problem--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
  Ammocoetes 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 Ammocoetes
  and their optic ganglia--Comparison with other median eyes--Lateral
  eyes of vertebrates 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 Ammocoetes; its topographical arrangement, its
  structure, its origin in muco-cartilage--The prosomatic skeleton of
  Ammocoetes; the trabeculæ and parachordals, their structure, their
  origin in white fibrous tissue--The mesosomatic skeleton of Limulus
  compared with that of Ammocoetes; similarity of position, of
  structure, of origin in muco-cartilage--The prosomatic skeleton of
  Limulus; the entosternite, or plastron, compared with the trabeculæ
  of Ammocoetes; similarity of position, of structure, of origin in
  fibrous tissue--Summary                                               119

  CHAPTER IV

  THE EVIDENCE OF THE RESPIRATORY APPARATUS

  Branchiæ 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 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           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 Ammocoetes--
  Its structure--Its openings--The nature of the thyroid segment--The
  uterus of the scorpion--Its glands--Comparison with the thyroid
  gland of Ammocoetes--Cephalic generative glands of Limulus--
  Interpretation of glandular tissue filling up the brain-case of
  Ammocoetes--Function of thyroid gland--Relation of thyroid gland
  to sexual functions--Summary                                          185

  CHAPTER VI

  THE EVIDENCE OF THE OLFACTORY APPARATUS

  Fishes divided into Amphirhinæ and Monorhinæ--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 antennæ--Its termination at the true
  mouth--Comparison 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                                        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 Ammocoetes--
  Prosomatic segmentation shown by marks on carapace--Evidence of
  coelomic cavities in Limulus--Summary                                 233

  CHAPTER VIII

  THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP

  The prosomatic 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 OF AMMOCOETES

  The prosomatic region in Ammocoetes--The suctorial apparatus of the
  adult Petromyzon--Its origin in Ammocoetes--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-pericardial muscles in Limulus--The old mouth
  or palæostoma--The pituitary gland--Its comparison with the coxal
  gland of Limulus--Summary                                             286

  CHAPTER X

  THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE
      OSTRACODERMATA

  The nose of the Osteostraci--Comparison of head-shield of Ammocoetes
  and of Cephalaspis--Ammocoetes 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 VIIth nerve
  passes in and out of capsule--Evidence of Ammocoetes--Intrusion of
  glandular mass round brain into auditory capsule--Intrusion of
  generative and hepatic mass round brain into base of flabellum--
  Summary                                                               355

  CHAPTER XII

  THE REGION OF THE SPINAL CORD

  Difference between cranial and spinal regions--Absence of lateral
  root--Meristic variation--Segmentation of coelom--Segmental
  excretory organs--Development of nephric organs; pronephric,
  mesonephric, metanephric--Excretory organs of Amphioxus--
  Solenocytes--Excretory organs of Branchipus and Peripatus,
  appendicular and somatic--Comparison of coelom of Peripatus and
  of vertebrate--Pronephric organs compared to coxal glands--Origin
  of vertebrate body-cavity (metacoele)--Segmental duct--Summary of
  formation of excretory organs--Origin of somatic trunk-musculature--
  Atrial cavity of Amphioxus--Pleural folds--Ventral growth of
  pleural folds and somatic musculature--Pleural folds of
  Cephalaspidæ and of Trilobita--Meaning of the ductless glands--
  Alteration in structure of excretory organs which have lost their
  duct in vertebrates and in invertebrates--Formation of lymphatic
  glands--Segmental coxal glands of arthropods and of vertebrates--
  Origin of adrenals, pituitary body, thymus, tonsils, thyroid, and
  other ductless glands--Summary                                        385

  CHAPTER XIII

  THE NOTOCHORD AND ALIMENTARY CANAL

  Relationship between notochord and gut--Position of unsegmented tube
  of notochord--Origin of notochord from a median groove--Its function
  as an accessory digestive tube--Formation of notochordal tissue in
  invertebrates from closed portions of the digestive tube--Digestive
  power of the skin of Ammocoetes--Formation of new gut in Ammocoetes
  at transformation--Innervation of the vertebrate gut--The three
  outflows of efferent nerves belonging to the 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 the theory
  advanced in this book--The germ-layer theory--Its present position--
  A physiological not a morphological conception--New fundamental law
  required--Composition of adult body--Neuro-epithelial 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 Amphioxus and of Lucifer--Summary                  455

  CHAPTER XV

  FINAL REMARKS

  Problems requiring investigation--

  Giant nerve-cells and giant nerve-fibres; their comparison in fishes
  and arthropods; 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 EARTHQUAKE
  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.




{1}THE

ORIGIN OF VERTEBRATES




_INTRODUCTION_


In former days it was possible for a man like Johannes Müller 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 comparative 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 {2}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 physiology, 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 communicantes_ 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 communicantes_, 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 {3}the
ventral roots, for the cells of the sympathetic system are 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, belonging 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
erigentes_, 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 musculature 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 associate 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}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 generation 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 infraoesophageal
ganglia, there in the cerebral hemispheres and optic and olfactory nerves
the supraoesophageal 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
morphological laboratory and taught that the one organ which is homologous
throughout the animal kingdom is the gut, and that therefore the gut 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 unsegmented 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.

{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 been 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}forward in this book, and to
the latter for his great kindness in undertaking the laborious task of
collecting the proofs.




LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CONCERNING 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
          Philosophical 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 Nerve," vol. xxxiv., p. 514.

  1900.   "    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 Palæontological Evidence: Ammocoetes a
                      Cephalaspid," vol. xxxiv., p. 562.

  1901.   "     IX. "The Origin of the Optic Apparatus: the Meaning of the
                      Optic Nerves," vol. xxxv., p. 224.

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




{8}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 Palæontology.--Reasons for
  choosing Ammocoetes 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 Ammocoetes 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 accumulation 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; {9}thus
zoologists are able to speak definitely of the Vertebrata, Arthropoda,
Annelida, Echinodermata, Porifera, Coelenterata, 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 palæontology 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 confronted 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 {10}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.

[Illustration: FIG. 1.--ARRANGEMENT OF ORGANS IN THE VERTEBRATE (A) AND
ARTHROPOD (B).

_Al_, gut; _H_, heart; _C.N.S._, central nervous system; V, ventral side;
D, dorsal side.]

Now, if we take a broad and comprehensive view of the invertebrate kingdom,
without arguing out each separate case, we find that it bears strongly the
stamp of a general plan of evolution derived from a coelenterate 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 {11}nervous masses are of necessity ventral to the digestive tube,
because the mouth of the coelenterate 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
oesophagus--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 alimentary 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 Vertebrates; 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 versâ_; 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 digestive 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}[Illustration: FIG. 2.--LARVAL BALANOGLOSSUS (from the Royal Natural
History).]

It has been supposed that the Tunicata and the Enteropneusta
(_Balanoglossus_) (Fig. 2) are members of this missing chain, and that in
Amphioxus the vertebrate approaches in organization to these low
invertebrate forms. The tunicates, indeed, are looked upon as degenerate
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 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, Rhabdopleura, 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
Balanoglossus 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 comparison of
the vertebrate central nervous system with that of the segmented
invertebrate, annelid or arthropod. In the latter the central nervous
system is composed of--

1.  The supra-oesophageal 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 antennæ are olfactory in function. These are connected with
the infra-oesophageal ganglia by the oesophageal commissures which encircle
the oesophagus.

2.  The infra-oesophageal ganglia and the two chains of ventral ganglia,
which are segmentally-arranged sets of ganglia. Of these, {13}each pair
gives rise to the nerves of its own segment, and these nerves are not
nerves of special sense as are the supra-oesophageal 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.

[Illustration: FIG. 3.--VERTEBRATE CENTRAL NERVOUS SYSTEM COMPARED WITH THE
CENTRAL NERVOUS SYSTEM AND ALIMENTARY CANAL OF THE ARTHROPOD.

A.  Vertebrate central nervous system. _S. 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; _Pn._, pineal gland.

B.  Invertebrate central nervous system. _S. Oes. G._, supra-oesophageal
ganglia; _I. Oes. G._, infra-oesophageal ganglia; _Oes. 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}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-oesophageal ganglia; the crura cerebri become the
oesophageal commissures, and the cranial and spinal segmental nerves are
respectively the nerves belonging to the infra-oesophageal 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 infundibulum, _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
ventricle. 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.

{15}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 explanation 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 explanation may still be found, and
must be diligently sought for, which shall take into account the strong
evidence of this relationship 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 {16}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.


IMPORTANCE 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 evolution depends,
it has gradually dawned upon the scientific mind that 'the fittest' may be
produced in two diametrically opposite ways: {17}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 enlargement 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 confirms 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 invertebrate
groups. Here also an upward progress is recognized as we {18}pass from the
sponges to the arthropods--a progress which is manifested, 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 transformation, 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 (_Ammocoetes_) 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 transformation 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-oesophageal 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 {19}results in the
formation of a brain region more like that of the higher vertebrates than
exists in Ammocoetes.

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 degeneration, not to a
higher type. Instances are seen in the Tunicata, and in various parasitic
arthropod forms, such as Lernæa, 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 immediate
ancestor of the vertebrate must have had a central nervous system nearly
approaching that of the lowest undegenerated vertebrate. 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 development 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}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 invertebrates 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 Ammocoetes, both of which are devoid of any marked medullary sheath, is
very apparent, and Retzius points out that the only evidence of
medullation, so characteristic of the vertebrates, is found in a species of
prawn (_Palæmon_). In all these cases the nearest resemblance to the
vertebrate tissues is to be found in the arthropod.


THE EVIDENCE OF PALÆONTOLOGY.

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
distribution 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 {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
accompanied by the rapid diminution and approaching extermination of the
larger mammals. No question can possibly arise as to the immediate 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 comparison 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 mammalian 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 reptiles arose from the race which was then predominant on
the earth--the Amphibia.

{22}[Illustration: FIG. 4.--PLAN OF GEOLOGICAL STRATA. (From LANKESTER.)]

{23}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 amphibians, 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}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. Henceforth, 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 organization 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 invertebrates; 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 discovered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc.
(Fig. 5 and Fig. 12). From them also arose the most striking group {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.

[Illustration: FIG. 5 (from H. WOODWARD).--1. _Limulus polyphemus_ (dorsal
aspect). 2. _Limulus,_ young, in trilobite stage. 3. _Prestwichia
rotundata._ 4. _Prestwichia Birtwelli._ 5. _Hemiaspis limuloides._ 6.
_Pseudoniscus aculeatus._]


The appearance of these sea-scorpions is given in Figs. 7 and 8,
representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They 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 trilobites {26}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,
Gigantostraca, Xiphosura, etc. Of all these the only member alive at the
present time is Limulus, or the King-Crab.

[Illustration: FIG. 6.--A TRILOBITE (_Dalmanites_) (after PICTET). Dorsal
view.]

[Illustration: FIG. 7.--_Eurypterus remipes_ (after NIESKOWSKI). Dorsal
view.]

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

[Illustration: FIG. 8.--A, _Pterygotus Osiliensis_ (from SCHMIDT). B,
_Stylonurus Logani_ (from WOODWARD). C, _Slimonia acuminata_ (from
WOODWARD).]

Korschelt and Heider have accordingly suggested the name _Palæostraca_ for
this whole group, and _Protostraca_ for the still earlier
{28}arthropod-like animals which gave origin to the trilobites themselves.
This name I shall adopt, and speak, therefore, of the _Palæostraca_ 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 palæostracan 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 appearance are the two
groups of animals, so different their methods of locomotion, that it is
apparently only an inmate of a lunatic asylum 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, wriggling, 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.

[Illustration: FIG. 9.--_Apus_ (from the Royal Natural History). Dorsal
view.]

[Illustration: FIG. 10.--_Branchipus stagnalis._ (From CLAUS.)]

{29}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 palæostracan 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, Heterostraci,
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 Cephalaspidæ, Tremataspidæ, 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 vertebræ 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 conspicuous 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. rostrata_, but
the rostrum was longer and the spine at the extremity of the head-shield
much longer and more conspicuous. 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.

{30}[Illustration: FIG. 11.--_Pteraspis dunensis_ (from DREVERMANN). Dorsal
view of body and spine on the right side. Head-end, showing long rostrum on
the left side.]

[Illustration: FIG. 12.--_Bunodes lunula._ (From SCHMIDT.)]

[Illustration: FIG. 13.--_Auchenaspis (Thyestes) verrucosus_, natural size.
(From WOODWARD.)]

{31}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 Cephalaspidian
fish known by the name of _Auchenaspis_ or _Thyestes verrucosa_, the other
a Palæostracan called _Bunodes lunula_.

[Illustration: FIG. 14.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis)
verrucosus_. (From ROHON.)

_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate
over brain; _Occ._, occipital region.]

[Illustration: FIG. 15.--_Pterichthys._]

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}earliest fishes and
members of the Palæostraca, 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 among living forms, but is immediately
revealed when the ages themselves are questioned.

[Illustration: FIG. 16.--_Bothriolepis._ (After PATTEN.)

_An._, position of anus.]

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

[Illustration: FIG. 17.--RESTORATION OF _Tremataspis_. (After ROHON,
slightly modified.)]

[Illustration: FIG. 18.--_Ammocoetes._]

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 {33}vertebrate, but also to a
distinct ancient type of arthropod, the Palæostracan, 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.


AMMOCOETES 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 Balanoglossus, 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:--

{34}Amphioxus, like the tunicates, does not possess the characteristics 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. In Amphioxus no eyes exist, for
the pigmented spot at the anterior extremity of the brain-region is no eye
but only a mass of pigment, 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
important 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 rudimentary 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.

{35}The next lowest group of living fishes is the Marsipobranchii 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 commencement of vertebræ.

Among these forms the lamprey is by far the best for investigation, 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 transformation 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 _Ammocoetes 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 Ammocoetes 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 investigation 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 peculiarities of the larva may be due to such
coenogenetic factors, yet on the whole many of them are due to ancestral
characters, which disappear 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}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 Ammocoetes, the larval form of the lamprey--a form which, as
I hope to satisfy my reader after perusal of subsequent pages, more nearly
resembles the ancient Cephalaspidian fishes than any other living
vertebrate.


COMPARISON OF CENTRAL NERVOUS SYSTEMS OF VERTEBRATE AND ARTHROPOD WITHOUT
REVERSAL 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 extraordinary fact that the infundibular tube is only a ventral
offshoot from the tube of the central nervous system. Yet this latter tube
{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 hypophysis, 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 infundibular 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 conclusion that the infundibulum was the
original oesophagus leads immediately to the further conclusion that the
ventricles of the vertebrate brain represent the original cephalic stomach,
and the central canal of the spinal cord the straight intestine of the
arthropod ancestor.

{38}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 oesophagus, 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 oesophagus,
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 composed 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 {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 epithelial 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
Ammocoetes, 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 Ammocoetes (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 oesophagus. 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.

{40}[Illustration: FIG. 19.--COMPARISON OF VERTEBRATE BRAINS.

_CB._, cerebellum; _PT._, pituitary body; _PN._, pineal body; _C. STR._,
corpus striatum; _G.H.R._, right ganglion habenulæ. _I._, olfactory; _II._,
optic nerves.]

{41}[Illustration: FIG. 20.--BRAIN OF AMMOCOETES.

A, dorsal view; B, lateral view; C, ventral view. _C.E.R._, cerebral
hemispheres; _G.H.R._, right ganglion habenulæ; _PN._, right pineal eye;
_CH_2_, _CH_3_, choroid plexuses; _I.-XII._ cranial nerves; _C.P._, _Conus
post-commissuralis_.]

{42}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 phylogenetic development,
the peculiarities of the formation of the central nervous system in the
vertebrate embryo ought to receive an appropriate 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 {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 pigmented 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 structures--an
epithelial non-nervous tube, on the outside of which the central nervous
system was originally grouped; how, as development 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 formation? 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 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.

[Illustration: FIG. 21.--A, METHOD OF FORMATION OF THE VERTEBRATE SPINAL
CORD FROM THE VENTRAL CHAIN OF GANGLIA AND THE INTESTINE OF AN ARTHROPOD,
REPRESENTED IN 1; B, METHOD OF FORMATION OF THE VERTEBRATE MEDULLA
OBLONGATA FROM THE INFRA-OESOPHAGEAL GANGLIA AND THE CEPHALIC STOMACH OF AN
ARTHROPOD.]

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 _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; {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 Ammocoetes 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-oesophageal
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 formation 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 Ammocoetes, in which animal a ventral portion of
the epithelial bag has been evidently compressed and its lumen finally
obliterated {46}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 _raphé_ or seam, which is so characteristic of the
infra-infundibular portion of the brain.

[Illustration: FIG. 22.--HORIZONTAL SECTION THROUGH THE BRAIN OF
AMMOCOETES.

_Cr._, membranous cranium; _I_, olfactory nerves; _l.v._, lateral
ventricles; _gl._, glandular tissue which fills up the cranial cavity.]

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 corresponding compression in the upper
brain-region forms the lateral ventricles, as is seen in the accompanying
figure of the brain of Ammocoetes (Fig. 22).

[Illustration: FIG. 23.--SECTION THROUGH RHOMBOIDAL SINUS OF BIRD.]

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 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 confined 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 {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-oesophageal 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}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 habenulæ_. 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 (Ammocoetes) 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 {49}region of the brain up to the region of the
pineal eye and _ganglion habenulæ_ is one large membranous bag, except for
the single constriction 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 quadrigemina 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 commissures 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 sign of the dorsal membranous roof,
those parts of the simple epithelial tube which still remain being tucked
away into the cavities to form the various choroid plexuses.

{50}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.

Especially 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 constriction 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 fimbriæ, is already commencing
to grow from the ventral mass of the medulla oblongata to surround these
lateral membranous folds. These _fimbriæ_ 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.

[Illustration: FIG. 24.--CEREBELLUM OF DOG-FISH.

_v_, worm of cerebellum; _IV._, membranous roof of fourth ventricle
continuous with the membranous folds on each side. Through these the
fimbriæ (_fb._) can be dimly seen.]

One of the striking facts about malformations 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, difficult 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 {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 termination 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, therefore, 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 CEPHALIZATION.

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 transversely 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-oesophageal 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}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-oesophageal 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 concentration 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-oesophageal ganglion-mass.

The infra-oesophageal 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 _prosoma_ 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 _mesosoma._ 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-oesophageal ganglia and ventral chain, which
consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia.

The infra-oesophageal ganglion-mass, then, in most of the Arthropoda 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
_neuromeres_, although all such indication has disappeared in the adult;
thus the {53}infra-oesophageal 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 Ammocoetes. In all the figures the
supra-oesophageal ganglia are lined horizontally, and their nerves shown,
viz. optic (lateral eyes (II) and median eyes (II[prime])), olfactory (I)
(first antennæ, camerostome, nose); then come the prosomatic ganglia
(dotted), with their nerves (A) supplying the mouth parts, and the second
antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their
nerves (B) supplying respiratory appendages. These figures show that the
concentrated brain mass around the oesophagus of an arthropod which has
arrived at the stage of Astacus, is represented by the supra-oesophageal
ganglia and the fused prosomatic ganglia.

The next stage in the evolution of the brain is seen in the gradual
inclusion of the mesosomatic ganglia, one after the other, into the
infra-oesophageal 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 consequence 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-oesophageal mass. It is also a fact that the
prosomatic 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 branchiæ, 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
together in the caudal region to form what is known as the caudal brain.

{54}[Illustration: FIG. 25.--COMPARISON OF INVERTEBRATE BRAINS FROM
BRANCHIPUS TO AMMOCOETES.]

{55}The brain in these animals may be spoken of as composed of three
parts--(1) the fused supra-oesophageal 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 corresponds, therefore, to
the supra-oesophageal 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 division, 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 metasomatic 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 illustrates 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}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 concentrated 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 Palæostracan age were Pterygotus,
Slimonia, etc., all animals of the scorpion-type--in fact, sea-scorpions.
Now, all these animals, spiders and scorpions, without exception, are
blood-suckers, and in all of them the concentrated cephalic mass of nervous
material surrounds an oesophagus 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 exceedingly narrow tube.

In Fig. 25 this increasing 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 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.

[Illustration: FIG. 26.--TRANSVERSE SECTION THROUGH THE BRAIN OF A YOUNG
THELYPHONUS.

_A_, supra-oesophageal ganglia; _B_, infra-oesophageal ganglia; _Al_,
oesophagus.]

{57}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 establishment 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 lung; 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
overcoming 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}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.


{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
Ammocoetes, 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 Ammocoetes stage is unknown; I myself have kept some
without transformation 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--Ammocoetes. 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
Ammocoetes 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 Ammocoetes more {60}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 be
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 statement that the
lamprey is a parasite, and that the eyes in Ammocoetes 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 gnathostomatous. 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, however, 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 gnathostomatous
condition either in Petromyzon or, in its larval state, Ammocoetes. 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 vertebrate
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 {61}resemblance to that
of the vertebrate. In the former case a transformation 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 vertebræ
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 branchiæ 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 Ammocoetes, the tentacles, the muco-cartilage, the tubular muscles, all
these structures, so striking in the Ammocoetes 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
Ammocoetes-like ancestors.

As to the eyes, they are functional in the adult form and as serviceable 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 AMMOCOETES 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 supra-infundibular portion of the brain in vertebrates corresponds
clearly to the supra-oesophageal portion of the invertebrate 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-oesophageal ganglia as to
the vertebrate deprived of its cerebrum. 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-oesophageal ganglia, for from a portion of these
ganglia arise the nerves to the eyes and the nerves to the first antennæ,
of which the latter are olfactory in function. Thus, in the accompanying
figure, taken from Bellonci, it is seen that the supra-oesophageal ganglia
consist of a superior segment corresponding to the cerebrum, a middle
segment from which arise the nerves to the lateral eyes and to the
olfactory antennæ, 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 antennæ. This last segment is not
supra-oesophageal 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-oesophageal
series, and not to belong to the supra-oesophageal group.

[Illustration: FIG. 27.--THE BRAIN OF _Sphæroma serratum_. (After
BELLONCI.)

_Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _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.]

Further, in Limulus, in the scorpion-group, and in all the extinct
{63}Eurypteridæ--in fact, in the Palæostraca generally--there are two
median eyes in addition to the lateral eyes, which were innervated from
these ganglia.

In Ammocoetes, then, if the supra-infundibular portion of the brain really
corresponds to the supra-oesophageal of the palæostracan 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, Ammocoetes.

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
palæostracan 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
  invertebrates the vertebrate arose; no attempt is made to speculate upon
  the causes of variation by means of which evolution takes place.

  {64}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 topographical 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-oesophageal ganglia. 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 represented the original
  invertebrate oesophagus which had become closed and no longer opened into
  the alimentary canal owing to the formation of a new mouth 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 groups
  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
  _Balanoglossus_; they are looked upon as aberrant annelid forms by many
  observers.

  This theory 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 nervous 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
  guiding principles in this investigation?

  {65}The evolution of animal life on this earth can clearly, on the whole,
  be 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
  themselves a similar evolution is apparent, and the evidence shows that
  the vertebrate tissues resemble more closely those of the arthropod than
  of any other invertebrate group.

  The evidence 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 Palæostracan age, using the term Palæostraca 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 Palæostraca--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 palæostracan group, so that again and again
  palæontologists have found great difficulty in determining 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 Ammocoetes stage of the lamprey, because it
  was formerly considered to be a separate species and received the name of
  Ammocoetes. 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
  Ammocoetes on the other are {66}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 coelenterate 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
  competition 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
  {67}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 Palæostracan
  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 organs in
  the arthropod and vertebrate respectively.




{68}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
  vertebrates 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; certain 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 the 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
{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 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_.

[Illustration: FIG. 28.--DIAGRAM OF FORMATION OF AN UPRIGHT SIMPLE 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 hypodermal 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 {70}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 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.

[Illustration: FIG. 29.--DIAGRAM OF 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.]

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 {71}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 'Punctsubstanz,' or in
modern terminology, 'Neuropil.' Fibres which pass into this 'neuropil' from
other parts of the brain connect them with the optic ganglion.

{72}At the present time, owing to the researches of Golgi, Ramón 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.'

[Illustration: FIG. 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, _Rt. g._]

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

{73}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 composed of a
series of layers, which may be analyzed into their several components as
follows:--

  Layer of rods and cones  }
  External nuclear layer   } retina proper          } Ectodermic part
  External molecular layer }}
  Internal nuclear layer    } ganglion of retina    }
  Internal molecular layer }}                       } retinal } neurodermic
  Optic nerve-cell layer   } ganglion of optic nerve} ganglion}   part
  Layer of optic nerve     }
    fibres

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
development 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 Korschelt 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}retina, as quoted from Bobretsky by Balfour, is therefore in
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 palæostracan 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 Palæostraca (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 palæontologists, {75}in
the best-preserved head-shields of the most ancient fishes, especially well
seen in the Osteostraci, in Cephalaspis, Tremataspis, 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 dwindling 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 Ammocoetes 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 passing to the consideration of the structure of the median eyes of
Ammocoetes, it is advisable to see whether these median eyes in other
animals, such as arachnids and crustaceans, belong to any particular type
of eyes, for then assuredly the median eyes of Ammocoetes 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 characteristics 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 {76}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 upright; 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 palæostracan group. What, then, is the
nature of the median eyes in the vertebrate?


THE MEDIAN EYES OF AMMOCOETES.

The evidence of Ammocoetes is so conclusive that I, for one, cannot
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 whiteness of its pigment. Upon
opening the brain-case the appearance as in Fig. 20 is seen, and the mass
of the right _ganglion habenulæ_ (_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 habenulæ_ is present, though much
smaller than on {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 called Meynert's bundles, which connect this region with
the infra-infundibular region of the brain; of these, the right Meynert's
bundle is much larger than the left.

[Illustration: FIG. 31.--ONE OF A SERIES OF HORIZONTAL SECTIONS THROUGH THE
HEAD OF AMMOCOETES.

_l.m._, upper lip muscles; _m.c._, muco-cartilage; _n._, nose; _na.c._,
nasal cartilage; _pn._, right pineal eye and nerve; _g.h.r._, right
_ganglion habenulæ_; _s.m._, somatic muscles; _cr._, membranous wall of
cranium; _ch._, choroid plexus; _gl._, glandular substance and pigment
filling up brain-case.]

{78}[Illustration: FIG. 32.--EYE OF ACILIUS LARVA, WITH ITS OPTIC GANGLION.

On the right side the nerve end-cells have been drawn free from pigment.]

[Illustration: FIG. 33.--PINEAL EYE OF AMMOCOETES, WITH ITS _Ganglion
Habenulæ._

On the left side the eye is drawn as it 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.]

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 habenulæ_ 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 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 Ammocoetes (Fig. 31).
Originally, as described by Scott, the eye stood vertically {79}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 horizontally over
the _ganglion habenulæ_. For this reason the eye, nerve, and ganglion are
better shown in a nearly horizontal than in a transverse section.

The optic nerve belonging 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 question having been the
nerve of the eye, even though, as is possible, it does not contain any
functional nerve-fibres.

[Illustration: FIG. 34.--HORIZONTAL SECTION THROUGH BRAIN OF AMMOCOETES, TO
SHOW THE LEFT, OR VENTRAL PINEAL EYE.

_pn._2_, left or ventral pineal eye; _pn._1_, last remnant of right, or
dorsal pineal eye; _g.h.r._, right _ganglion habenulæ_; _g.h.l._1_,
_g.h.l._3_, parts of left _ganglion habenulæ_; _pi._, fold of _pia mater_
which separates the left _ganglion habenulæ_ from the left pineal eye;
_f._, strands of nerve-fibres connecting the left eye with its ganglion,
_g.h.l._3_; _V_3_, third ventricle; _V.aq._, ventricle of aquæduct.]

The second, ventral or left, eye, belonging to the left ganglion habenulæ
is very different in appearance, being much less evidently an eye. Fig. 34
is one of the same series of horizontal sections as Fig. 31, _pn._1_ being
the last remnant of the right, or dorsal, eye, while _pn._2_ shows the
left, or ventral, eye with its connection with the left _ganglion
habenulæ_.

{80}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
habenulæ_ just as the nerve to any simple eye enters into its optic
ganglion. This nerve, as I have shown, is a very distinct, 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 defined nerve at all; but the cells of the
_ganglion habenulæ_ 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 degeneration than on the right side, it may be fairly
concluded that the original condition of these two median eyes is more
nearly represented by the right eye, with its well-defined nerve and optic
ganglion, 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 Ammocoetes.
Quite recently Dendy has examined the two median eyes in the New Zealand
lamprey _Geotria australis_. In this species the second eye is much better
defined than in the European lamprey, and its connection with the _ganglion
habenulæ_ 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
Ammocoetes stage of _Petromyzon Planeri_; in both, cells resembling those
of the cortex of the _ganglion habenulæ_ and connective tissues are mixed
up with the nerve-fibres which pass from each eye to its respective optic
ganglion.


THE RIGHT PINEAL EYE OF AMMOCOETES.

The optic fibres of the right median eye of Ammocoetes are connected 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 pigment, were described by me in
1890 as the retinal end-cells with large nuclei. In 1893, Studniçka
examined them more closely, and concluded 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 {81}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 Studniçka. These terminations are free from pigment,
and contain strikingly translucent 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. Studniçka has also seen these appearances, and figures them in his
second paper on the nerve end-cells of the pineal eye of Ammocoetes.

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

  1.  Ammocoetes possesses a pair of median eyes, just as was the case with
  the most ancient fishes, and with the members of the contemporary
  palæostracan 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 translucent 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 Studniçka'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 Studniçka 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.

{82}On the other hand, Dendy describes in the New Zealand lamprey, _Geotria
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 habenulæ_. 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 Ammocoetes of _Petromyzon Planeri_.
Taking also into consideration the continuity of the mass of small
ganglion-cells which surround this atrium with the cells of the _ganglion
habenulæ_ 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 habenulæ_
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
Ammocoetes 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 _Geotria_ worked
at by Dendy were in the 'Velasia' stage of the New Zealand {83}lamprey, and
correspond, therefore, more nearly to the Petromyzon than to the Ammocoetes
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 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.

[Illustration: FIG. 35.--EYE OF ACILIUS LARVÆ. (After PATTEN.)

_l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._,
rhabdites; _ret._, retinal end-cells.]

This distinction is not considered valid by other observers. Thus, {84}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
concerned 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.

[Illustration: FIG. 36.--EYE OF HYDROPHILUS LARVA, WITH THE PIGMENT OVER
THE RETINAL END-CELLS.

_l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._,
rhabdites; _ret._, retinal end-cells.]

This layer (_c._, Fig. 35) is composed of much-elongated cells of the
hypodermal layer, in each of which the large nucleus is always situated
towards the base of the cell. The space between it and the retina contains,
according to 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 Ammocoetes (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 hæmatoxylin 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 {85}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 translucent rhabdite-like bodies, such as are seen
in the termination 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.

Retina, 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?

[Illustration: FIG. 37.--PINEAL EYE OF AMMOCOETES, WITH ITS _Ganglion
Habenulæ_.]

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 {86}thickening of the cuticular
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 be 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
Ammocoetes, 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 apparently, 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 subdermal 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.

Studniçka, 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 {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 crustaceans 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 Palæostraca, 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 palæostracan 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 convincing 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 crustaceans 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 palæostracan epoch {88}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 characteristic 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. _Harpes 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, "Palæontologists 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 conjectured 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 Gammaridæ, 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 be 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.

{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.

[Illustration: 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 the ganglion of the retina;
_f.br.r._, terminal fibre-layer of retina; _r._, layer of retinal end-cells
(indicated only).]


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 connect the peripheral part of the optic
ganglion with the brain, the so-called optic nerve, are by no means
homologous in the different groups; for in some cases, as in many of the
stalk-eyed crustaceans, the whole optic ganglion is at the periphery, 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 {90}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.

[Illustration: FIG. 39.--THE BRAIN OF _Sphæroma serratum_. (After
BELLONCI.)

_Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _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.]

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 Sphæroma, it would almost appear as though the cerebral part
of the retina in the higher forms originated from two ganglionic
enlargements, an external and 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 (_cf._ 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 {91}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, together with smaller ones, form the ganglionic 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 vertebrate
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 substance called by
Leydig in arthropods 'Punktsubstanz,' and known in vertebrates and in
invertebrates at the present time as 'neuropil.' A further analysis
resolves the whole system into a combination 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.

[Illustration: FIG. 40.--BIPOLAR CELLS OF NUCLEAR LAYER IN RETINA OF
BRANCHIPUS. (After CLAUS.)

_f.br.r._, terminal fibre-layer of retina; _n.l.r.g._, bipolar cells of the
ganglion of the retina = inner nuclear layer; _m.l._, Punktsubstanz = inner
molecular layer; _b.m._, basement membrane formed by neurilemma round
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 {92}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 consists 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.

{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 description 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 RETINA OF THE LATERAL EYE OF AMMOCOETES.

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 Ammocoetes 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 clear
away a remarkable misconception, shared among others by {94}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
Ammocoete 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 Ammocoete. The development of such aborted eyes
has as yet been studied only in the Ammocoete, 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 development is not followed by a stoppage, but on
the contrary by a period of very highly intensified progressive development
during the metamorphosis," with the result that "the adult eye of
_Petromyzon Planeri_ does not diverge from the ordinary type."

Referring 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 ammocoete 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.
   (3) Optic fibre layer.
   (4) Thick inner nuclear layer.
   (5) Peculiar double-layered ganglionic layer.
   (6) External molecular layer.
   (7) External nuclear layer.
   (8) _Membrana limitans externa._
   (9) Layer of rods.
  (10) Pigment-epithelium.

{95}[Illustration: FIG. 41.--RETINA AND OPTIC NERVE OF PETROMYZON. (AFTER
MÜLLER AND LANGERHANS.)

On the left side the Müllerian 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 ganglion 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.]

He points out especially the peculiarity of layer (2) (2, Fig. 41), the
inner molecular, in which two rows of nuclei are arranged with great
regularity, the one row closely touching the _membrana limitans interna_,
the other at the inner boundary of the middle third of the {96}molecular
layer. Of these two rows of nuclei, he describes the innermost 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. Müller, in 1874, gave a most careful description of the eye of
Ammocoetes 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
Müller's illustrations. In the accompanying figure (Fig. 41) I reproduce on
the right-hand side Müller'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 neurodermal (cerebral) part which forms the rest of the retina.

Further, Müller points out that the neuroderm gives origin throughout 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 _membrana
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
Müllerian 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 {97}optic ganglion
and of the retinal ganglion ramify to form the molecular layers.

It is evident from Fig. 41 that the retina of Ammocoetes 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 Ammocoetes 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 Ammocoetes 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 habenulæ_), in the {98}optic lobes and other parts of
the Ammocoetes 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 Ammocoetes 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.
Müller. Here, then, is a striking peculiarity of the retina of the lamprey,
and according to Müller 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 (Nervenansätze), 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 Ammocoetes, 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.

Müller 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 regularity of the
cells and their lighter appearance when stained. Thus in his description of
the retina of an Ammocoetes 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 characteristic 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 Reichenbach, {99}according to whom the
earliest sign of the formation of the retina is an ectodermic involution
(Augen-einstülpung), 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 consequence 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.

Reichenbach 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 thickening, 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 Reichenbach. 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 Reichenbach 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 Ammocoetes and in crustaceans is remarkably close, and in both cases
observers have found the same difficulty in interpreting their meaning. In
each case one group of observers looks upon them as ganglion-cells, the
other as supporting structures. Thus in the lamprey, Müller considers them
to belong to the supporting elements, while Langerhans and Kohl describe
them as a double {100}layer of ganglion-cells. In the crustacean, Berger in
Squilla, Grenacher 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 interpretation 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, Reichenbach, and Parker
in the following figure.

[Illustration: FIG. 42.--IDEAL DIAGRAM 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 cerebral 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 comparison of this figure (Fig. 42) with that of the Petromyzon retina
(Fig. 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 terminations of the retinal cells
belonging to Parker's neurones of the first order, is very much longer than
in Petromyzon or in any vertebrate, for the external molecular layer (6,
Fig. 41) (Müller's layer of Nervenansätze) 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 Ammocoetes, as pointed out by
{101}Langerhans and carefully figured by Kohl, a crossing of the fibres of
the optic nerve occurs as the nerve leaves the retina, just as is so
universally 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 arthropod brain; thus
Bellonci in Sphæroma, 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 Sphæroma 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
retina and retinal ganglion of the type found in arthropods. 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 Götte'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}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 the
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, Götte's
observation 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 vertebrates, 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 vertebrate eye differs
from that of a compound retina of an invertebrate; in the former, a
striking supporting tissue exists, known as Müller'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 explanation 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?

{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
walls 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}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 equally 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 with 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 understand. 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 Kölliker and His, who state
that the optic nerve-fibres are formed outside the {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 represented by the upper diagram in Fig.
43; at a later stage, when the choroidal slit is closed, by the lower
diagram.

[Illustration: FIG. 43.--DIAGRAM OF THE RELATION 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.]

The evident truth of this manner of looking at the origin of the optic
nerve is demonstrated by the appearance of the optic nerve in Ammocoetes
and Petromyzon. 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
Müllerian fibres.

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

"The development of the optic nerve shows peculiarities in {106}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 proliferation 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 supporting 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 Ammocoetes 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 narrowness 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 {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 ganglionic 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
corresponding 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
Müllerian 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, forming 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 retinæ_; then suddenly, at the
_ora {108}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 Müller. 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 membrane in all
regions of the retina are certain peculiar-shaped bodies known as the
radial fibres of Müller. 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, indeed, at the _ora serrata_ the fibres of Müller may
be seen suddenly to lose their peculiar features and to pass into the
ordinary columnar cells which form the _pars ciliaris retinæ_."

[Illustration: 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; _p._, pigment epithelium;
_p.c.r., pars ciliaris retinæ_; _m.f._, Müllerian fibres; _l._, lens.]

It is then absolutely clear that the essential parts of the eye may be
considered as composed of two parts--

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
Müllerian 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 Müllerian fibres.

{109}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-oesophageal 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 conclusions 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 consideration 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 Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most marked
characteristic of the stomach-region is the presence of four pairs of
coecal diverticula, which spread laterally over the prosomatic region. In
the spiders the number of such diverticula increases, and the whole
prosomatic region becomes filled 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 {110}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 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.

[Illustration: 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.]

{111}[Illustration: FIG. 46.--THE BRAIN, EYES, AND ANTERIOR TERMINATION OF
THE ALIMENTARY CANAL OF ARTEMIA, VIEWED FROM THE DORSAL ASPECT.

_Br._, brain; _l.e._, lateral eyes; _c.e._, median eyes; _Al._, alimentary
canal.]

[Illustration: FIG. 47.--A, THE FORMATION OF THE RETINA OF THE EYE OF
AMMOCOETES (after SCOTT); B, THE FORMATION OF THE RETINA OF THE EYE OF
AMMOCOETES, ON MY THEORY.

_R._, retina; _l._, lens; _O.n._, optic nerve fibres; _Al._, cephalic end
of invertebrate alimentary canal; _V._, cavity of ventricles of brain;
_Al.d._, anterior diverticulum of alimentary canal; _op.d._, optic
diverticulum.]

My friend Mr. W. B. Hardy has especially investigated the nervous system of
Artemia. In the course of his work he cut serial 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 lateral eye is
so closely attached to the end of the corresponding diverticulum of the gut
that the lining cells of the ventral part of the diverticulum form a lining
to the retinal ganglion (Fig. 45). In this animal there are only two
gut-diverticula, which are situated most anteriorly. 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 Fig. 46. It is instructive to
compare with this figure Scott's picture of the developing eye in
Ammocoetes, where he figures the retina as {112}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 I 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 surrounded 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^cDougall, 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 contractions of the anterior diverticula to
the middle region of the gut.

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

Such an origin for the lateral eyes of the vertebrate explains in a simple
and satisfactory manner why the vertebrate retina is a compound 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 {114}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 Reichenbach 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 Götte'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 Bütschli 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 without 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 Cephalaspid, the
lateral eyes remained throughout functional. I therefore, for my own part,
would say that the inversion of the {115}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 'lentigen'),
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
{116}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 Palæostraca (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,
palæostracan branch, leads through the Cephalaspidæ 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 coelom and
urinary apparatus.


{117}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 vertebrate, seeing 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
  Palæostracans 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 underlying 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 invagination, 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 Palæostracan group, which
  is regarded as the ancestor of both the crustaceans and arachnids.
  {118}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 Müllerian 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 Müllerian 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 explanation 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 Artemia, 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 Palæostraca.




{119}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 Ammocoetes; its topographical arrangement, its
  structure, its origin in muco-cartilage.--The prosomatic skeleton of
  Ammocoetes; the trabeculæ and parachordals, their structure, their origin
  in white fibrous tissue.--The mesosomatic skeleton of Limulus compared
  with that of Ammocoetes; similarity of position, of structure, of origin
  in muco-cartilage.--The prosomatic skeleton of Limulus; the entosternite
  or plastron compared with the trabeculæ 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 epithelial tube which
formed the alimentary canal of the ancient type of marine arthropods such
as were dominant in the seas when the vertebrates 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 vertebrates from
arthropods, the one skeleton being an endo-skeleton composed of cartilage
and bone, and the other an exo-skeleton composed 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}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 _sui 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 phylogenetic
origin: as we pass from the higher vertebrates to the lower these
structures can be traced back to a cartilaginous and membranous 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 {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-brain.

[Illustration: 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.-XII._, cranial nerves.]

[Illustration: FIG. 49.--HEAD OF EMBRYO DOG-FISH (from PARKER), BASAL VIEW
OF CRANIUM FROM ABOVE.

_ol._, olfactory sacs; _au._, auditory capsule; _py._, pituitary body;
_pa.ch._, parachordal cartilage; _tr._, trabecula; _inf._, infundibulum;
_pt.s._, pituitary space; _e._, eye.]

2. A pair of bars forming the floor for the fore-brain, known as the
trabeculæ (_tr_). These bars are continued forward from the parachordals.
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}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
(_au._) and the olfactory capsules (_ol._) unite more or less intimately
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 trabeculæ, so far as their mere anatomical relations are concerned,
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 trabeculæ always enclose a pituitary space, in
which lies the infundibulum (_inf._) and the pituitary body (_py._).

In the majority of the lower forms the trabeculæ arise quite independently
of the parachordals, though the two sets of elements soon unite.

The trabeculæ 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 may be distinguished, viz. an alisphenoidal
region behind, growing out from what is known as the basisphenoidal region
of the primitive trabeculæ, and an orbito-sphenoidal region in front,
growing out from the presphenoidal region of the trabeculæ. These plates
form at first a continuous lateral wall of the cranium. The cartilaginous
walls 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
trabeculæ and parachordals with olfactory and auditory cartilaginous
capsules.

{123}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. An anterior arch known as the mandibular
arch (Fig. 50, _Mn._), placed in front of the hyo-mandibular 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 branchial arches (_Br._), and are only fully developed in
the Ichthyopsida. In all cases of jaw-bearing (gnathostomatous) vertebrates
the first arch has become a supporting 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.

[Illustration: FIG. 50.--HEAD OF EMBRYO DOG-FISH, ELEVEN LINES LONG. (From
PARKER.)

_Tr._, trabecula; _Mn._, mandibular cartilage; _Hy._, hyoid arch; _Br_1._,
first branchial arch; _Na._, olfactory sac; _E._, eye; _Au._, auditory
capsule; _Hm._, hemisphere; _C_1_, _C_2_, _C_3_, cerebral vesicles.]

[Illustration: FIG. 51.--SKULL OF ADULT DOG-FISH, SIDE VIEW. (From PARKER.)

_cr._, cranium; _Br._, branchial arches; _Mn._ + _Hy._, mandibular and
hyoid arches.]

The true branchial arches persist, to a certain extent, in the Amphibia,
and become still more degenerated in the Amniota (reptiles, birds, and
mammals) in correlation with the total disappearance of a branchial
respiration at all periods of their life. {124}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 respiratory 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
trabeculæ, 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 branchiæ, 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 vertebræ 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 vertebræ. Owing especially to the
labours of Huxley, who showed that the segmentation 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 vertebræ 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 {125}structures in the fossils of the
Silurian age either among the vertebrate 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 Ammocoetes.

The Elasmobranchs represent the most primitive of the gnathostomatous
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 trabeculæ
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 contradistinction to this elaborate
cranio-facial skeleton, the spinal vertebral skeleton is represented only
by segmentally arranged small pieces of cartilage formed in the connective
tissue dissepiments between segmented sheets of body-muscles (myotomes).

[Illustration: FIG. 52.--SKELETON OF PETROMYZON. (From PARKER.)

_na._, nasal capsule; _au._, auditory capsule; _nc._, notochord.]

But Petromyzon is derived from Ammocoetes by a remarkable process of
transformation, and a most important part of that transformation is the
formation of new cartilaginous structures. Thus we see that in Ammocoetes
there is no sign of a cartilaginous vertebral column; at transformation the
rudimentary vertebræ of Petromyzon are formed. In Ammocoetes the brain-case
is a simple fibrous membranous covering; at transformation this becomes
cartilaginous. In Ammocoetes 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 Ammocoetes, and we may therefore
conclude {126}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 trabeculæ and parachordals, the auditory and
nasal capsules--a clear proof that the cranial skeleton is older than the
spinal. Of these structures the branchial bars are the only evidently
segmented parts.

[Illustration: FIG. 53.--COMPARISON OF CARTILAGINOUS SKELETON OF LIMULUS
AND AMMOCOETES.

A, Diagram of cartilaginous skeleton of Limulus. _Soft cartilage_,
entapophysial ligaments, deep black; branchial bars simply hatched; _hard
cartilage_, lateral trabeculæ of entosternite, netted; _Ph._, position of
pharynx.

B, Diagram of cartilaginous skeleton of Ammocoetes. _Soft cartilage_,
sub-chordal cartilaginous bands, deep black; branchial basket-work (first
formed part), simply hatched; _hard cartilage_, cranio-facial skeleton,
trabeculæ, parachordals and auditory capsules, netted; _Inf._, position of
tube of infundibulum (old oesophagus).]


THE SOFT CARTILAGE OF THE BRANCHIAL SKELETON OF AMMOCOETES.

The study of Ammocoetes 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 topographical position. J. Müller was the first to point
out that these two sets of cartilages differ in appearance and
constitution, 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 skeleton is composed of soft
cartilage, while the basi-cranial skeleton, consisting of trabeculæ,
parachordals, and auditory capsule, is composed {127}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 trabeculæ 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 continuous: 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 primitive 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 Ammocoetes 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 trabeculæ."

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

Underlying the skin of Ammocoetes 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 characteristic 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 superjacent 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
branchiæ 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}bars of other higher fishes, in that it forms a system of cartilages
which lie external to the branchiæ--an extra-branchial system.

This branchial basket-work is simpler in Ammocoetes than in Petromyzon, and
its actual starting-point consists of a main transverse 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 Ammocoetes examined
by Schaffer, he could find no absolute discontinuity of the cartilage
except between 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 trabeculæ and parachordals, and only later do the two
systems join.

[Illustration: FIG. 54.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES.

_Th._, thyroid gland; _M._, lower lip, with its muscles.]

These observations on Ammocoetes 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 connected with two
axial longitudinal cartilaginous rods, which at first contained cartilage
only near the places of junction of the branchial {129}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 trabeculæ
and parachordals with the auditory capsules: a system which was at first
entirely separated from the mesosomatic, 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 cartilaginous skull and the rudimentary vertebræ, 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 which 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 cartilage.
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 cartilage, and the last observer
(Schaffer) 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 {130}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.

[Illustration: FIG. 55.--A, BRANCHIAL CARTILAGE OF AMMOCOETES, STAINED WITH
THIONIN. B, BRANCHIAL CARTILAGE OF LIMULUS, STAINED WITH THIONIN.]

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


ON THE STRUCTURE OF THE MUCO-CARTILAGE IN AMMOCOETES.

We can, however, go further than this, and ask how this cartilage itself is
formed in Ammocoetes? The answer is most definite, most instructive and
suggestive, for in all cases this particular kind of cartilage 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 perichondrium in
Ammocoetes 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 fibrillæ, 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 fibrillæ filled up
with a semi-fluid mass.

{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.

[Illustration: FIG. 56.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE
OF AMMOCOETES.]

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 disregarded
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 entirely confined to the head-region, and its distribution there is most
suggestive, for, as will be described fully later on, it forms a skeleton
which both in structure and position resembles very closely the
head-shields of cephalaspidian fishes. At the present part of my argument
its more immediate interest lies in the method 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 substances a bright
purple. Schaffer made use of a corresponding basophil stain, hæmalum. 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}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 Ammocoetes.

The conclusions to which we are led by the study of the structure,
position, and mode of origin of these primitive cartilages of Ammocoetes
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 AMMOCOETES.

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 Ammocoetes, which consists of the trabeculæ, 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 composed of a
foremost part, the trabeculæ (_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 Ammocoetes 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 {133}brain
can be clearly seen. In an earlier stage of Ammocoetes 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 presence 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.

[Illustration: FIG. 57.--A, CARTILAGE OF TRABECULÆ OF AMMOCOETES, STAINED
WITH HÆMATOXYLIN AND PICRIC ACID. B, NESTS OF CARTILAGE CELLS IN
ENTOSTERNITE OF HYPOCTONUS, STAINED WITH HÆMATOXYLIN AND PICRIC ACID.]


STRUCTURE OF THE HARD CARTILAGES.

The structure of this hard cartilage of the trabeculæ and auditory capsules
resembles that of the soft, in so far that it consists of large 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 hæmatoxylin
and picric acid {134}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 trabeculæ 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 hæmalum-eosin the hard cartilage stains red, the
soft blue; and he points out that even with over-staining by hæmalum 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
tropoeolin-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 difference 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 substance, 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
Ammocoetes--_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, consequently we cannot be said to
possess any really exhaustive and {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 chondroblasts 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 Ammocoetes, 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 trabeculæ 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 themselves 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}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 vertebrate
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 LIMULUS.

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
palæostracan 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
Ammocoetes, cartilaginous bars supporting the branchiæ, and these bars are
situated externally to the branchiæ, as in Ammocoetes. In addition this
animal possesses a basi-cranial internal semi-cartilaginous unsegmented
plate known as the entosternite or plastron situated, with respect to the
oesophagus, similarly to the position of the trabeculæ 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 Ammocoetes.

{137}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 OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED OF SOFT
CARTILAGE.

Searching through the literature of the histology of the cartilaginous
tissues in invertebrate animals, to see whether any cartilage had been
described similar to that seen in the branchial cartilages of Ammocoetes,
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 Ammocoetes. 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 XI.). 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 prosoma.

{138}[Illustration: FIG. 58.--TRANSVERSE SECTION THROUGH THE MESOSOMA OF
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._, longitudinal venous sinus; _Dv._,
dorso-ventral somatic muscle; _Vp._, veno-pericardial muscle.]

At the base of each of these appendages, where it is attached to the body
of the animal, the external chitinous surface is characterized 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 {139}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 mesosomatic carapace indicates, by the number of
entapophyses, the number of segments in that region, in contradistinction
to the prosomatic 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.l._) (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 entapophysis, 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 parenchymatous
cartilage, as it occurs in the branchial cartilages of Ammocoetes, 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
Ammocoetes 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 Ammocoetes. 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
Ammocoetes, does not stain, or gives only a light-blue tinge with thionin.
The tissue of {140}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.

[Illustration: 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 branchiæ red. _gl._, generative and hepatic glands surrounding the
central nervous system and passing into the base of the flabellum (_fl._).]

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

{141}I have attempted in Fig. 53 to represent this close resemblance
between the segmented branchial skeleton of Limulus and of Ammocoetes, a
resemblance so close as to reach even to minute details, such as the
thinning out of the cartilage in the subchordal bands and entapophysial
ligaments respectively between the places where the branchial bars come
off.

[Illustration: FIG. 60.--DIAGRAM OF AMMOCOETES 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
branchiæ red. _gl._, glandular substance surrounding the central nervous
system and passing into the auditory capsule with the auditory nerve
(_VIII._).]

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

In Fig. 60 I have drawn a corresponding picture of the prosomatic and
mesosomatic region of Ammocoetes with the corresponding nerves {142}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 prosomatic
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 vertebrate
ancestor, that they do possess a remarkable internal semi-cartilaginous
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
preponderance 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 hæmatoxylin and picric acid, while
the fibres between the cell-nests stain a blue-brown colour, partly from
the hæmatoxylin, 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 {143}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-oesophageal portion of the central nervous system, and in all
cases it possesses two anterior horns which pass around the oesophagus and
the nerve-masses which immediately enclose the oesophagus (Fig. 61, A).
These lateral horns, then, which lie laterally and slightly ventral to the
central nervous system, and are called by Ray Lankester and Benham the
sub-neural portion of the entosternite, are very nearly in exactly the
position of the racquet-shaped head of the trabeculæ in Ammocoetes. 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 position. Now, these two lateral horns of the plastron of Limulus
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 Thelyphonidæ, the plastron consists
mainly of these two lateral ridges or trabeculæ, as they might be called,
and Schimkéwitsch, who more than any one else has made a comparative study
of the entosternite, describes it as composed in these animals of two
lateral trabeculæ crossed by three transverse trabeculæ. I myself can
confirm his description, and give in Fig. 61, B, the appearance of the
entosternite of Thelyphonus or of Hypoctonus. The supra-oesophageal ganglia
and part of the infra-oesophageal ganglia fill up the space _Ph._;
stretching over the rest of the infra-oesophageal mass is a transverse
trabecula, which is very thin; then comes a space in which is seen the rest
of the infra-oesophageal mass, and then the posterior part of the plastron,
ventrally to which lies the commencement of the ventral nerve-cord.

[Illustration: FIG. 61.--A, ENTOSTERNITE OF LIMULUS; B, ENTOSTERNITE OF
THELYPHONUS.

_Ph._, position of pharynx.]

{144}In these forms, in which the central nervous system is more
concentrated towards the cephalic end than in Limulus, 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 trabeculæ, which would then
take up the ventro-lateral position of the two trabeculæ of Ammocoetes.

On the other hand, it might be that two lateral trabeculæ, 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, Schimkéwitsch has come to the
conclusion that this latter explanation is the true one. He points out that
the lateral trabeculæ 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 trabeculæ are
more cartilaginous, while the transverse are more fibrous. Schimkéwitsch,
from observations of structure and from embryological investigations, 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 segmentally arranged mesodermic tendons, and
are crossed by the fibrils of the transverse muscular bundles.

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

It is clear from these observations of Schimkéwitsch, that the essential
part of the entosternite consists of two lateral trabeculæ, 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 trabeculæ function as supports for the attachment of
muscles, which are specially attached at certain places. At these places
transverse fibres belonging to some {145}of the muscular attachments cross
between the two longitudinal trabeculæ, and so form the transverse
trabeculæ.

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

From this evidence it is easy to see that that portion of the basi-cranial
skeleton known as the trabeculæ may have originated from the formation of
cartilage in the plastron or entosternite of a palæostracan 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 Schimkéwitsch thinks, the plastron may
have been originally segmented, it has long lost all sign of segmentation.
The latter would be derived from a second entosternite 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 palæostracan
stock on the invertebrate side and to the Cyclostomata on that of the
vertebrate; for in Limulus, the only living representative of the
Palæostraca, and in Limulus alone, we find a skeleton marvellously 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 Cephalaspidæ, 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}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 (Ammocoetes), in which vertebræ are not
  yet formed, but the cranial and branchial skeleton is well marked.

  The embryological 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 Ammocoetes 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 trabeculæ 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
  Ammocoetes we find the manifest natural outcome of the embryological
  history in the shape of simple trabeculæ and parachordals, from which the
  whole complicated basi-cranial skeleton of the higher vertebrates arose.

  The mesosomatic skeleton, which is formed before the prosomatic,
  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 branchiæ are situated.

  The cartilages which compose these two skeletons respectively are
  markedly different in chemical constitution, in that the first (hard
  cartilage) 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 Ammocoetes 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, which 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 Ammocoetes 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.

  {147}In the invertebrate kingdom true cartilage occurs but scantily.
  There is a cartilaginous covering of the brain of cephalopods. 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, and 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 Ammocoetes is extraordinarily great.

  Here, in Limulus, just as in Ammocoetes, 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 ligament containing 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 difficulty 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 cartilaginous
  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-formation 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
  Palæostraca, and is of so strong a character that, taken alone, it may
  almost be considered as proof of such origin.




{148}CHAPTER IV

_THE EVIDENCE OF THE RESPIRATORY APPARATUS_

  Branchiæ 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 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 branchiæ, 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 development 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 {149}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 evolution 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 Ammocoetes, 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 consequences which logically follow upon the supposition
that the branchial cartilaginous bars of Limulus are homologous with the
branchial basket-work of Ammocoetes.


INTERNAL BRANCHIAL APPENDAGES.

Seeing that in both cases the cartilaginous bars of Limulus and Ammocoetes
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 branchiæ of the vertebrate were derived from the
branchiæ of the arthropod, a conclusion which, according to the generally
accepted view of the origin of the respiratory region in the vertebrate, is
extremely difficult to accept; for the branchiæ of Limulus and of the
Arthropoda in general are part of the mesosomatic appendages, while the
branchiæ of vertebrates are derived from the anterior part of the
alimentary canal. This conclusion, therefore, implies that the vertebrate
has utilized in the formation of the anterior portion of its new alimentary
canal the branchial appendages of the palæostracan ancestor.

{150}[Illustration: FIG. 62.--_Eurypterus._

The segments and appendages on the right are numbered in correspondence
with the cranial system of lateral nerve-roots as found in vertebrates.
_M._, metastoma. The surface ornamentation is represented on the first
segment posterior to the branchial segments. The opercular appendage is
marked out by dots.]

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 occurred in that lower group.
On the assumption, therefore, that the vertebrate branchiæ 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 branchiæ. In this way the so-called lung-books of the scorpion
are formed, which are in all respects homologous with the branchiæ or
gill-books of Limulus. Now, as already mentioned, the lords of creation in
the palæostracan 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 appendages on the segments of the mesosomatic region. As
they lived in the sea, they must have breathed with gills, and those
branchial appendages 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 branchiæ, and
these segments are therefore supposed to have borne the branchiæ. 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 (_Palæophonus 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
{151}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 branchiæ, which had been directly derived
from the branchiæ-bearing appendages of their Limulus-like kinsfolk.

This abolition of the branchiæ-bearing appendages as external organs of
locomotion, with the retention of the important branchial portion of the
appendage as internal branchiæ, 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 antennæ; the important excretory organs, the coxal glands, and
important respiratory organs, the branchiæ, are still present in the
vertebrate, although the appendages to which they originally belonged have
dwindled away, or, at all events, are no longer recognizable as arthropod
appendages.


INNERVATION OF BRANCHIAL 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}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
pronounced 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-oesophageal
and infra-oesophageal 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-oesophageal 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 contains 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 arrangement 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.

{153}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 ventral 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
segmental nerves admits of two solutions--either the cranial arrangement
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 phylogenetic
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 posteriorly to the one first formed. We
see how, in Ammocoetes, the first formed parts of the skeleton are the
branchial bars and the basi-cranial system, while the rudiments of the
vertebræ 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}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 unincubated blastoderm which lies
anterior to the centre of the blastoderm, 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 distinct 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 {155}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 extraordinary 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 vertebrate 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 innervated by the IIIrd (oculomotor), IVth (trochlearis), and
VIth (abducens) nerves, while the posterior segments give origin to the
{156}muscles from the cranium to the shoulder-girdle, innervated by the
XIIth (hypoglossal) nerve. The ventral series of segments give origin to
the musculature supplied by the trigeminal, facial, glossopharyngeal, 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:--

  ---------+---------------------------------
  Segments.| Ventral nerve-roots and muscles
           |      derived from myotomes.
  ---------+---------+-----------------------
      1    |  III.   |M. rectus superior,
           |         | m. rectus
           |         | internus, m.
           |         | rectus inferior,
           |         | m. obliquus inferior
      2    |   IV.   |M. obliquus
           |         | superior
      3    |   VI.   |M. rectus externus
      4    |   --    |      --
      5    |   --    |      --
      6    |   --    |      --
      7    |   XII.  |} Muscles from
      8    |   XII.  |}  cranium to
      9    |   XII.  |}shoulder-girdle
  ---------+---------+-----------------------

  ---------+------------------+--------------------------------
  Segments.|  Visceral clefts.| Dorsal nerve-roots and muscles.
           |                  |
  ---------+------------------+--------------+-----------------
      1    |                  |  V. N. op-   |
           |                  |  thalmicus   |
           |                  |  profundus   |
      2    |                  |     V.       |   Masticating
           | 1st Mandibular   |              |     muscles.
      3    |                  |     VII._1   |{Facial muscles
           |                  |              |{(VIII. is dorsal
           | 2nd {Hyoid_1     |     VII._2   |{branch of VII.)
      4    |     {Hyoid_2     |              |
      5    |3rd 1st Branchial |    IX.       |
      6    |4th 2nd    "      |     X._1     |
      7    |{5th 3rd   "      |     X._2     |Branchial and
      8    |{6th 4th   "      |     X._3}    |visceral muscles.
      9    |{7th 5th   "      |     X._4     |
  ---------+------------------+--------------+-----------------

As is seen in the table, van Wijhe attempts to arrange the cranial
segmental nerves into dorsal and ventral roots, in accordance with the
arrangement in the spinal region. In order to do this he calls the Vth,
VIIth, 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.

{157}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 may be called somatic and splanchnic. The somatic segments,
consisting of the outer skin and the body musculature, are _doubly_
innervated as are those of the spinal cord by a series of ventral motor
roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the
abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series
of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve.
But the splanchnic segments are innervated by _single_ roots, the vagus or
Xth nerve, glossopharyngeal or IXth nerve, facial or VIIth 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 connected 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 LIMULUS AND BRANCHIPUS TO THE LATERAL
ROOT 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 palæostracan 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 mesosomatic 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 of the nerve-roots {158}in
Limulus as in other arthropods is the large 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-muscles, 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
Ammocoetes, 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 {159}appendage-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 segmented 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 segmentation may be spoken of as twofold, yet the
number of segments in the prosomatic and mesosomatic regions are much more
clearly marked out by the appendages than by the divisions of the soma;
for, in the prosomatic region such a fusion of somatic segments to form the
tergal prosomatic carapace has taken place that the segments of which it is
composed are visible only in the young condition, 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 _branchiomeric segmentation_,
but yet would show {160}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 palæostracan 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 apparatus in the
extinct fishes of Silurian times, we have still living, in the shape of
Ammocoetes, 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 Ammocoetes,
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.


{161}THE RESPIRATORY CHAMBER OF AMMOCOETES.

In order to make clear the nature of the branchial segments in Ammocoetes,
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 Ammocoetes consists of two chambers, the
contents of which are different. In front, an oral or stomodæal 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 branchiæ. A glance at the two
drawings shows clearly that Rathke's original description of this chamber
is the natural one, for he at that time, looking upon _Ammocoetes
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 represented
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
Rathke'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 branchial 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 (_Sp._) 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 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 the
diaphragms; each of these nerves possesses its own ganglion--the
epibranchial ganglion.

{162}[Illustration: FIG. 63.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.

Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
Tubular constrictor muscles distinguished from striated constrictor muscles
by simple hatching. _Tent._, tentacles; _Tent. m.c._, muco-cartilage of
tentacles; _Vel. m.c._, muco-cartilage of the velum; _Hy. m.c._,
muco-cartilage of the hyoid segment; _Ps. br._, pseudo-branchial groove;
_Br. cart._, branchial cartilages; _Sp._, space between somatic and
splanchnic muscles; _Th. op._, orifice of thyroid; _H._, heart.]

{163}[Illustration: FIG. 64.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.

_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._,
median serrated flange of velar folds.]

{164}[Illustration: FIG. 65.--SECTION THROUGH BRANCHIAL APPENDAGE OF
AMMOCOETES.

_br. cart._, branchial cartilage; _v. br._, branchial vein; _a. br._,
branchial artery; _b.s._, blood-spaces; _p._, pigment; _S._, sense-organ;
_c._, ciliated 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.]

[Illustration: FIG. 66.--SECTION THROUGH BRANCHIAL APPENDAGE OF LIMULUS.

_br. cart._, branchial cartilage; _v.br._, branchial vein; _b.s._,
blood-spaces formed by branchial artery; _P._, pigment; _m_1_, posterior
entapophysio-branchial muscle; _m_2_, anterior entapophysio-branchial
muscle; _m_3_, external branchial muscle.]

The work of Miss Alcock has shown that the segmental branchial nerve
supplies solely and absolutely such an appendage or branchial {165}segment,
and does not supply any portion of the neighbouring branchial segments. The
nerve-supply in Ammocoetes 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 Ammocoetes, 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 embryological 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 embryology, and is
the outcome of the gastrula theory--a theory which {166}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 coelomatous 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--

[Illustration: FIG. 67.--DIAGRAMS TO SHOW THE TWO METHODS OF
POUCH-FORMATION.

A, by the thinning of the mesoblast at intervals. B, by the ingrowth of
mesoblast 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 case (B) the formation of a pouch is {167}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 formation. 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 Ammocoetes 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 coelomic 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 coelom, of which the walls form a series of muscular and
skeletal segments. In the head-region, as already mentioned, such coelomic
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 coelomic cavities and Kishinouye has described in Limulus a
separate coelomic cavity for every one of the mesosomatic or branchial
segments, and he states that in Arachnida {168}the segmental coelomic
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 appearance 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 Ammocoetes 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 transformation, (A)
representing three branchial segments of Ammocoetes, 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 morphologists, 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 Ammocoetes 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 {169}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 versâ_, 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
branchiæ of the scorpion group are directly derived from branchial
appendages of a kind similar to those of Limulus, it is evident that a
branchial appendage such as that of Ammocoetes might also have arisen from
such an appendage, because in various respects it is easier to compare the
branchial appendage of Ammocoetes, than that of the scorpion group, with
that of Limulus.

[Illustration: FIG. 68.--DIAGRAM OF THREE BRANCHIAL SEGMENTS OF AMMOCOETES
(A) COMPARED WITH THREE BRANCHIAL SEGMENTS AFTER TRANSFORMATION (B) TO SHOW
HOW THE BRANCHIAL APPENDAGES OF AMMOCOETES 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.]

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}each of the branchiæ 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 branchiæ,
their nerves, and their afferent and efferent blood-vessels.

In the case of Ammocoetes 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 which 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 Ammocoetes, 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 Ammocoetes
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.

{171}The paper by Benham 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_1_); a third muscle
is the anterior entapophysio-branchial (Fig. 66, _m_2_). Of these muscles,
the posterior entapophysio-branchial (_m_1_) 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 branchiæ, 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 homologous 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 _rôle_ of respiratory muscles, and so have given origin to the
respiratory muscles of the ancestors of Ammocoetes.

The respiratory muscles of Ammocoetes 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
ventrally to the connective tissue in the neighbourhood {172}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 cartilaginous
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 (_m_1_, Fig. 66) of the Limulus appendage,
especially when the homology of the Ammocoetes 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 Ammocoetes 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 Ammocoetes, 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, VIIth, IXth, and Xth nerves, which ought on this theory to be derived
from the musculature of the corresponding appendages, is, speaking
generally, dorso-ventral in direction in Ammocoetes and of the same
character throughout; 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 Ammocoetes most sharply defined from all the other muscles of the
body. They form the great dorsal and ventral longitudinal
{173}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 (Kästchen). 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 Ammocoetes 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 Ammocoetes. 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 Ammocoetes 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 Ammocoetes--the so-called
tubular muscles. These muscles are of great interest, but as they are
especially connected with the VIIth nerve, their consideration 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}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 AMMOCOETES AND LIMULUS.

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 branchiæ,
it is worth while to compare the circulation of the blood in the gills of
Limulus and of Ammocoetes 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 lacunæ, 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 Ammocoetes; 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 Ammocoetes 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
Ammocoetes there are no well-defined branchial capillaries, but the blood
circulates, as in {175}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 precisely
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 invertebrate; but
the former is situated 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.

[Illustration: FIG. 69.--COMPARISON OF BRANCHIAL LAMELLÆ OF LIMULUS AND
SCORPIO WITH BRANCHIAL LAMELLÆ OF AMMOCOETES.

A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of
Ammocoetes (after Nestler).]

The only exception to this scheme is found in the vertebrate where the
heart is essentially a branchial heart, the blood being {176}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.

[Illustration: FIG. 70.--LONGITUDINAL DIAGRAMMATIC SECTION THROUGH THE
MESOSOMATIC REGION OF LIMULUS, TO SHOW THE ORIGIN OF THE BRANCHIAL
ARTERIES. (After BENHAM.)

_L.V.S._, longitudinal venous sinus, or collecting sinus; _a. br._,
branchial arteries; _V.p._, veno-pericardial muscles; _P._, pericardium.]

In Limulus the blood flows into the lamellæ from sinuses or blood-spaces
(_b.s._, Fig. 66) at the base of each of the lamellæ, which sinuses are
filled by a vessel which may be called the branchial 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. br._).
The blood of the branchial artery flows into the blood-spaces between the
anterior and posterior laminæ of the appendage and thence into the
gill-lamellæ, 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 {177}branchial appendage of Ammocoetes at right angles to the
cartilaginous branchial bar.

Further, the observations of Blanchard, Milne-Edwards, Ray 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 contractiles,' and considers that they
play an important part in assisting the pulmonary circulation; for, he
says, "en mettant a nu une portion du coeur, on remarque que ces battements
se font sentir sur les ligaments 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 system. 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 consideration of this group of
muscles will be given in Chapter IX.

Passing now to the condition of the branchial blood-vessels of Ammocoetes,
we see that the blood passes into the gill-lamellæ 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-lamellæ the blood is collected into an
efferent or branchial vein (_v. br._), which {178}runs, as seen 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
commencement of the thyroid it divides into two, and so forms two ventral
longitudinal vessels, from which the branchial arteries arise segmentally.

[Illustration: FIG. 71.--DIAGRAM 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._, epibranchial ganglia of
vagus; _R. 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 Ammocoetes 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, homologous with the paired
longitudinal venous sinuses of the latter.

{179}_A priori_, such a derivation seems highly improbable; and yet it is
precisely the manner in which embryology teaches us 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 invertebrate, 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 Appendiculata 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 Neal 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 {180}line of
two longitudinal venous channels, which were originally situated one on
each side of the notochord, in what was then the ventral surface of this
part of the embryo. This history is especially instructive 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 phylogenetic interpretation of this embryological 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, which one after the other arise
in harmonious succession as the necessary 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.

[Illustration: FIG. 72.--DIAGRAM (UPPER HALF OF FIGURE) OF THE ORIGINAL
POSITION OF VEINS (H) WHICH COME TOGETHER TO FORM THE HEART OF A
VERTEBRATE.

_C.N.S._, central nervous system; _nc._, notochord; _m._, myotome.

The lower half of figure shows comparative position of the longitudinal
venous sinus (_L.V.S._) in Limulus. _C.N.S._, central nervous system;
_Al._, alimentary canal; _H._, heart; _m._, body-muscles.]

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, {181}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 prosomatic 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, _f_. 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 Ammocoetes 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
Ammocoetes 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 part of the vertebral column, but
is simply fat-cells, such as might easily have taken the place of some
other previously existing organ.

{182}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 be as to
this? The heart _ex hypothesi_ having ceased to function, the muscular
tissue would not remain, and the space would be 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 Ammocoetes, is not found in the
higher vertebrates, so that it possesses, at all events, the significance
of being a peculiarity of ancient times before the vertebrate skeletal
column was formed.

I mention it here in connection with my view as to the origin of
vertebrates, 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.

[Illustration: FIG. 73.--SECTION THROUGH THE NOTOCHORD (_NC._), THE SPINAL
CANAL AND THE FAT-COLUMN (_F._), OF AMMOCOETES, DRAWN FROM AN OSMIC
PREPARATION.

_sp. c._, spinal cord; _gl._, glandular tissue filling the spinal canal;
_sk._, Gegenbaur's skeletogenous cells; _p._, pigment.]


{183}SUMMARY.

  From the close similarity of structure and position between the branchial
  skeleton of Limulus and of Ammocoetes, as given in the preceding chapter,
  it logically follows that the branchiæ of Ammocoetes must be homologous
  with the branchiæ of Limulus. But the respiratory apparatus of Limulus
  consists of branchial appendages. It follows, therefore, that the
  branchiæ 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 Palæostraca, 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
  composed of (1) somatic or body-segments, constituting the mesomeric
  segmentation; (2) appendage-segments, which, seeing that they carry the
  branchiæ, 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
  branchiæ.

  Although the evidence of such appendages has entirely disappeared in the
  higher vertebrates, together with the disappearance of branchiæ, 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}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 coelomic cavity is formed in it,
  similarly to the ingrowing of the branchial appendage in scorpions.

  We do not know how the palæostracan 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 appendage-muscles after
  the Limulus fashion, being water-breathers, even although their
  respiratory appendages 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 Ammocoetes, 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 Ammocoetes 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 longitudinal veins.

  The origin of these two longitudinal veins is immediately apparent if the
  vertebrate arose from a palæostracan, for in Limulus and the whole
  scorpion tribe, in which the heart is a systemic heart, the branchiæ 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 Palæostraca.




{185}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 Ammocoetes--Its
  structure.--Its openings.--The nature of the thyroid segment.--The uterus
  of the scorpion.--Its glands.--Comparison with the thyroid gland of
  Ammocoetes.--Cephalic genital glands of Limulus.--Interpretation of
  glandular tissue filling up the brain-case of Ammocoetes.--Function of
  thyroid gland.--Relation of thyroid gland to sexual functions.--Summary.


I have now given my reasons why I consider that the glossopharyngeal and
vagus nerves were originally the nerves belonging to a series of
mesosomatic branchial appendages, each of which is still traceable in the
respiratory chamber of Ammocoetes, 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 branchiæ we may still have
segments {186}homologous to the branchial segments, originally
characterized by the presence of appendages, but that such appendages need
never have carried branchiæ. 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 RESPIRATORY SEGMENT.

This simple alteration of the branchiomeric unit from a gill-pouch to an
appendage, which may or may not bear branchiæ, immediately 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 glossopharyngeal 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 branchiæ.

When, however, we examine Ammocoetes (_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 VIIth or facial nerves. It is so much like the segments
belonging to the glossopharyngeal 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
cartilaginous bar is here replaced by a bar of muco-cartilage and the
branchiæ 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 Rinne.' 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.

{187}[Illustration: FIG. 74.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.

Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
Tubular constrictor muscles distinguished from striated constrictor muscles
by simple hatching. _Tent._, tentacles; _Tent. m.c._, muco-cartilage of
tentacles; _Vel. m.c._, muco-cartilage of the velum; _Hy. m.c._,
muco-cartilage of the hyoid segment; _Ps. br._, pseudo-branchial groove;
_Br. cart._, branchial cartilages; _Sp._, space between somatic and
splanchnic muscles; _Th. op._, orifice of thyroid; _H._, heart.]

{188}The evidence, then, of Ammocoetes points directly to this conclusion:
The facial nerves represent the foremost of the mesosomatic group of
nerves, and supply two segments, which have amalgamated 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 branchiæ, 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 branchiæ, 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 Palæostraca was really composed of two pairs
of appendages, of which the most posterior carried branchiæ, while the
anterior pair had amalgamated in the mid-line ventrally, and carried some
special organ instead of branchiæ, 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 branchiæ, and instead of being {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 and always
carries the genital ducts.

A survey of the nature of the opercular appendage demonstrates the
existence of three different types--

1.  That of Limulus, 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).

[Illustration: FIG. 75.--OPERCULUM OF LIMULUS TO SHOW THE TWO SEPARATE
GENITAL DUCTS.]

[Illustration: FIG. 76.--OPERCULUM OF MALE SCORPION.

_Ut._, terminal chamber, or uterus.]

2.  The type of Scorpio, Androctonus, Buthus, etc., in which the 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, {190}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 common chamber has been called in the female scorpion the
vagina, or in Thelyphonus the uterus. I shall use the latter term, in
accordance with Tarnani's work, and the corresponding chamber in the male
will be the _uterus masculinus_.

A considerable discussion 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.

[Illustration: FIG. 77.--OPERCULUM AND FOLLOWING SEGMENTS 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 observations of Schmidt, Zittel, and others show that the
{191}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 branchiæ-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,
according 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 branchiæ, so Schmidt thinks from the evidence of markings on some
specimens.

[Illustration: FIG. 78.--_Eurypterus._

The segments and appendages on the right are numbered in correspondence
with the cranial system of lateral nerve-roots as found in vertebrates.
_M._, metastoma. The surface ornamentation is represented on the first
segment posterior to the branchial segments. The opercular appendage is
marked out by dots.]

Apparently an opercular appendage 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 {192}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 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.

[Illustration: FIG. 79.--DIAGRAM TO INDICATE THE PROBABLE NATURE OF THE
MESOSOMATIC SEGMENTS OF EURYPTERUS.

The opercular segment is marked out by the thick black line. The segments
_II.-VI._ bear branchiæ, and segment _I._ is supposed in the male to carry
the uterus masculinus (_Ut. Masc._) and the genital ducts.]

It is possible that in Holm's representation of Eurypterus, Fig. 104, the
genital duct on each side is indicated.


THE THYROID GLAND OF AMMOCOETES.

If we compare this mesosomatic region of Eurypterus with that of
Ammocoetes, the resemblance is most striking, and gives a meaning 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 branchiæ, 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 Ammocoetes, 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 Ammocoetes.
What, then, is the nature of the thyroid gland in Ammocoetes?

{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 Ammocoetes, we find that the thyroid
has suddenly become an organ of much more complicated structure, covering a
much larger space, and bearing no resemblance to the thyroid glands of the
higher forms. At transformation the thyroid of Ammocoetes is largely
destroyed, 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 Ammocoetes 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 underlying laminated layer has been removed 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.

[Illustration: FIG. 80.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES.

_Th._, thyroid gland; _M._, lower lip, with its muscles.]

The series of transverse sections in Fig. 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 termination.

{194}[Illustration: FIG. 81.--SAMPLES FROM A COMPLETE SERIES OF TRANSVERSE
SECTIONS THROUGH THE THYROID GLAND OF AMMOCOETES.

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.]

{195}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. o._) into the pharyngeal 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[prime]
(Fig. 82), giving origin to two tubes, B, B[prime], 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 Fig. 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 absorption of their walls, the remains of the wall being
still visible as the septum, which partially divides the chamber, C, 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 gland 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[prime] (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[prime] b[prime]_ (Fig. 81), and on the right side _c d, c[prime]
d[prime]_, then the central chamber has opening into it the glands _a b, c
d_, while the chambers of A and A[prime] have opening into them
respectively _a b, a[prime] b[prime]_, and _c d, c[prime] d[prime]_.
Further, the same series of sections shows that the glands _a_ and _b_ are
continuous with the glands _a[prime]_ and _b[prime]_ 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[prime] b[prime]_,
and we see that the {196}cavity of the portion A or A[prime] is formed by
the bending over of the tube or horn, B or B[prime], 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
Ammocoetes may be represented as in Fig. 83, _i.e._ it consists of a long,
common chamber, C, which, for reasons apparent afterwards, I will call the
_palæo-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[prime], 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 _palæo-hysteron_, the
glandular lines are symmetrically situated on each side.

[Illustration: FIG. 82.--DIAGRAMMATIC REPRESENTATION OF THE SO-CALLED
THYROID GLAND OF AMMOCOETES.

_C_, central chamber; _A, A[prime]_, anterior extremity; _B, B[prime]_,
posterior extremity; _Th. o._, thyroid opening into respiratory chamber;
_Ps. br., Ps. br[prime]._, ciliated grooves, Dohrn's pseudo-branchial
grooves.]

[Illustration: FIG. 83.--THYROID GLAND AS IT WOULD APPEAR IF THE CENTRAL
CHAMBER WERE UNCURLED AND THE TWO HORNS, _B_, _B[prime]_, SEPARATED FROM
THE CENTRAL CHAMBER.]

Any explanation, then, of the thyroid gland of Ammocoetes, must {197}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. o._). In the second place, there exist two ciliated grooves
(_Ps. br._, _Ps. br[prime]._), 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 previously 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
palæo-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 Ammocoetes 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 headwards 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}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 direction 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 Ammocoetes 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 somewhere 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 Ammocoetes. Dohrn
concludes, in my opinion rightly, that the endostyle in the Tunicata and in
Amphioxus represents the remnants of the more elaborate organ in
Ammocoetes, and that, therefore, in order to explain the meaning of these
organs in the former animals, we must first find out their meaning in
Ammocoetes. Dohrn, however, goes further than this; for just as he
considers Amphioxus and the Tunicata to have arisen by degeneration from an
Ammocoetes-like form, so he considers Ammocoetes to have arisen {199}from a
degenerated Selachian; therefore, in order to be logical, he ought to show
that the thyroid of Ammocoetes is an intermediate downward 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 immeasurably more complicated and elaborate a
structure than is that of Petromyzon, as to make it impossible to believe
that the Ammocoetes 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 conclusion that the
starting-point of the thyroid gland in the vertebrate series is to be found
in the thyroid of Ammocoetes, which has given rise, on the one hand, to the
endostyle of Amphioxus and the Tunicata, 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 morphologically 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 VIIth 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 concerned,
but is not right, and is not proven, in 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 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 cartilaginous 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 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.

[Illustration: FIG. 84.--DIAGRAM OF (A) VENTRAL SURFACE AND (B) LATERAL
SURFACE OF AMMOCOETES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PITS ON
THE BRANCHIAL REGION, AND THEIR INNERVATION BY _VII._, THE FACIAL, _IX._,
THE GLOSSOPHARYNGEAL, AND _X^1_-_X^6_, THE VAGUS NERVES.]

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 {201}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 of transformation here is the destruction of this
interpolated segment, and the resulting necessary drawing together
ventrally of the branchial segments on each side.

[Illustration: FIG. 85.--FACIAL SEGMENT OF AMMOCOETES MARKED 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.]

Moreover, another most instructive piece of evidence pointing in the same
direction is afforded by the behaviour of the ventral epithelial {202}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 Ammocoetes 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 Ammocoetes, disappearing entirely as such at transformation, we
may feel perfectly certain that a corresponding, 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 Ammocoetes. We may therefore
assert with considerable confidence that the thyroid gland was the
_palæo-hysteron_, and was derived from the uterus of the ancient
palæostracan 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 {203}on
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 Ammocoetes. I then turned my
attention to the organs of the scorpion, in which the two ducts have fused
to form a single uterus.

[Illustration: 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.]

{204}[Illustration: FIG. 87.--LONGITUDINAL SECTION THROUGH THREE OF THE
CONES OF THE UTERINE GLANDS OF THE SCORPION.]

[Illustration: 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 SURFACE.]

I there found that both in the male and in the female the genital 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 interrupted by two large, remarkable
glandular structures. The structure of these glands is better shown by
means of sagittal sections. They are composed of very long, wedge-shaped
cells, each of which possesses a large, round nucleus at the basal end of
the cell (Fig. 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. 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 (_a_), 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 {205}is seen in Fig. 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, represented 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
Ammocoetes. This uterus and glandular arrangement is found in both sexes;
the gland is, however, more developed in the male than in the female
scorpion.

[Illustration: FIG. 89.--TRANSVERSE SECTION THROUGH THE BASAL PART OF THE
UTERINE GLANDS OF THE SCORPION.]

The resemblance between the structure of the thyroid of Ammocoetes 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 Ammocoetes opening into the respiratory chamber, while the
uterus of Scorpio opens direct to the exterior.

[Illustration: FIG. 90.--SECTION OF CENTRAL CHAMBER OF THYROID OF
AMMOCOETES AND SECTION OF UTERUS OF SCORPION.]

With respect to the first difference, the same difficulty is met {206}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 Peripatus. 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, Branchipus,
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 Ammocoetes, 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 Ammocoetes, 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 {207}epithelium of the uterus masculinus and its processes is
extraordinarily 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 conspicuous 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 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 pulmonary
or gill cavities on each side, so that instead of a single opening for the
genital products and one on each side for each gill-pouch, as would 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.

[Illustration: FIG. 91.--SAGITTAL MEDIAN DIAGRAMMATIC SECTION THROUGH THE
OPERCULUM OF THE MALE THELYPHONUS. (From TARNANI.)

The thick line is the operculum, composed of two segments, _I._ and _II._
_Ut. Masc._, uterus masculinus; _Gen. Ch._, genital chamber; _Int. Op._,
internal opening; _Ext. Op._, external opening common to the genital and
respiratory organs.]

This genital chamber, according to Tarnani, opens to the exterior by a
single median opening between the operculum and the succeeding segment;
similarly, a communication from side to side exists between the second pair
of gill-pouches. I have been able to examine _Hypoctonus formosus_ and
_Thelyphonus 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}not a single opening, as described by Tarnani in
_Thelyphonus asperatus_, 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 Ammocoetes 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-oesophageal 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 mesosomatic 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.


{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
Ammocoetes 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 palæo-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
represented in the figure of Eurypterus (Fig. 79), a course which would
take much the same direction as the ciliated groove in Ammocoetes.

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 composed 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 generative 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 Ammocoetes 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 _conus
post-commissuralis_, which can actually be traced right into this tissue on
the outside of the brain (see Fig. 13, _a-e_, Pl. XXVI., in my paper in the
_Quarterly Journal of Microscopical Science_). {210}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.

[Illustration: FIG. 92.--DRAWING OF THE TISSUE WHICH SURROUNDS THE BRAIN OF
AMMOCOETES.]

This tissue has been largely described as a peculiar kind of connective
tissue, which is there as packing material, 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 connective 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 {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
Ammocoetes, 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 Ammocoetes 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 Ammocoetes, 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 {212}original opercular
ducts by which the generative products were conveyed 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
Ammocoetes, 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
Ammocoetes 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 Ammocoetes the remains of the closed genital duct of Limulus and
its allies.

{213}Such is the conclusion to which the study of the thyroid gland in
Ammocoetes 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 Ammocoetes?


THE FUNCTION OF THE THYROID GLAND IN AMMOCOETES.

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 thyroid, 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 Ammocoetes, 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 branchiæ, 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 required 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 Ammocoetes
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}may contain mucin in the younger
developmental stages, and therefore 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 Amphioxus and the Tunicata.

[Illustration: Fig. 93.--A, PORTION OF A GILL OF AMMOCOETES WITH ORDINARY
RESPIRATORY EPITHELIUM; B, CORRESPONDING PORTION OF THE FIRST OR HYOID
GILL.]

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 corresponding 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 generative
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 {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 Vertebrata down to and including Petromyzon, but, as is seen
at transformation, is all that remains of the more elaborate, more
extensive organ of Ammocoetes. 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 Ammocoetes,
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 palæostracan 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 extreme 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.


{216}SUMMARY.

  Step by step in the preceding chapters the evidence is accumulating in
  favour of the origin of vertebrates from a member of the palæostracan
  group. In a continuously complete and harmonious manner the evidence has
  throughout been most convincing when the vertebrate chosen for the
  purpose of my arguments has been Ammocoetes.

  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 Palæostraca
  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
  palæostracan ancestor. Yet so clear is the evidence that it is difficult
  to see how the homology can be denied.

  In the one animal (Palæostraca) 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
  branchiæ.

  In the other animal (Ammocoetes) 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 branchiæ.

  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 Ammocoetes 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 complicated 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 Palæostraca the generative mass was situated in the
  cephalic region as in Limulus, and it is probable that the remnant of it
  still exists in {217}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 palæostracan ancestor.

  The consideration of the facial nerve, and the segments it supplies,
  still further points to the origin of the Vertebrata from the
  Palæostraca.




{218}CHAPTER VI

_THE EVIDENCE OF THE OLFACTORY APPARATUS_

  Fishes divided into Amphirhinæ and Monorhinæ.--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 antennæ.--Its termination at the true
  mouth.--Comparison 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 vertebrates arose from some member of the
Palæostraca, 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, suggested 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 {219}in the vertebrate
from the supra-infundibular nerve-mass, and in the invertebrate from the
supra-oesophageal 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, Monorhinæ and Amphirhinæ, according as they possess
a median unpaired olfactory opening, or a paired opening. The Monorhinæ
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 Ammocoetes, 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 {220}same position is
always found in the dorsal head-shields of all the Cephalaspidæ and
Tremataspidæ, 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, containing 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 infundibular tube terminates
blindly and the notochord ends. After transformation, when the Ammocoete
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-oesophageal 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 antennæ, often called the antennules, are
olfactory in function, and these are free-moving, bilaterally
{221}situated, independent appendages. Still, even here there is the
striking fact that the nerves of these olfactory organs always arise from
the supra-oesophageal ganglia, although those to the second pair of antennæ
arise from the infra-oesophageal 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 character 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 vertebrates. 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
palæostracan 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
antennæ are not present, either in the living land-forms or the extinct
sea-scorpions, for all the antennæ-like, frequently chelate, appendages
seen in Pterygotus, etc. (Fig. 8), represent the cheliceræ, and correspond,
therefore, to the second pair of antennæ in the crustaceans.

What, then, represents the olfactory antennæ in the scorpions? The answer
to this question has been given by Croneberg, and very striking it is. The
two olfactory antennæ 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}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 antennæ-like organ (Fig. 94).

Croneberg has also shown that this rostrum, or camerostome, arises
embryologically as a pair of appendages similar to the other appendages.
This last observation of Croneberg has been confirmed by Brauer in 1894,
who describes the origin of the upper lip, as he calls it, in very similar
terms, without, however, referring to Croneberg's paper. Croneberg further
shows that this foremost pair of antennæ 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 portion, 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.

[Illustration: FIG. 94.--DORSAL VIEW OF BRAIN AND CAMEROSTOME OF GALEODES.

_cam._, camerostome; _pr. ent._, pre-oral entosclerite; _l.l._, dependent
portion of camerostome; _ph._, pharynx; _al._, alimentary canal; _n. op._,
median optic nerves; _pl._, plastron; _v.c._, ventral nerve chain; 2, 3,
second and third appendages.]

At the base of the tubular passage formed by this modified first pair of
antennæ 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).

{223}In order to convey to my readers the antennæ-like character of the
camerostome in Galeodes (Fig. 101), and its position, I give a figure (Fig.
94) of the organ from its dorsal aspect, after removal of the cheliceræ and
their muscles. A side view of the same organ is given in Fig. 95 to show
the feathered termination of the camerostome, and the position of the
dependent accessory portion (_l.l._) (Croneberg'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-oesophageal
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 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 Croneberg's figures, the rostrum
or camerostome is built up on the same plan as in Galeodes, though the
antenna-like character may not be so evident.

[Illustration: FIG. 95.--LATERAL VIEW OF BRAIN AND CAMEROSTOME OF GALEODES.

_gl. supr. oes._, supra-oesophageal ganglion; _gl. infr. oes._,
infra-oesophageal ganglion. The rest of the lettering same as in Fig. 94.]

When we consider that the first pair of antennæ in the crustaceans are
olfactory in function, Croneberg's observations amount to this--

In the arachnids and their allies the first pair of antennæ 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}Croneberg's observations and conclusions are distinctly of very great
importance in bringing the arachnids into line with the crustaceans, 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).

[Illustration: 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 conception. At the
front end of the body is seen the median eye (_ce._), _o_ is the mouth,
_Ph._ the pharynx, _oes._ the narrow oesophagus, compressed between the
supra-oesophageal (_supr. oes._) and infra-oesophageal (_infr. oes._) brain
mass, which opens into the large alimentary canal (_Al._); _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 camerostome and the median eye is filled up by the
massive cheliceræ, which are not shown in this section, as they begin to
appear in the {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 Fig. 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._), well 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 antennæ of Sphæroma. Finally,
we have the observation of Blanchard quoted by Huxley, to the effect that
this camerostome is innervated by nerves from the supra-oesophageal 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-oesophageal ganglia.

These facts demonstrate with wonderful clearness that in one group of the
Arthropoda the olfactory antennæ have been so modified 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 _o_ in Fig. 96 were to be closed, then the
olfactory tube (_olf. pass._) might still remain, owing to its importance
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 AMMOCOETES.

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.

{226}[Illustration: FIG. 97.--TRANSVERSE SECTION THROUGH THE OLFACTORY
PASSAGE OF A YOUNG THELYPHONUS.

1 and 2, sections of first and second appendages.]

[Illustration: FIG. 98.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE
OF PETROMYZON.

_cart._, nasal cartilage.]

{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
Ammocoetes 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 Ammocoetes, 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
more valuable information as to vertebrate ancestry than all the rest of
the vertebrates put together.

[Illustration: FIG. 99.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE
OF AMMOCOETES.

_cart._, nasal cartilage.]

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, tremataspids, 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 {228}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 larval 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 Ammocoetes 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
considerable rapidity, so that we may regard the transformation which takes
place at this stage, as in some degree comparable with the great
transformation which occurs when the Ammocoetes 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 Palæostracan 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 Ammocoetes 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, _vel._ Opening 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 {229}rapidity and thickens considerably, thus forcing the opening of
the hypophysial tube more and more dorsalwards, until at last, in the
full-grown Ammocoetes, 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 Cyclostomata, in
contradistinction to its paired character in the rest of the 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.

[Illustration: FIG. 100.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4
MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)

_A-B_, the line of epibranchial ganglia; _au._, 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.]

On the other hand, the two olfactory nerves of Ammocoetes compare
absolutely with the olfactory nerves of other vertebrates, and force one to
the conclusion that this median organ of Ammocoetes arose from a pair of
bilateral organs, which have fused in the middle line.

{230}[Illustration: FIG. 101.--_Galeodes._ (From the Royal Natural
History.)]

{231}The comparison of this olfactory organ with the camerostome gives a
satisfactory reason for its appearance in the lowest vertebrates as an
unpaired median organ; equally so, the history of the camerostome itself
supplies the reason why the olfactory nerves are double, why the organ is
in reality a paired organ and not a single median one. Thus, in a sense,
the grouping of the fishes into Monorhinæ and Amphirhinæ has not much
meaning, seeing that the olfactory organ is in all cases double.

[Illustration: FIG. 102.--_Thelyphonus._ (From the Royal Natural History.)]

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}vertebrate
from the Palæostracan, but points indubitably to an origin from a
scorpion-like rather than a crustacean-like stock. To complete 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 Palæostracan 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 palæostracan ancestor.

  Strikingly enough, although in the crustaceans the first pair of antennæ
  form the olfactory organs, no such free antennæ 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 antennæ
  in crustaceans as to their origin from the supra-oesophageal ganglia.

  This nasal passage, or tube of the hypophysis, corresponds in structure
  and in 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 mouth, 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 palæostracan group with marked
  scorpion-like affinities.




{233}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 Ammocoetes.--Prosomatic segmentation
  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
antennæ 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 palæostracan 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-oesophageal segments belonging to the prosoma and mesosoma
respectively, the correspondence between the mesosomatic segments carrying
the branchial appendages and the uterus, with those in the vertebrate
carrying the branchiæ 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 invertebrate, is
never respiratory in function but always masticatory, such {234}mastication
being performed in Limulus 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 invertebrate, are supplied by the
trigeminal nerves, while the dorsal or somatic segments, corresponding to
the somatic segments in the invertebrate, are supplied by the IIIrd 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 appendages 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 originally 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 knowledge 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 coelomic 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 {235}away
and become insignificant, we still have the muscular, coelomic, 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 carapace; (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 concentration 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 cheliceræ.

(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
cheliceræ, the question has arisen whether their nerves belong to the
infra-oesophageal group, or are in reality supra-oesophageal.

It is instructive to observe the nature and the anterior position of this
pair of appendages in the allied sea-scorpions, especially in Pterygotus,
where the only chelate organs are found in these long, antennæ-like
cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to
be represented by the small non-chelate antennæ 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}of these forms, for they
possess only five free prosomatic appendages instead of six, as in Limulus
and in Pterygotus. Similarly, Woodward only allowed five appendages for
Pterygotus, so that his restorations 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).

[Illustration: FIG. 103.--VENTRAL SURFACE OF 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._).]

With respect to Eurypterus, Schmidt figures an exceedingly minute pair of
antennæ 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
Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus
were of the same kind as those of Limulus. I reproduce his figure (Fig.
104) showing the small chelate cheliceræ (1) overhanging the mouth orifice,
just as in Limulus or in Scorpio.

{237}So, also, since Woodward's monograph, Laurie has discovered in
_Slimonia acuminata_ a small median pair of chelate appendages exactly
corresponding to the cheliceræ 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 prosomatic appendages in all
these extinct sea-scorpions were in the same position and of the same
character as the cheliceræ of the scorpions.

[Illustration: FIG. 104.--_Eurypterus Fischeri._ (From HOLM.)]

In the living scorpion and in Limulus the nerves to this pair of appendages
undoubtedly arise from the foremost prosomatic ganglia, and the reason why
they appear to belong 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-oesophageal, though in reality belonging to the
infra-oesophageal ganglia. This cheliceral pair of appendages is, in all
probability, homologous with the second pair of antennæ in the crustacea.

{238}I conclude, then, that the cheliceræ must truly be included in the
prosomatic 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 has found embryologically that they arise
in the same way as the rest of the prosomatic appendages, and belong to a
distinct prosomatic segment, viz. the seventh segment. In accordance with
this, Brauer has found that in the scorpion there is in the embryo a
segment, whose appendages degenerate, which is situated between the segment
bearing the last pair of thoracic appendages and the genital operculum--a
segment, therefore, comparable in position to the chilarial segment of
Limulus.

Coming now to the five locomotor appendages, we find that they resemble
each other to a considerable extent in most cases, with, however, certain
striking differences. Thus in Limulus they are chelate, with their basal
joints formed as gnathites, except in the case of the fifth appendage, in
which the extremity is modified for the purpose of digging in the sand. In
Pterygotus, Slimonia, Eurypterus, the first four of these appendages are
very similar, and are called by Huxley and Woodward endognaths; in all
cases they possess a basal part or sterno-coxal process, which acts as a
gnathite or foot-jaw, and a non-chelate tactile part, which possesses no
prehensile power, and in most cases could have had no appreciable share in
locomotion, called by Huxley and Woodward the palpus. These small palps
were probably retractile, and capable of being withdrawn entirely under the
hood. The fifth appendage is usually different, being a large swimming
organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is
known as the ectognath.

Finally, in _Drepanopterus Bembycoides_, as stated by Laurie, all five
locomotor appendages are built up after the same fashion, the last one not
being formed as a paddle-shaped organ or elongated as {239}in Stylonurus,
but all five possess no special locomotor or prehensile power. According to
Laurie this is a specially primitive form of the group.

It is significant to notice from this sketch that with the absence of
special prehensile terminations such as chelæ, or the absence of special
locomotor functions such as walking or swimming, these appendages tend to
dwindle and become insignificant, taking up the position of mere feelers
round the mouth, and at the same time are concentrated and pressed closely
together, so that their appendage-nerves must also be close together.

This sketch therefore shows us that--

Of the six foremost prosomatic appendages, the cheliceræ and the four
endognaths were, at the time when the vertebrates first appeared, in very
many cases dwindling away; the latter especially no longer functioned as
locomotor appendages, but were becoming more and more mere palps or
tentacles situated round the mouth, which could by no possibility afford
any help to locomotion.

On the contrary, the sixth pair of appendages--the ectognaths--remained
powerful, being modified in many cases into large oar-like limbs by which
the animal propelled itself through the water.

It is a striking coincidence that those ancient fishes, Pterichthys and
Bothriolepis, should have possessed a pair of large oar-like appendages.

At this time, then, in strong contrast to the endognaths, the ectognaths,
or sixth pair of appendages, remained strong and vigorous. What about the
seventh pair, the chilaria of Limulus?

Of all the prosomatic appendages these are the most interesting from the
point of view of my theory, for whereas in the scorpion of the present day
they have dwindled away and left no trace except in the embryo, in the
sea-scorpions of old, far from dwindling, they had developed and become a
much more important organ than the chilaria of Limulus.

In all these animals a peculiarly striking and unique structure is found in
this region known by the name of the metastoma, or lip-plate (Figs. 8 and
104 (7)); it is universally considered to be formed by the fusion of the
two chilarial appendages.

All observers are agreed that this lip-plate was freely movable. Nieskowski
considers that the movement of the metastoma was entirely in a vertical
direction, whereby the cleft which is seen {240}between the basal joints of
all the pairs of locomotor appendages could be closed from behind. Woodward
says it no doubt represents the labium, and served more effectually to
enclose the posterior part of the buccal orifice, being found exteriorly to
the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with
Nieskowski, and looks on the mestasoma as forming a lower lip within which
the bases of the ectognaths worked.

[Illustration: FIG. 105.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH A,
LIMULUS, B, EURYPTERUS.]

Quite recently Gerhard Holm has worked over again the very numerous
specimens of _Eurypterus Fischeri_, which are obtainable at Rootziküll, and
has thrown new light on the relation of the metastoma to the mouth-parts.
His preparations show clearly that the true lower lip of Eurypterus was not
the metastoma, for when the metastoma is removed another plate (_End._,
Fig. 105, B) situated {241}internally to it is disclosed, which, in his
view, corresponds to the sternite between the bases of the pro-somatic
appendages in Limulus, _i.e._ to the sternite called by Lankester, the
pro-mesosternite (_End._, Fig. 103). This inner plate formed with the
metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed
posteriorly, within which the bases of the ectognaths worked. In other
words, the removal of the metastoma discloses in Eurypterus the true
anterior ventral surface of the animal which corresponds to that of
Limulus, or of the scorpion group, with its pro-mesosternite and laterally
attached gnathites or sterno-coxal processes. To this inner plate or
pro-mesosternite Holm gives the name of _endostoma_.

To the anterior edge of the endostoma a thinner membrane is attached which
passes inwards in the direction of the throat, and forms, therefore, the
lower lip (_Hyp._, Fig. 105, B) of the passage of the mouth (_olf. p._).
This membrane bears upon its surface a tuft of hairs, which he thought were
probably olfactory in function. Consequently, in his preliminary
communication, he describes this lower lip as forming, in all probability,
an olfactory organ; in his full communication he repudiates this
suggestion, because he thinks it unlikely that such an organ would be
situated within the mouth. I feel sure that if Holm had referred to
Croneberg's paper, and seen how the true mouth in all the scorpion group is
situated at the base of an olfactory passage, he would have recognized that
his first suggestion is in striking accordance with the nature of the
entrance to the mouth in other scorpions.

That Eurypterus also possessed a camerostome (_cam._) seems to follow of
necessity from its evident affinities both with Limulus and the scorpions.
We see, in fact, that the mouth of these old sea-scorpions was formed after
the fashion of Limulus, surrounded by masticatory organs in the shape of
foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal
sagittal section of one of these old palæostracan forms would be obtained
by the combination of actual sagittal sections through Limulus and a member
of the scorpion group, with, at the same time, a due recognition of Holm's
researches. Such a section is represented in Fig. 105, B, in which I have
drawn the central nervous system and its nerves, the median eyes (_C.E._),
the olfactory organs (_Cam._), the pharynx (_Ph._), oesophagus (_oes._),
and alimentary canal (_Al._), but have not tried to indicate the lateral
eyes. I have represented the prosomatic appendages by numbers (1-7), and
{242}the foremost mesosomatic segments by numbers (8-13). I have placed the
four endognaths and the nerves going to them close together, and made them
small, mere tentacles, in recognition of the character of these appendages
in Eurypterus, and have indicated the position and size of the large
ectognath, with its separate nerve, by (6). If among the ancient
Eurypterus-like forms, which were living at the time when vertebrates first
appeared, there were some in which the ectognaths also had dwindled to a
pair of tentacles, then such animals would possess a prosomatic chamber
formed by a metastoma or accessory lip, within which were situated five
pairs of short tactile appendages or tentacles. If the vertebrate were
derived from such an animal, then the trigeminal nerve, as the
representative of these prosomatic appendage-nerves, ought to be found to
supply the muscles of this accessory lip and of these five pairs of
tentacles in the lowest vertebrate.

This prosomatic or oral chamber, as it might be called, was limited
posteriorly by the fused metastoma (7) and operculum (8), so that if in the
same imaginary animal one imagines that the gill-chambers, instead of being
separate, are united to form one large respiratory chamber, then, in such
an animal, a prosomatic oral chamber, in which the prosomatic appendages
worked, would be separated from a mesosomatic respiratory chamber by a
septum composed of the conjoined basal portions of the mesosomatic
operculum and the prosomatic metastoma, as indicated in the diagram. In
this septum the nerves to the last prosomatic appendage (equivalent to the
last part of the trigeminal in the vertebrate) and to the first mesosomatic
(equivalent to the thyroid part of the facial) would run, as shown in the
figure, close together in the first part of their course, and would
separate when the ventral surface was reached, to pass headwards and
tailwards respectively.


THE COXAL GLANDS.

One more characteristic of these appendages requires mention, and that is
the excretory glands situated at the base of the four endognaths known as
the coxal glands. These glands are the main excretory organs in Limulus and
the scorpions, and extend into the basal segments or coxæ of the four
endognaths, not into those of the ectognaths or the chilaria (or
metastoma). Hence their name, coxal {243}glands; and, seeing the importance
of the excretory function, it is likely enough that they would remain, even
when the appendages themselves had dwindled away. With the concentration
and dwindling of the endognaths these coxal glands would also be
concentrated, so that in the diagram (Fig. 105) they would rightly be
grouped together in the position indicated (_cox. gl._).

Such a diagram indicates the position of all the important organs of the
head-region except the special organs for taste and hearing. These, for the
sake of convenience, I propose to take separately, in order at this stage
of my argument not to overburden the simplicity of the comparison I desire
to make with too much unavoidable detail.


THE PROSOMATIC REGION OF AMMOCOETES.

Let us now compare this diagram with that of the corresponding region in
Ammocoetes and see whether or no any points of similarity exist.

With respect to this region, as in so many other instances already
mentioned, Ammocoetes occupies an almost unique position among vertebrates,
for the region supplied by the trigeminal nerve--the prosomatic
region--consists of a large oral chamber which was separated from the
respiratory chamber in the very young stage by a septum which is
subsequently broken through, and so the two chambers communicate.

This chamber is bounded by the lower lip ventrally, the upper lip and
trabecular region dorsally, and the remains of the septum or velum
laterally and posteriorly. It contains a number of tentacles arranged in
pairs within the chamber so as to form a sieve-like fringe inside the
circular mouth; of these, the ventral pair are large, fused together, and
attached to the lower lip.

All the muscles belonging to this oral chamber are of the visceral type,
and are innervated by the trigeminal nerve. In accordance with the evidence
obtained up to this point this means that such an oral chamber was formed
by the prosomatic appendages of the invertebrate ancestor, similarly to the
oral chamber just figured for Eurypterus.

This chamber in the full-grown Ammocoetes is not only open to the
respiratory chamber, but is bounded by the large upper lip (_U.L._, Fig.
106, D). On the dorsal surface of this region, in front of the {244}pineal
eye (_C.E._), is the most conspicuous opening of the olfactory tube
(_Na._), which olfactory tube passes from the dorsal region to the ventral
side to terminate blindly at the very spot where the infundibulum comes to
the surface of the brain. Here, also, is situated that extraordinary
glandular organ known as the pituitary body (_Pit._). A sagittal section,
then, in diagram form, of the position of parts in the full-grown
Ammocoetes, would be represented as in Fig. 106, D.

But, as argued out in the last chapter, the diagram of the adult Ammocoetes
must be compared with that of a cephalaspidian fish; the diagram of the
palæostracan must be compared with the larval condition of Ammocoetes. In
other words, Fig. 106, B, must be compared with Fig. 106, C, which
represents a section through the larval Ammocoetes as it would appear if it
reached the adult condition without any forward growth of the upper lip or
any breaking through of the septum between the oral and respiratory
chambers. The striking similarity between this diagram and that of
Eurypterus becomes immediately manifest even to the smallest details. The
only difference between the two, except, of course, the notochord, consists
in the closure of the mouth opening (_o_), in Fig. 106, B, by which the
olfactory passage (_olf. p._) of the scorpion becomes converted into the
hypophysial tube (_Hy._), Fig. 106, C, and later into the nasal tube
(_Na._), Fig. 106, D, of the full-grown Ammocoetes. That single closure of
the old mouth is absolutely all that is required to convert the Eurypterus
diagram into the Ammocoetes diagram.

Such a comparison immediately explains in the simplest manner a number of
anatomical peculiarities which have hitherto been among the great mysteries
of the vertebrate organization. For not only do the median eyes (_C.E._)
correspond in position in the two diagrams, and the infundibular tube
(_Inf._) and the ventricles of the brain (_C.C._) correspond to the
oesophagus (_oes._) and the cephalic stomach (_Al._), as already fully
discussed; but even in the very place where the narrow oesophagus opened
into the wider chamber of the pharynx (_Ph._), there, in all the lower
vertebrates, the narrow infundibular tube opens into the wider chamber of
the membranous _saccus vasculosus_ (_sac. vasc._). This is the last portion
of the membranous part of the tube of the central nervous system which has
not received explanation in the previous chapters, and now it is seen how
simple its explanation is, how natural its presence--it represents the old
pharyngeal chamber of the palæostracan ancestor.

{245}[Illustration: FIG. 106.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH
B, EURYPTERUS; C, LARVAL AMMOCOETES; D, FULL-GROWN AMMOCOETES.]

{246}Next among the mysteries requiring explanation is the pituitary body,
that strange glandular organ always found so closely attached to the brain
in the infundibular region that when it is detached in taking out the brain
it leaves the infundibular canal patent right into the IIIrd ventricle. A
comparison of the two diagrams indicates that such a glandular organ
(_Pit._), Fig. 106, C, was there because the coxal excretory glands (_cox.
gl._), Fig. 106, B, were in a similar position in the palæostracan
ancestor--that, indeed, the pituitary body is the descendant of the coxal
glands.

Finally, the diagrams not only indicate how the mesosomatic
appendage-nerves supplying in the one case the operculum and the
respiratory appendages correspond to the respiratory group of nerves, VII.,
IX., X., supplying in the other case the thyroid, hyoid, and branchial
segments, but also that a similar correspondence exists between the
prosomatic appendage-nerves in the one case and the trigeminal nerve in the
other; a correspondence which supplies the reason why in the vertebrate a
septum originally existed between an oral and respiratory chamber.

Such a comparison, then, leads directly to the suggestion that the
trigeminal nerve originally supplied the prosomatic appendages, such
appendages being: 1. The metastoma, which has become in Ammocoetes the
lower lip supplied by the velar or mandibular branch of the trigeminal
nerve (7); 2. The ectognath, which has become the large median ventral
tentacle, called by Rathke the tongue, supplied by the tongue nerve (6); 3.
The endognaths, which have been reduced to tentacles and are supplied by
the tentacular branch of the trigeminal nerve (2, 3, 4, 5).

I have purposely put these two diagrams of the larval Ammocoetes and of
Eurypterus before the minds of my readers at this early stage of my
argument, so as to make what follows more understandable. I propose now to
consider fully each one of these suggestive comparisons, and to see whether
or no they are in accordance with the results of modern research.

In the first instance, the diagrams suggest that the trigeminal nerve
originally supplied the prosomatic appendages of the palæostracan ancestor,
while the eye-muscle nerves supplied the body-muscles of the prosoma.

{247}As these appendages did not carry any vital organs such as branchiæ,
but were mainly locomotor and masticatory in function, it follows that
their disappearance as such would be much more complete than that of the
mesosomatic branchial appendages. Most probably, then, in the higher
vertebrates no trace of such appendages might be left; consequently the
segmentation due to their presence would be very obscure, so that in this
region the very reverse of what is found in the region of the vagus nerve
would be the rule. There branchiomeric segmentation is especially evident,
owing to the persistence of the branchial part of the branchial appendages;
here, owing to the disappearance of the appendages, the segmentation is no
longer branchiomeric, but essentially mesomeric in consequence of the
persistence of the somatic eye-muscles.

In addition to the evidence of the appendages themselves, the number of
prosomatic segments is well marked out in all the members of the scorpion
group by the divisions of the central nervous system into well-defined
neuromeres in accordance with the appendages, a segmentation the
reminiscence of which may still persist after the appendages themselves
have dwindled or disappeared. In accordance with this possibility we see
that one of the most recent discoveries in favour of a number of segments
in the head-region of the vertebrate is the discovery in the early embryo
of a number of partial divisions in the brain-mass, forming a system of
cephalic neuromeres which may well be the rudiments of the well-defined
cephalic neuromeres of animals such as the scorpion.


THE EVIDENCE OF THE PROSOMATIC MUSCULATURE.

Even if the appendages as such become obscure, yet their muscles might
remain and show evidence of their presence. The most persistent of all the
appendage-muscles are the basal muscles which pass from coxa to carapace
and are known by the name of tergo-coxal muscles. They are large, well
marked, segmentally arranged muscles, dorso-ventral in direction, and,
owing to their connecting the limb with the carapace, are likely to be
retained even if the appendage dwindles away.

The muscular system of Limulus and Scorpio has been investigated by Benham
and Miss Beck under Lankester's direction, and the conclusions to which
Lankester comes are these--

{248}The simple musculature of the primitive animal from which both Limulus
and the scorpions arose consisted of--

  1. A series of paired longitudinal dorsal muscles passing from tergite to
  tergite of each successive segment.

  2. A similar series of paired longitudinal ventral muscles.

  3. A pair of dorso-ventral muscles passing from tergite to sternite in
  each segment.

  4. A set of dorso-ventral muscles moving the coxa of each limb in its
  socket.

  5. A pair of veno-pericardial muscles in each segment.

Of these groups of muscles, any one of which would indicate the number of
segments, Groups 1 and 2 do not extend into the prosomatic region, and
Group 5 extends only as far as the heart extends in the case of both
Limulus and the Scorpion group; so that we may safely conclude that in the
Palæostraca the evidence of somatic segmentation in the prosomatic region
would be given, as far as the musculature is concerned, by the
dorso-ventral somatic muscles (Group 3), and of segmentation due to the
appendages by the dorso-ventral appendage musculature (Group 4).

Therefore, if, as the evidence so far indicates, the vertebrate has arisen
from a palæostracan stock, we should expect to find that the musculature of
the somatic segments in the region of the trigeminal nerve did not resemble
the segmental muscles of the spinal region, was not, therefore, the
continuation of the longitudinal musculature of the body, but was
dorso-ventral in position, and that the musculature of the splanchic
segments resembled that of the vagus region, where, as pointed out in
Chapter IV., the respiratory muscles arose from the dorso-ventral muscles
of the mesosomatic appendages. This is, of course, exactly what is found
for the muscles which move the lateral eyes of the vertebrate; these
muscles, innervated by the IIIrd, IVth, and VIth nerves, afford one of the
main evidences of segmentation in this region, are always grouped in line
with the somatic muscles of spinal segments, and yet cannot be classed as
longitudinal muscles. They are dorso-ventral in direction, and yet belong
to the somatic system; they are exactly what one ought to find if they
represent Group 3--the dorso-ventral body-muscles of the prosomatic
segments of the invertebrate ancestor.

The interpretation of these muscles will be given immediately; at present I
want to pass in review all the different kinds of evidence {249}of
segmentation in this region afforded by the examination of the
invertebrate, whether living or fossil, so as to see what clues are left if
the evidence of appendages fails us. I will take in the first instance the
evidence of segmentation afforded by the presence of the musculature of
Group 4, even when, as in the case of many fossils, no appendages have yet
been found. In such animals as Mygale and Phrynus the prosomatic carapace
is seen to be marked out into a series of elevations and depressions, and
upon removing the carapace we see that these elevations correspond with and
are due to the large tergo-coxal muscles of the appendages; so that if such
carapace alone were found fossilized we could say with certainty: this
animal possessed prosomatic appendages the number of which can be guessed
with more or less certainty by these indications of segments on the
carapace.

In those forms, then, which are only known to us in the fossil condition,
in which no prosomatic appendages have been found, but which possess, more
or less clearly, radial markings on the prosomatic carapace resembling
those of Phrynus or Mygale, such radial markings may be interpreted as due
to the presence of prosomatic appendages, which are either entirely
concealed by the prosomatic carapace or dorsal head-plate, or were of such
a nature as not to have been capable of fossilization.

The group of animals in question forms the great group of animals, chiefly
extinct, classified by H. Woodward under the order of Merostomata. They are
divided by him into the sub-order of Eurypteridæ, which includes--(1)
Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, (5)
Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9)
Exapinurus, (10) Pseudoniscus; and the sub-order Xiphosura, which
includes--(1) Belinurus, (2) Prestwichia, (3) Limulus.

{250}[Illustration: FIG. 107.--_Phrynus Margine-Maculata._

_Ce._, median eyes; _le._, lateral eyes; _glab._, median plate over brain;
_Fo._, fovea.]

[Illustration: FIG. 108.--_Phrynus sp._ (?). CARAPACE REMOVED.

_cam._, camerostome; _pl._, plastron.]

{251}The evidence of the Xiphosura and of the Hemiaspidæ conclusively
shows, in Woodward's opinion, that the Merostomata are closely related to
the Trilobita, and the Hemiaspidæ especially are supposed to be
intermediate between the trilobites and the king-crabs. They are
characterized, as also Belinurus and Prestwichia, by the absence of any
prosomatic appendages, so that in these cases, as is seen in Fig. 12 (p.
30), representing _Bunodes lunula_, found in the Eurypterus layer at
Rootziküll, we have an animal somewhat resembling Limulus in which the
prosomatic appendages have either dwindled away and are completely hidden
by the prosomatic carapace, or became so soft as not to be preserved in the
fossilized condition. The appearance of the prosomatic carapace is, to my
mind, suggestive of the presence of such appendages, for it is marked out
radially, as is seen in the figure, in a manner resembling somewhat the
markings on the prosomatic carapace of Mygale or Phrynus; the latter
markings, as already mentioned, are due to the aponeuroses between the
tergo-coxal muscles of the prosomatic appendages which lie underneath and
are attached to the carapace.

A very similar radial marking is shown by Woodward in his picture of
_Hemiaspis limuloides_, reproduced in Fig. 109, found in the Lower Ludlow
beds at Leintwardine. This species has yielded the most perfect specimens
of the genus Hemiaspis, which is recognized as differing from Bunodes by
the possession of a telson.

It is striking to find that similar indications of segments have been found
on the dorsal surface of the head-region in many of the most ancient
extinct fishes, as will be fully discussed later on.

[Illustration: FIG.109.--_Hemiaspis limuloides._ (From WOODWARD.)

_gl._, glabellum.]


THE EVIDENCE OF COELOMIC CAVITIES.

In the head-region of the vertebrate, morphologists depend largely upon the
embryonic divisions of the mesoderm for the estimation of the number of
segments, and, therefore, upon the number of coelomic cavities in this
region, the walls of which give origin to the striated muscles of the head,
so that the question of the number of segments depends very largely upon
the origin of the muscles from the walls of these head-cavities. It is
therefore interesting to examine whether a similar criterion of
segmentation holds good in such a segmented {252}animal as Limulus, or in
the members of the scorpion group, in which the number of segments are
known definitely by the presence of the appendages. In Limulus we know,
from the observations of Kishinouye, that a series of coelomic cavities are
formed embryologically in the various segments of the mesosoma and prosoma,
in a manner exceedingly similar to their mode of formation in the
head-region of the vertebrate, and he has shown that in the mesosoma a
separate coelomic cavity exists for each segment, so that just as the
dorso-ventral somatic muscles are regularly segmentally arranged in this
region, so are the coelomic cavities, and we should be right in our
estimation of the number of segments in this region by the consideration of
the numerical correspondence of these cavities with the mesomatic
appendages. Similarly, in the vertebrate, we find every reason to believe
that a single, separate head-cavity corresponds to each of the branchial
segments in the opisthotic region, and therefore we should estimate rightly
the number of segments by the division of the mesoderm in this region.

In the prosomatic region of Limulus, the dorso-ventral muscles are not
arranged with such absolute segmental regularity as in the mesosomatic
region, and Kishinouye's observations show that the coelomic cavities in
this region do not correspond absolutely with the number of prosomatic
appendages. His words are:--

A pair of coelomic cavities appears in every segment except the segments of
the 2nd, 3rd, and 4th appendages, in which coelomic cavities do not appear
at all. At least eleven pairs of these cavities are produced. The eleventh
pair belongs to the seventh abdominal segment.

The first pair of coelomic cavities is common to the cephalic lobe and the
segment of the first appendage (_i.e._ the cheliceræ).

The second coelomic cavity belongs to the segment of the fifth appendage.
It is well developed.

The ventral portion of the second coelomic cavity remains as the coxal
gland.

*       *       *       *       *       *

Consequently, if we were to estimate the number of segments in this region
by the number of coelomic cavities we should not judge rightly, for we
should find only four cavities and seven appendages, as is seen in the
following table:--

  Key:                                                                {253}
  A Prosomatic.
  B Mesosomatic.
  C.c. Coelomic cavities.

  ---------------------------------------------------------+---------------
                             LIMULUS.                      |  VERTEBRATE.
  ---------+----------------+-----------------------+------+---------------
  Segments.|   Appendages.  | Eurypterid appendages.| C.c. |   Coelomic
           |                |                       |      |   cavities.
  ---+-----+----------------+-----------------------+------+---------------
     |  1  | Cheliceræ or   |  Cheliceræ            |   1  |  Anterior
     |     |  1st locomotor.|                       |      |
     |  2  | 2nd locomotor  |}                      |      |
   A |  3  | 3rd     "      |} Endognaths           |   2  |  Premandibular
     |  4  | 4th     "      |}                      |      |
     |  5  | 5th     "      |}                      |      |
     |  6  | 6th     "      |  Ectognath            |   3  |} Mandibular
     |  7  | Chilaria       |  Metastoma            |   4  |}
  ---+-----+----------------+-----------------------+------+---------------
     |  8  | Operculum      |} Operculum (Genital)  |   5  |} Hyoid
     |  9  | 1st branchial  |}    " (1st branchial) |   6  |}
     | 10  | 2nd    "       |  2nd branchial        |   7  |  1st branchial
   B | 11  | 3rd    "       |  3rd     "            |   8  |  2nd    "
     | 12  | 4th    "       |  4th     "            |   9  |  3rd    "
     | 13  | 5th    "       |  5th     "            |  10  |  4th    "
     | 14  | 6th    "       |                       |  11  |
  ---+-----+----------------+-----------------------+------+---------------

The second cavity would in reality represent four segments belonging to the
2nd, 3rd, 4th, 5th locomotor appendages, _i.e._ the very four segments
which in the Eurypteridæ are concentrated together to form the endognaths,
and we should be justified in putting this interpretation on it, because,
according to Kishinouye, its ventral portion forms the coxal gland, and,
according to Lankester, the coxal gland sends prolongations into the coxa
of the 2nd, 3rd, 4th, 5th locomotor appendages. Similarly in the
vertebrate, we find three head-cavities in the region which corresponds, on
my theory, to the prosomatic region of Limulus, (1) the anterior cavity
discovered by Miss Platt, (2) the premandibular cavity, and (3) the
mandibular cavity, which, if they corresponded with the prosomatic coelomic
cavities of Limulus, would represent not three segments but seven segments,
as follows:--the anterior cavity would correspond to the first coelomic
cavity, _i.e._ the cavity of the cheliceral segments in both Limulus and
the Eurypteridæ; the premandibular, to the second coelomic cavity,
representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in
Limulus and the endognathal segments in the Eurypteridæ; and the mandibular
to the 3rd and 4th coelomic cavities, representing the last locomotor and
chilarial segments in Limulus, _i.e._ the ectognathal and metastomal
segments in the Eurypteridæ.

{254}It is worthy of note that, in respect to their coelomic cavities, as
in the position and origin of their nerves in the central nervous system,
the first pair of appendages, the cheliceræ, retain a unique position,
differing from the rest of the prosomatic appendages.

In the table I have shown how the vertebrate coelomic cavities may be
compared with those of Limulus. The next question to consider is the
evidence obtained by morphologists and anatomists as to the number of
segments supplied by the trigeminal nerve-group; this question will be
considered in the next chapter.


SUMMARY.

  In Chapters IV. and V. I have dealt with the opisthotic segments of the
  vertebrate, including therein the segments supplied by the facial nerve,
  and shown that they correspond to the mesosomatic segments of the
  palæostracan; consequently the facial (VII.), glossopharyngeal (IX.), and
  vagus (X.) nerves originally supplied the branchial and opercular
  appendages.

  In this chapter the consideration of the pro-otic segments is commenced,
  that is, the segments supplied by the trigeminal (V.) and the eye-muscle
  nerves (III., IV., VI.). I have considered the VIth nerve with the rest
  of the eye-muscle nerves for convenience' sake, though in reality it
  belongs to the same segment as the facial. Of these, that part of the
  trigeminal which innervates the muscles of mastication corresponds to the
  splanchnic segments, while the eye-muscle nerves belong to the
  corresponding somatic segments; but the pro-otic segments of the
  vertebrate ought to correspond to the prosomatic segments of the
  invertebrate, just as the opisthotic correspond to the mesosomatic.
  Therefore the motor part of the trigeminal ought to supply muscles which
  originally moved the prosomatic appendages, while the eye-muscles ought
  to have belonged to the somatic part of the same segments.

  The first question considered is the number of segments which ought to be
  found in this region. In Limulus, the Eurypteridæ, and the scorpions
  there are seven prosomatic segments which carry (1) the cheliceræ, (2, 3,
  4, 5) the four first locomotor appendages--the endognaths, (6) the large
  special appendage--the ectognath--and (7) the appendages, which in
  Limulus are known as the chilaria, and are small and insignificant, but
  in Eurypterus and other forms grow forwards, fuse together, and form a
  single median lip to an accessory oral chamber, which lip is known as the
  metastoma. Of these appendages the cheliceræ and endognaths tend to
  dwindle away and become mere tentacles, while the large swimming
  ectognath and metastoma remain strong and vigorous.

  In this, the prosomatic region, the somatic segmentation is not
  characterized by the presence of the longitudinal muscle segments, for
  they do not extend into this head-region, but only by the presence of the
  segmental somatic {255}ventro-dorsal muscles. Among the muscles of the
  appendages the system of large tergo-coxal muscles is especially
  apparent.

  From these considerations it follows that the number of segments in this
  region in the vertebrate ought to be seven; that the musculature supplied
  by the trigeminal nerve ought to represent seven ventral or splanchnic
  segments, of which only the last two are likely to be conspicuous; and
  that the musculature supplied by the eye-muscle nerves ought to be
  dorso-ventral in direction, which it is, and represent seven dorsal or
  somatic segments.

  A further peculiarity of this region, both in Limulus and the scorpions,
  is found in the excretory organs which are known by the name of coxal
  glands, because they extend into the basal joint, or coxa, of certain of
  the prosomatic limbs. The appendages so characterized are always the four
  endognaths, and it follows that if these four endognaths lose their
  locomotor power, become reduced in size, and concentrated together to
  form mere tentacles, then of necessity the coxal glands will be
  concentrated together, and tend to form a glandular mass in the region of
  the mouth; in fact, take up a position corresponding to that of the
  pituitary body in vertebrates.

  Taking all these facts into consideration, it is possible to construct a
  drawing of a sagittal section through the head-region of Eurypterus,
  which will represent, with considerable probability, the arrangement of
  parts in that animal. This can be compared with the corresponding section
  through the head of Ammocoetes.

  Now, as pointed out in the last chapter, the early stage of Ammocoetes is
  remarkably different from the more advanced stage; at that time the
  septum between the oral and respiratory chambers has not yet broken
  through, and the olfactory or nasal tube, known at this stage as the tube
  of the hypophysis, is directed ventrally, not dorsally.

  The comparison of the diagram of Eurypterus with that of the early stage
  of Ammocoetes is remarkably close, and immediately suggests not only that
  the single nose of the former is derived from the corresponding organ in
  the palæostracan, but that the pituitary body is derived from the
  concentrated coxal glands, and the lower lip from the metastoma. The
  further working out of these homologies will be discussed in the next
  chapter.

  In addition to the evidence of segmentation afforded by the appendages,
  there are in this region, in Limulus and the scorpion group, three other
  criteria of segmentation available to us, if from any cause the evidence
  of appendages fails us. These are--

  1. The number of neuromeres are marked out in this region of the brain
  more or less plainly, especially in the young animal, just as they are
  also in the embryo of the vertebrate.

  2. The segmentation is represented here, just as in the mesosomatic
  region, by two sets of muscle-segments; the one _somatic_, consisting of
  the segmentally arranged dorso-ventral muscles, the continuation of the
  group already discussed in connection with the mesosomatic segmentation,
  and the other _appendicular_ characterized by the tergo-coxal muscles.
  These latter segmental muscles are especially valuable, for in such forms
  as Mygale, Phrynus, etc., their presence is indicated externally by
  markings on the prosomatic carapace, and thus corresponding markings
  found on fossil carapaces or on dorsal head-shields can be
  {256}interpreted. These two sets of muscle-segments correspond in the
  vertebrate to the somatic and splanchnic segmentations.

  3. In the vertebrate the segmentation in this region is indicated by the
  coelomic or head-cavities, which are cavities formed in the mesoderm of
  the embryo, the walls of which give origin to the striated muscles of the
  head. In Limulus corresponding coelomic cavities are found, which are
  directly comparable with those found in the vertebrate.




{257}CHAPTER VIII

_THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP_

  The prosomatic 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 trochlearis 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.


From the evidence given in the last chapter, combined with that given in
Chapter IV., the probability of the theory that the trigeminal group of
nerves of the vertebrate have been derived from the prosomatic group of
nerves of the invertebrate can be put to the test by the answers to the
following morphological and anatomical questions:--

1. Do we find in the vertebrate two segmentations in this region
corresponding to the two segmentations in the branchial region, _i.e._ a
somatic or dorsal series of segments, and a splanchnic or ventral series of
segments? The latter would not be branchial, but rather of the nature of
free tactile appendages; so that it is useless to look for or talk about
gill-slits, although such appendages, being serially homologous with the
branchial mesosomatic appendages, would readily give rise to the conception
of branchial segments.

2. Is there morphological evidence that the trigeminal nerve is not the
nerve belonging to a single segment, or even to two segments, but is really
a concentration of at least six, probably seven, segmental nerves?

3. Is there morphological evidence that the oculomotor and trochlear
nerves, which on all sides are regarded as belonging to the trigeminal
segments, are not single nerves corresponding each {258}to a single
segment, but are the somatic motor roots belonging to the same segments as
those to which the trigeminal supplies the splanchnic roots?

4. Do the mesoderm segments, which give origin to the eye-muscles, and
therefore do the head-cavities of this region, correspond with the
trigeminal segments? Considering the concentration of parts in this region
and the difficulty already presented by the want of numerical agreement
between the prosomatic appendages and the prosomatic coelomic cavities in
Limulus, it may very probably be difficult to determine the actual number
of the mesoderm segments.

5. Is there anatomical evidence that the ganglion of origin of the motor
part of the trigeminal nerve is not a single ganglion, but a representative
of many, probably seven?

6. Is there anatomical evidence that the ganglia of origin of the
oculomotor and trochlear nerves represent many ganglia?

7. Is there any evidence that the organs originally supplied by the motor
part of the trigeminal nerve are directly comparable with prosomatic
appendages?

It is agreed on all sides that in this region of the head there is distinct
evidence of double segmentation, the dorsal mesoderm segments giving origin
to the eye-muscles, and the ventral segments to the musculature innervated
by the trigeminal nerve. Originally, according to the scheme of van Wijhe,
two segments only were recognized, the dorsal parts of which were
innervated by the IIIrd and IVth nerves respectively. Since his paper, the
tendency has been to increase the number of segments in this region, as is
seen in the following sketch, taken from Rabl, of the history of cranial
segmentation.


HISTORY OF CRANIAL SEGMENTATION.

The first attempt to deal with this question was made by Goethe and Oken.
They considered that the cranial skeleton was composed of a series of
vertebræ, but as early as 1842 Vogt pointed out that only the occipital
segments could be reduced to vertebræ. In 1869, Huxley showed that vertebræ
were insufficient to explain the cranial segmentation, and that the nerves
must be specially considered. The olfactory and optic nerves he regarded as
parts of the brain, not true segmental nerves; the rest of the cranial
nerves {259}were segmental, with special reference to branchial arches and
clefts, the facial, glossopharyngeal, and separate vagus branches supplying
the walls of the various branchial pouches. In a similar manner, the supra-
and infra-maxillary branches of the trigeminal were arranged on each side
of the mouth, and the inner and outer twigs of the first (ophthalmic)
branch of the trigeminal on each side of the orbito-nasal cleft, the
trabecular and the supra-maxillary arches being those on each side of this
cleft. Thus Huxley considered that there was evidence of a series of pairs
of ventral arches belonging to the skull, viz. the trabecular and maxillary
in front of the mouth, the mandibular, hyoid, and branchial arches behind,
and that the Vth, VIIth, IXth, and Xth nerves were segmental in relation to
these arches and clefts. Gegenbaur, in 1871 and 1872, considered that the
branchial arches represented the lower arches of cranial vertebræ, and
therefore corresponded to lower arches in the spinal region, _i.e._ the
skull was composed of as many vertebræ as there are branchial arches. These
vertebræ were confined to the notochordal part of the skull, the prechordal
part having arisen secondarily from the vertebral part, while the number of
vertebræ are at least nine, possibly more. The nerves which could be
homologized with spinal nerves were, he thought, divisible into two great
groups--(1) the trigeminal group, which included the eye-muscle nerves, the
facial, and its dorsal branch, the auditory; (2) the vagus group, which
included the glossopharyngeal and vagus.

Such was the outcome of the purely comparative anatomical work of Huxley
and Gegenbaur--work that has profoundly influenced all the views of
segmentation up to the present day.

Now came the investigations of the embryologists, of whom I will take, in
the first instance, Balfour, whose observations on the embryology of the
Selachians led him to the conclusion that besides the evidence of
segmentation to be found in the cranial nerves and in the branchial clefts,
further evidence was afforded by the existence of head-cavities, the walls
of which formed muscles just as they do in the spinal region. He came to
the conclusion that the first head-cavity belonged to one or more pre-oral
segments, of which the nerves were the oculomotor, trochlearis, and
possibly abducens; while there were seven post-oral segments, each with its
head-cavity and its visceral arch, of which the trigeminal, facial,
glossopharyngeal, and the four parts of the vagus were the respective
nerves.

{260}Marshall, in 1882, considered that the cranial segments were all
originally respiratory, and that all the segmental nerves are arranged
uniformly with respect to a series of gill-clefts which have become
modified anteriorly and have been lost, to a certain extent, posteriorly.
He included the olfactory nerves among the segmental nerves, and looked
upon the olfactory pit, the orbito-nasal lacrymal duct, the mouth, and the
spiracle as all modified gill-slits, so that he reckoned three pre-oral and
oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and eight
post-oral segments belonging respectively to the VIIth and VIth nerves, and
to the IXth nerve, and six segments belonging to the Xth nerve. He pointed
out that muscles supplied by the oculomotor nerve develop from the outer
wall of the first head-cavity; not, however, the _obliquus superior_ and
_rectus externus_, the latter originating probably from the walls of the
third cavity.

In the same year, 1882, came van Wijhe's well-known paper, in which he
showed that the mesoderm of the head in the selachian divided into two sets
of segments, dorsal and ventral; that the dorsal segments were continuous
with the body-somites, and that the ventral segments formed the lateral
plates of mesoblast between each of the visceral and branchial pouches. He
concluded that the dorsal somites were originally nine in number, that each
was supplied with a ventral nerve-root, in the same way as the somites in
the trunk, and that to each one a visceral pouch corresponded, whose walls
were supplied by the corresponding dorsal nerve-root; of these nine
segments, the ventral nerve-roots of the first three segments were
respectively the oculomotor, trochlearis, and abducens nerves. The next
three segments possessed no definable ventral root or muscles, and the
seventh, eighth, and ninth segments possessed as ventral roots the
hypoglossal nerve, with its muscular supply. The corresponding dorsal
nerve-roots were the trigeminal, facial, auditory, glossopharyngeal and
vagus nerves, the difference between cranial and spinal dorsal roots being
that the former contain motor fibres.

Ahlborn, in 1884, drew a sharp distinction between the segments of the
mesoderm and those of the endoderm. The former segmentation he called
mesomeric, the latter branchiomeric. He considered the two segmentations to
be independent, and concluded that the branchiomeric was secondary to the
mesomeric, and therefore not of {261}segmental value. As to the segments of
the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ
remain permanently, and correspond to the three last segments in the
selachian head. Of the anterior mesoderm segments, he considered that there
were originally six, and that there are six typical eye-muscles in all
Craniota, which have been compressed into three segments, as in Selachia.

Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal
nerve belongs to three proto-vertebræ posterior to the vagus region, which
were true spinal segments. He therefore modified Gegenbaur's conceptions to
this extent: that portion of the skull designated by Gegenbaur as vertebral
must be divided into two parts--a hind or occipital region, which is
clearly composed of modified vertebræ and is the region of the hypoglossal
nerves, and a front region, extending from the oculomotor to the
accessorius nerves, which is characterized segmentally by the formation of
branchial arches, but in which there is no evidence that proto-vertebræ
were ever formed. He therefore divides the head-skeleton into three parts--

1. Gegenbaur's evertebral part--the region of the olfactory and optic
nerves--which cannot be referred to any metameric segmentation.

2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be
segmented from the consideration of the nerves and branchial arches, but
not referable to proto-vertebræ--the region of the trigeminal and vagus
nerves.

3. The vertebral spinal part--the region of the hypoglossal nerves.

He further showed that the ganglia of the specially branchial nerves, the
facial, glossopharyngeal, and vagus, are at one stage in connection with
the epidermis, so that these parts of the epidermis represent sense-organs
which do not develop; these organs probably belonged to the lateral line
system. As the connection takes place at the dorsal edge of the gill-slits,
they may also be called rudimentary branchial sense-organs.

Since this paper of Froriep's, it has been generally recognized, and
Gegenbaur has accepted Froriep's view, that the three hindmost metameres,
which distinctly show the characteristics of vertebræ, belong to the spinal
and not to the cranial region, so that the metameric segmentation of the
cranial region proper has become {262}more and more associated with the
branchial segmentation. Froriep's discovery of the rudimentary branchial
sense-organs as a factor in the segmentation question has led Beard to the
conclusion that the olfactory and auditory organs represent in a permanent
form two of these rudimentary branchial sense-organs. He therefore includes
both the olfactory and auditory nerves in his list of cranial segmental
nerves, and makes eleven cranial branchial segments in front of the spinal
segments represented by the hypoglossal.

A still larger number of cranial segments is supposed to exist, according
to the researches of Dohrn and Killian, in the embryos of _Torpedo
ocellata_. The former, holding to the view that vertebrates arose from
annelids, considered that the head was formed of a series of metameres, to
each one of which a mesoderm-segment, a gill-arch, a gill-cleft, a
segmental nerve and vessel belonged. He found in the front head-region of a
Torpedo embryo, corresponding to van Wijhe's first four somites, no less
than twelve to fifteen mesoderm segments, and concluded, therefore, that
the eye-muscle nerves, especially the oculomotor, represented many
segmental nerves, and were not the nerves of single segments; so, also,
that the inferior maxillary part of the trigeminal and the hyoid nerve of
the facial are probably not single nerves, but a fusion of several. Killian
comes to much the same conclusion as Dohrn, for he finds seventeen to
eighteen separate mesoderm segments in the head, of which twelve belong to
the trigeminal and facial region.

Since Rabl's paper, a number of papers have appeared, especially from
America, dealing with yet another criterion of the original segmentation of
the head, viz. a series of divisions of the central nervous system itself,
which are seen at a very early stage of development, and are called
neuromeres; the divisions in the cranial region being known as
encephalomeres, and those of the spinal region as myomeres. Locy's paper
has especially brought these divisions into prominence as a factor in the
question of segmentation. They are essentially segments of the epiblast and
not of the mesoblast; they are conspicuous in very early stages, and appear
to be in relation with the cranial nerves, according to Locy. He recognizes
in _Squalus acanthias_, in front of the spino-occipital region, fourteen
pairs of such encephalomeres and a median unsegmented termination, which
may represent one more pair fused in the middle line, making at least
fifteen. He distributes these fifteen segments as follows: {263}fore-brain
three and unsegmented termination, mid-brain two, and hind-brain nine.

Again, Kupffer, in his recent papers on the embryology of Ammocoetes,
asserts that especial information as to the number of primitive segments is
afforded by the appearance in the early stages of a series of epibranchial
ganglia in connection with the cranial nerves, which remain permanently in
the case of the vagus nerves, but disappear in the case of pro-otic nerves.
He considers that the evidence points to the number of segments in the mid-
and hind-brain region as being primitively fifteen, viz. six segments
belonging to the trigeminal and abducens group, three segments belonging
respectively to the facial, auditory, and glossopharyngeal, and six to the
vagus.

From this sketch we see that the modern tendency is to make six segments at
least out of the region of the trigeminal nerves rather than two. In this
region, as already mentioned, the evidence of segmentation is based more
clearly on the somatic than on the splanchnic segments. We ought,
therefore, in the first place, to consider the teaching of the eye-muscles
and their nerves and the coelomic cavities in connection with them, and see
whether the hypothesis that such muscles represent the original
dorso-ventral somatic muscles of the palæostracan ancestor is in harmony
with and explains the facts of modern research.


EYE-MUSCLES AND THEIR NERVES.

The only universally recognized somatic nerves belonging to these segments
which exist in the adult are the nerves to the eye-muscles, of which,
according to van Wijhe, the oculomotor is the nerve of the 1st segment, the
trochlearis of the 2nd, and the abducens of the 3rd; while the nerves and
muscles belonging to the 4th and 5th segments, _i.e._ the 2nd facial and
glossopharyngeal segments respectively, show only the merest rudiments, and
do not exist in the adult. One significant fact appears in this statement
of van Wijhe, and is accepted by all those who follow him, viz. that the
oculomotor nerve has equal segmental value with the trochlearis and the
abducens, although it supplies a number of muscles, each of which, on the
face of it, has the same anatomical value as the superior oblique or
external rectus. Dohrn alone, as far as I know, as already pointed out,
insists upon the multiple character of the oculomotor nerve.

{264}As far as the anatomist is concerned, the evidence is becoming clearer
and clearer that the nucleus of the IIIrd nerve is a composite ganglion
composed of a number of nuclei, each similar to that of the trochlearis, so
that if the trochlearis nucleus is a segmental motor nucleus, then the
oculomotor nucleus is a combined nucleus belonging to at least four
segmental nerves, each of which has the same value as that of the
trochlearis.

The investigations of a number of anatomists, among whom may be mentioned
Gudden, Obersteiner, Edinger, Kölliker, Gehuchten, all lead directly to the
conclusion that this oculomotor nucleus is composed of a number of separate
nuclei, of which the most anterior as also the Edinger-Westphal nucleus
contains small cells, while the others contain large cells. Thus Edinger
divides the origin of the oculomotor nerve into a small-celled anterior
part and a larger posterior part, of which the cells are larger and
distinctly arranged in three groups--(1) dorsal, (2) ventral, and (3)
median. Between the anterior and posterior groups lies the Edinger-Westphal
nucleus, which is small-celled; naturally, the large-celled group is that
which gives origin to the motor nerves of the eye-muscles, the small-celled
being possibly concerned with the motor nerves of the pupillary and ciliary
muscles. I may mention that Kölliker considers that the anterior lateral
nucleus has nothing to do with the oculomotor nerve, but is a group of
cells in which the fibres of the posterior longitudinal bundle and of the
deep part of the posterior commissure terminate.

These conclusions of Edinger are the outcome of work done in his laboratory
by Perlia, who says that in new-born animals the nucleus of origin of the
oculomotor nerve is made up of a number of groups quite distinct from each
other, each group being of the same character as that of the trochlearis.
He finds the same arrangement in various mammals and birds. Further, he
finds that some of the fibres arise from the nucleus of the opposite side,
thus crossing, as in the trochlearis; these crossing fibres belong to the
most posterior of the dorsal group of nuclei, _i.e._ to the nerve to the
inferior oblique muscle.

The evidence, therefore, points to the conclusion that the oculomotor
nucleus is a multiple nucleus, each part of which gives origin to one of
the nerves of one of the eye-muscles.

Edinger says that such an array of clinical observations exists, {265}and
of facts derived from post-mortem dissections, that one may venture to
designate the portion of the nucleus from which the innervation of each
individual ocular muscle comes. He gives Starr's table, the latest of these
numerous attempts, begun by Pick. According to Starr, the nuclei of the
nerves to the individual muscles are arranged from before backward, thus--

  _m. sphincter iridis._    _m. ciliaris._
  _m. levator palpebræ._    _m. rectus internus._
  _m. rectus superior._     _m. rectus inferior._
  _m. obliquus inferior._

Further, the evidence of the well-known physiological experiments of Hensen
and Völckers that the terminal branches of the oculomotor nerve arise from
a series of segments of the nucleus, arranged more or less one behind the
other in a longitudinal row, leads them to the conclusion that the nuclei
of origin are arranged as follows, proceeding from head to tail:--

  Nearest brain.   1. _m. ciliaris._
                   2. _m. sphincter iridis._
                   3. _m. rectus internus._
                   4. _m. rectus superior._
                   5. _m. levator palpebræ._
                   6. _m. rectus inferior._
  Most posterior.  7. _m. obliquus inferior._

It is instructive to compare this arrangement of Hensen and Völckers with
the arrangement of the origin of these muscles from the premandibular
cavity as given by Miss Platt.

Thus she states that the most posterior part of the premandibular cavity is
cut off so as to form a separate cavity, resembling, except in position,
the anterior cavity; this separate, most posterior part gives origin to the
inferior oblique muscle. She then goes on to describe how the dorsal wall
of the remainder of the premandibular cavity becomes thickened, to form
posteriorly the rudiment of the inferior rectus and anteriorly the
rudiments of the superior and internal recti, a slight depression in the
wall of the cavity separating these rudiments. The internal rectus is the
more median of the two anterior muscles. In other words, her evidence
points not only to a fusion of somites to form the premandibular cavity,
but also to the arrangement of these somites as follows, from head to tail:
(1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior
{266}oblique--an order precisely the same as that of Hensen and Völckers,
and of Starr.

I conclude, from the agreement between the anatomical, physiological, and
morphological evidence, that the IIIrd and IVth nerves contain the motor
somatic nerves belonging to the same segments as the motor trigeminal, in
other words, to the prosomatic segments, so that the eye-muscles,
innervated by III. and IV., represent segmental muscles belonging to the
prosoma. Further, I conclude that originally there were seven prosomatic
segments, the first of which is represented by the anterior cavity
described by Miss Platt, and does not form any permanent muscles; that the
next four belong to the premandibular cavity, and the muscles formed are
the superior rectus, internal rectus, inferior rectus, and inferior
oblique; and that the last two belong to the mandibular cavity, the muscles
formed being Miss Platt's mandibular muscle and the superior oblique. It
is, to say the least of it, a striking coincidence that such an arrangement
of the coelomic cavities as here given should be so closely mimicked by the
arrangement in the prosomatic region of Limulus as already mentioned; it
suggests inevitably that the head-cavities of the vertebrate are nothing
more than the prosomatic and mesosomatic segmental coelomic cavities, as
found in animals such as Limulus. In the table on p. 253, I have inserted
the segments in the vertebrate for comparison with those of Limulus.

Before we can come to any conclusion as to the original position of these
eye-muscles, it is necessary to consider the VIth nerve and the external
rectus muscle. This nerve and this muscle belong to van Wijhe's 4th
segment. The muscle is, therefore, the somatic segmental muscle belonging
to the same segment as the facial and is, in fact, a segmental muscle
belonging not to the prosoma, but to the mesosoma. Neal comes to the
conclusion that the existing abducens is the only root which remains
permanent among a whole series of corresponding ventral roots belonging to
the opisthotic segments, and further points out that the external rectus
was originally an opisthotic muscle which has taken up a pro-otic position,
or, translating this statement into the language of Limulus, etc., it is a
mesosomatic muscle which has taken up a prosomatic position.

There is, however, another muscle--the _Retractor oculi_--belonging to the
same group which is innervated by the VIth nerve. Quite recently Edgeworth
has shown that in birds and reptiles this muscle {267}belongs to the hyoid
segment; so that in this respect also the hyoid segment proclaims its
double nature.

With respect to the external rectus muscle, Miss Platt has shown that the
mandibular muscle is formed close alongside the external rectus, so that
the two are in close relationship as long as the former exists.

Further, as already mentioned, the eye-muscles in Ammocoetes must be
considered by themselves; they do not belong in structure or position to
the longitudinal somatic muscles innervated by the spinal nerves; their
structure is not the same as that of the tubular constrictor or branchial
muscles, but resembles that structure somewhat; their position is
dorso-ventral rather than longitudinal; they may be looked upon as a
primitive type of somatic muscles segmentally arranged, the direction of
which was dorso-ventral.

Anderson also has shown that the time of medullation of the nerves
supplying these muscles is much earlier than that of the nerves belonging
to the somatic trunk-muscles, their medullation taking place at the same
time as that of the motor nerves supplying the striated visceral muscles;
and Sherrington has observed that these muscles do not possess
muscle-spindles, while all somatic trunk-muscles do. Both these
observations are strong confirmation of the view that the eye-muscles must
be classified in a different category to the ordinary somatic trunk muscle
group.

What, then, is the interpretation of these various embryological and
anatomical facts?

Remembering the tripartite division of each segmental nerve-group in
Limulus into (1) dorsal or sensory somatic nerve, (2) appendage-nerve, and
(3) ventral somatic nerve, I venture to suggest that the three nerves--the
_oculomotorius_, the _trochlearis_, and the _abducens_--represent the
ventral somatic nerves of the prosoma, and partly also of the mesosoma;
that they are nerves, therefore, which may have originally contained
sensory fibres, and which still contain the sensory fibres of the
eye-muscles themselves, as stated by Sherrington. According to this
suggestion, the eye-muscles are the sole survivors of the segmental
dorso-ventral somatic muscles, so characteristic of the group from which I
imagine the vertebrates to have sprung. In the mesosomatic region the
dorso-ventral muscles which were retained were those of the appendages and
not of the mesosoma itself, because the presumed ancestor breathed after
the fashion of the water-breathing Limulus, by means of the dorso-ventral
muscles of its {268}branchial appendages, and not after the fashion of the
air-breathing scorpion, by means of the dorso-ventral muscles of the
mesosoma. The only mesosomatic dorso-ventral muscles which were retained
were those of the foremost mesosomatic segments, _i.e._ those supplied by
the VIth nerve, which were preserved owing to their having taken on a
prosomatic position and become utilized to assist in the movements of the
lateral eyes.

Let us turn now to the consideration of the corresponding musculature in
Limulus and in the scorpion group. These muscles constitute the markedly
segmental muscles to which I have given the name 'dorso-ventral somatic
muscles.' They are most markedly segmental in the mesosomatic region, both
in Limulus and in Scorpio, each mesosomatic segment possessing a single
pair of these vertical mesosomatic muscles, as Benham calls them (_cf._
Fig. 58 (_Dv._)). In the prosomatic region the corresponding muscles are
not so clearly defined in Limulus; they are apparently attached to the
plastron forming the group of plastro-tergal muscles. From Benham's
description it is sufficiently evident that they formed originally a single
pair to each prosomatic segment.

In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles
are situated near the middle line on each side and form the following
well-marked series of pairs of muscles, shown in Fig. 110, A, taken from
her paper, and thus described by her:--

1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the
dorso-ventral muscles. It is very small, and is attached to the carapace
near the median line anteriorly to the central eyes.

2. The median dorso-preoral-entosclerite muscle (62) is a large muscle,
between which and its fellow of the opposite side the eyes are situated. It
is attached dorsally to the carapace and ventrally to the pre-oral
entosclerite.

3. The anterior dorso-plastron muscle (63) is attached dorsally to the
carapace in the middle line, being joined to its fellow of the opposite
side. They separate, and are attached ventrally to the plastron. Through
the arch thus formed the alimentary canal and the dorsal vessel pass.

4. The median dorso-plastron muscle (64) is attached dorsally to the
posterior part of the carapace. It runs forward on the anterior surface of
the posterior flap of the plastron to the body of the plastron, to which it
is attached.

{269}[Illustration] A.

DORSO-VENTRAL MUSCLES ON CARAPACE OF SCORPION. (From MISS BECK.)


[Illustration] B.

SIMILAR MUSCLES ON CARAPACE OF EURYPTERUS.


[Illustration] C.

SIMILAR MUSCLES ON HEAD-SHIELD OF A CEPHALASPID.

_l.e._, lateral eyes; _c.e._, central eyes; _Fro._, narial opening.

62-65 refer to Miss Beck's catalogue of the scorpion muscles.


FIG. 110.


{270}To these may be added, owing to its attachment to the plastron,

5. The posterior dorso-plastron muscle (65). This is the first of the
dorso-ventral muscles attached to the mesosomatic tergites, being attached
to the tergite of the first segment of the mesosoma.

This muscle is of interest, in connection with the prosomatic dorso-ventral
muscles, because it is attached to the plastron, and runs a course in close
contact with the muscle (64), the two muscles being attached dorsally close
together, on each side of the middle line, the one at the very posterior
edge of the prosomatic carapace, and the other at the very anterior edge of
the mesosomatic carapace.

Taking these muscles separately into consideration, it may be remarked with
respect to (61) that the cheliceral segment in its paired dorso-ventral
muscles, as in its tergo-coxal muscles, takes up a separate position
isolated from the rest of the prosomatic segments.

Next comes (62) the median dorso-preoral-entosclerite muscle, which is
strikingly different from all the other dorso-ventral muscles in its large
size and the extent of its attachment to the dorsal carapace, according to
Miss Beck's figures. The reason of its large size is clearly seen upon
dissection of the muscles in _Buthus_, for I find that, strictly speaking,
it is not a single muscle, but is composed of a series of muscle-bundles,
separated from each other by connective tissue. There are certainly three
separate muscles included in this large muscle, which are attached in a
distinct series along the pre-oral entosclerite, and present the appearance
given in Fig. 110, A, at their attachment to the prosomatic carapace. Of
this muscle-group the most anterior and the most posterior bundle are
distinctly separate muscles; I am not, however, clear whether the middle
bundle represents one or two muscles.

This division of Miss Beck's muscle (62) into three or four muscles brings
the prosomatic region of the scorpion into line with the mesosomatic, and
enables us to feel sure that a single pair of dorso-ventral somatic muscles
belongs to each prosomatic segment just as to each mesosomatic, and,
conversely, that each such single pair of muscles possesses segmental value
in this region as much as in the mesosomatic.

It is very striking to see how in all the Scorpionidæ, in which the two
median eyes are the principal eyes, this muscle group (62) on the two sides
closely surrounds these two eyes, so that with a fixed {271}pre-oral
entosclerite, a slight movement of the eyes, laterally or anteriorly, owing
to the flexibility of the carapace, might result as the consequence of
their contraction. But this cannot be the main object of these muscles. The
pre-oral entosclerite is firmly fixed to the camerostome, as is seen in
Fig. 94, _pr. ent._, so that the main object of these muscles is, as Huxley
has pointed out, the movement of this organ.

In order to avoid repetition of the long name given to this muscle group
(62) by Miss Beck, because of their position, and for other reasons which
will appear in the sequel, I will call this group of muscles the group of
recti muscles. These recti muscles belong clearly to the segments posterior
to the first prosomatic or cheliceral segment, and represent certainly
three, probably four, of these segments, _i.e._ belong to the segments
corresponding to the second, third, fourth, and fifth prosomatic locomotor
appendages--the endognaths of the old Eurypterids.

The next pair of muscles is the pair of anterior dorso-plastron muscles
(63). This muscle-pair evidently belongs to a segment posterior to the
segments represented by the group already discussed, and belongs,
therefore, in all probability to the same segment as the sixth pair of
prosomatic appendages--the ectognaths of the old Eurypterids. This can be
settled by considering either the nerve-supply or the embryological
development. In the Eurypteridæ it seems most highly probable that the
dorso-ventral muscles of each half of the segments belonging to the
endognaths should be compressed together and separate from the
dorso-ventral muscle belonging to the ectognathal segment, on account of
the evident concentration and small size of the endognathal segments in
contradistinction to the separateness and large size of the ectognathal
segment.

The striking peculiarity of this muscle-pair, which distinguishes it from
all other muscles in the scorpion, is the common attachment of the muscles
of the two sides in the mid-dorsal line, so that the pair of muscles forms
an arch through which the alimentary canal and dorsal blood-vessel pass.

The same dorso-ventral muscles are present in _Phrynus_, and in this animal
the fibres of this pair of muscles (63) actually interlace before the
attachment to the prosomatic carapace, so that the attachment of the muscle
on each side overpasses the mid-dorsal line, and a true crossing occurs. In
Fig. 108 the position of this pair of {272}muscles is shown just
posteriorly to the brain-mass. This muscle I will call the oblique muscle.

Finally we come to the muscles (64) and (65), the median and posterior
dorso-plastron muscles, which run close together. Both muscles are attached
to the plastron, and, therefore, to that extent belong to the prosomatic
region; they are attached dorsally close to the junction of the prosoma and
mesosoma. This position of the first mesosomatic dorso-ventral muscle
belonging to the opercular segment may be compared with the position of the
first mesosomatic dorso-ventral muscle in Limulus which has become attached
to the prosomatic carapace; in both cases we see an indication that the
foremost pair of mesosomatic dorso-ventral somatic muscles tend to take up
a prosomatic position.

As to the pair of small muscles (64), I believe that they represent the
dorso-ventral muscles of the seventh prosomatic segment (if the pair of
muscles (63) belongs to the segment of the sixth locomotor prosomatic
appendages), _i.e._ they belong to the chilarial segment or metastoma.

I desire to draw especial attention to the fact that the dorso-ventral
muscle (64), which represents the seventh segment, always runs close
alongside the dorso-ventral muscle (65), which represents the first
mesosomatic or opercular segment.

The comparison, then, of these two sets of facts leads to the following
conclusions:--

The foremost prosomatic or trigeminal segment stood separate and apart,
being situated most anteriorly; the musculature of this segment does not
develop, so that the only evidence of its presence is given by the anterior
coelomic cavity. This corresponds, according to my scheme, with the first
or anterior coelomic cavity of Limulus, and therefore represents, as far as
the prosomatic appendages are concerned, the first prosomatic
appendage-pair, or the cheliceræ; the appendage-muscles being the muscles
of the cheliceræ, and the dorso-ventral somatic muscles the pair of
dorso-cheliceral sternal muscles (61) in the scorpion. Both these sets of
muscles, therefore, dwindle and disappear in the vertebrate.

Then came four segments fused together to form the premandibular segment,
the characteristic of which is the apparent non-formation of any permanent
musculature from the ventral mesoderm-segments, and the formation of the
eye-muscles innervated by the {273}oculomotor nerve from the dorsal
mesoderm segments. These four segments have been so fused together that van
Wijhe looked upon them as a single segment, and the premandibular cavity as
the cavity of a single segment. They represent, according to my scheme, the
segments belonging to the endognaths, _i.e._ the second, third, fourth,
fifth pairs of prosomatic appendages; the premandibular cavity, therefore,
represents the second coelomic cavity in Limulus, which, according to
Kishinouye, is the sole representative of the coelomic cavities of the
second, third, fourth, fifth prosomatic segments. The muscles derived from
the ventral mesoderm-segments represent the muscles of these appendages,
which therefore dwindle and disappear in the vertebrate, with the possible
exception of the muscles innervated by the descending root of the
trigeminal. The muscles derived from the dorsal mesoderm-segments, _i.e._
the eye-muscles supplied by the oculomotor nerve, represent the
dorso-ventral somatic muscles of these four segments, muscles which are
represented in the scorpion by the recti group of muscles, _i.e._ the
median dorso-preoral-entosclerite muscles (62).

Then came two segments, the mandibular, in which muscles are formed both
from the ventral and from the dorsal mesoderm-segments. From the former
arose the main mass of muscles innervated by the motor root of the
trigeminal, from the latter the superior oblique muscle and the mandibular
muscle of Miss Platt, of which the former alone survives in the adult
condition. These two segments are looked upon as a single segment by van
Wijhe, of which the mandibular cavity is the coelomic cavity. They
represent, according to my scheme, the segments belonging to the sixth pair
of prosomatic appendages or ectognaths, and the seventh pair, _i.e._ the
chilaria or metastoma.

The first part, then, of the mandibular cavity represents the third
coelomic cavity in Limulus and the muscles derived from the ventral
mesoderm, in all probability the muscles of the tongue in the lamprey
(_cf._ Chap. IX.), which represents the ectognaths or sixth pair of
prosomatic appendages, while the muscles derived from the dorsal mesoderm,
_i.e._ the superior oblique muscles, represent the dorso-ventral somatic
muscles of this segment, muscles which are represented in the scorpion
group by the pair of anterior dorso-plastron or oblique muscles (63).

The second part of the mandibular cavity represents the 4th {274}coelomic
cavity in Limulus and the muscles derived from the ventral mesoderm, in all
probability the muscles of the lower lip in the lamprey (_cf._ Chap. IX.),
which represents the metastoma; while the muscles derived from the dorsal
mesoderm, _i.e._ Miss Platt's pair of mandibular muscles, represent the
dorso-ventral somatic muscles of this segment, muscles which are
represented in the scorpion group by the pair of median dorso-plastron
muscles (64).

In connection with this last pair of muscles we find that the external
rectus in the vertebrate represents the first dorso-ventral mesosomatic
muscle in the scorpion, _i.e._ the posterior dorso-plastron muscle (65),
and, as already mentioned (p. 267), that it always lies closely alongside
the mandibular muscle, just as in the scorpion group muscle (65) always
lies alongside muscle (64).

In the invertebrate as well as in the vertebrate this muscle is a
mesosomatic muscle which has taken up a prosomatic position.

The question naturally arises, what explanation can be given of the fact
that these dorso-ventral muscles attached on each side of the mid-dorsal
line to the prosomatic carapace became converted into the muscles moving
the eyeballs of the two lateral eyes? An explanation which must take into
account not only the isolated position of the abducens nerve, but also the
extraordinary course of the trochlearis. The natural and straightforward
answer to this question appears to me quite satisfactory, and I therefore
venture to commend it to my readers.

I have argued the case out to myself as follows: The lateral eyes must have
been originally situated externally to the group of muscles innervated by
the oculomotor nerve, for a sheet of muscle representing the superior
_internal_ and inferior rectus muscles could only wrap round the internal
surface of each lateral eye; _i.e._ the arrangement of the muscle-sheet, as
in the scorpion, about two median eyes, is in the wrong position, for if
those two eyes, which are the main eyes in the scorpion, were to move
outwards to become two lateral eyes, then such a muscle-group would form a
superior _external_ and inferior rectus group. The evidence, however, of
Eurypterus and similar forms is to the effect that the lateral eyes became
big and the median eyes insignificant and degenerate. If, then, with the
degeneration of the one and the increasing importance of the other, these
lateral eyes came near the middle line, then the muscular group (62), which
I have called the recti group, would naturally be pressed into their
{275}service, and would form an internal and not an external group of
eye-muscles.

In Fig. 110, A, taken from Miss Beck's paper, I have shown the relative
position of the eyes and the segmental dorso-ventral prosomatic muscles on
the carapace of the scorpion. In Fig. 110, B, I have drawn the prosomatic
carapace of _Eurypterus Scouleri_, taken from Woodward's paper, with the
eyes as represented there; in this I have inserted the segmental
dorso-ventral muscles as met with in the scorpion, thereby demonstrating
how, with the degeneration of the median eyes and the large size of the
lateral eyes, the recti muscles of the scorpion would approach the position
of an internal recti group to the lateral eyes, and so give origin to the
group of muscles innervated by the oculomotor nerve. In the Eurypterus
these large eyes are large single eyes, not separate ocelli, as in the
scorpion.

All, then, that is required is that in the first formed fishes, which still
possessed the dorso-ventral muscles of their Eurypterid ancestors, the
lateral eyes should be the important organs of sight, large and near the
mid-dorsal line. Such, indeed, is found to be the case. In amongst the
masses of Eurypterids found in the upper Silurian deposits at Oesel, as
described by Rohon, numbers of the most ancient forms of fish are found
belonging to the genera Thyestes and Tremataspis. The nature of the dorsal
head-shields of these fishes is shown in Fig. 14, which represents the
dorsal head-shield of _Thyestes verrucosus_, and Fig. 111 that of
_Tremataspis Mickwitzi_. They show how the two lateral eyes were situated
close on each side of the mid-dorsal line in these Eurypterus-like fishes,
in the very position where they must have been if the eye-muscles were
derived from the dorso-ventral somatic muscles of a Eurypterid ancestor.

[Illustration: FIG. 111.--DORSAL HEAD-SHIELD OF _Tremataspis Mickwitzi_.
(From ROHON.)

_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum plate over
brain; _Occ._, occipital spine.]

In Lankester's words, one of the characteristics of the Osteostraci
(Cephalaspis, Auchenaspis, etc.), as distinguished from the Heterostraci
(Pteraspis), are the large orbits placed near the centre of the shield. The
apparent exception of Thyestes mentioned by him is no {276}exception, for
orbits of the same character have since been discovered, as is seen in
Rohon's figure (Fig. 14). In Fig. 110, C, I give an outline of the frontal
part of the head-shield of a Cephalaspid, in which I have drawn the
eye-muscles as in the other two figures.

Although all the members of the Osteostraci possess large lateral eyes
towards the centre of the head-shield, the other group of ancient fishes,
the Heterostraci, are characterized by the presence of lateral eyes far
apart, situated on the margin of the head-shield on each side (_cf._ Fig.
142, _o_, p. 350).

So, also, on the invertebrate side, the lateral eyes of Pterygotus and
Slimonia are situated on the margin of the prosomatic carapace, while those
of Eurypterus and Stylonurus are situated much nearer the middle line of
the prosomatic carapace.

Next comes the question of the superior oblique muscle and the trochlearis
nerve. Why does this nerve (_n.IV._ in Fig. 106, C and D) alone of all the
nerves in the body take the peculiar position it always does take? The only
suggestion that I know of which sounds reasonable and worth consideration
is that put forward by Fürbringer, which is an elaboration of the original
suggestion of Hoffmann. Hoffmann suggested in 1889 that the trochlearis
nerve represented originally a nerve for a protecting organ of the pineal
eye, which became secondarily a motor nerve for the lateral eye as the
pineal eye degenerated. Fürbringer differs from Hoffmann in that he
considers that the nerve was originally a motor nerve, and was not
transformed from sensory to motor, yet thinks Hoffmann's suggestion is in
the right direction.

He points out that the crossing of the trochlearis is not a crossing of
fibres between two centres in the central nervous system, but may be
explained by the shifting of the peripheral organ, _i.e._ the muscle, from
one side to the other, and the nerve following this shift. Consequently,
says Fürbringer, the course of the nerve indicates the original position of
the muscle, and therefore he imagines that the ancestor of the superior
oblique muscle was a muscle the fibres of which were attached in the
mid-dorsal line, and interlaced with those of the other side, the two
muscles thus forming an arch through which the nervous system with its
central canal passed. Then, for the sake of getting a more efficient pull,
the crossing muscle-fibres became more definitely attached to the opposite
side of the middle line, and finally obtained a new attachment on the
opposite side, with the {277}obliteration of the muscular arch; the nerve
on each side, following the shifts of the muscle, naturally took up the
position of the original muscular arch, and so formed the trochlear nerve,
with its dorsal crossing. This explanation of Fürbringer's was associated
by him with movements of the median pineal eyes, the length of their nerve,
according to him, even yet indicating their previous mobility. This
assumption is not, it seems to me, necessary. The length of the nerve is
certainly no indication of mobility, for in Limulus and the scorpion group
the nerve to each median eye is remarkably long, yet these eyes are
immovably fixed in the carapace. All that is required is a pair of
dorso-ventral muscles belonging to the segment immediately following the
group of segments represented by the oculomotor nerves, the fibres of which
should cross the mid-dorsal line at their attachment; for, seeing that the
lateral eyes were originally so near this position, it follows that such
muscles might form part of the muscular group belonging to the lateral eye
without having previously moved the pineal eyes. In fact, Fürbringer's
explanation requires as starting-point that the pair of muscles which
ultimately become the superior oblique should have the exact position of
the pair of dorso-ventral muscles in the scorpion, called by Miss Beck the
anterior dorso-plastron muscles (63), which I have named the oblique
muscles. Here, and here only, do we find an interlacement, across the
mid-dorsal line, of the fibres of attachment of the muscles on the two
sides, in consequence of which this pair of muscles is described by her as
forming an arch encircling the alimentary canal and dorsal vessel. If,
then, as I have previously argued, the primitive plastron formed a pair of
trabeculæ, and the nervous system grew round the alimentary canal, such an
arch would encircle the tubular central nervous system of the vertebrate.

Still more striking is this pair of muscles (63) in Phrynus (Fig. 108),
where we see how the arch formed by them almost touches the posterior
extremity of the supra-oesophageal brain-mass, crossing, therefore, over
the beginning of the stomach region of the animal. The angle formed by the
arch is much more obtuse than that formed in Scorpio, so that an actual
crossing of the muscle-fibres has taken place at the point of attachment to
the carapace. Also, only the part nearest the carapace is muscular, the
rest forming a long tendinous prolongation of the plastron wall (the
primordial cranium), as seen in the figure.

{278}This muscle-pair is, as it should be, the pair of dorso-ventral
muscles belonging to the segment immediately following on the group of
segments represented by the recti muscles, _i.e._ according to previous
argument, the segment belonging to the sixth pair of locomotor appendages
or ectognaths; a muscle, therefore, which would arise in the vertebrate
from the mandibular, and not from the premandibular cavity. A similar
muscle probably existed in Eurypterus (_M.obl._ in Fig. 106, B), and, as in
the case of the formation of the oculomotor group, derived from the recti
group of the scorpion, would form the commencement of the superior oblique
muscle in Thyestes and Tremataspis.

[Illustration: FIG. 112.--A, DIAGRAM OF POSITION OF OBLIQUE MUSCLE IN
SCORPION; B, DIAGRAM OF TRANSITION STAGE; C, DIAGRAM OF SUPERIOR OBLIQUE
MUSCLE IN VERTEBRATE.

_l.e._, lateral eyes; _c.e._, central eyes; _C.N._, central nervous system;
_Al._, alimentary canal; _c._, _aqueductus Sylvii_.]

It is instructive to notice that the original position of attachment of
this muscle is naturally posterior to that of the oculomotor group of
muscles, and that Fürbringer, in his description of the eye-muscles of
Petromyzon, asserts that this muscle in this primitive vertebrate {279}form
is not attached as in other vertebrates, but is posterior to the other
muscles, so that he calls it the posterior rather than the superior
oblique. The nature of the change by which the muscle known in the scorpion
as the anterior dorso-plastron muscle (63) was probably converted into the
superior oblique muscle of the vertebrate, is represented in the drawings
Fig. 112, in which also are indicated the dwindling of the median eyes, and
the progressive superiority of the lateral eyes, as well as the
transformation of the recti muscle-group of the scorpion into the muscles
supplied by the oculomotor nerve of the vertebrate.

With respect to the external rectus muscle, it follows naturally that if
the muscles (64) and (65) are to follow suit with the rest of the group and
become attached to the lateral eyes, they must take up an external
position. These two muscles, which always run together, as seen in Fig.
110, A, the one belonging to the prosoma and the other to the mesosoma, are
represented by the mandibular muscle of Miss Platt and the external rectus,
the former derived from the walls of the last pro-otic head-cavity, the
latter from the foremost of the opisthotic head-cavities.

Such, then, is the simple explanation of the origin of the eye-muscles
which follows from my theory, and we see that the successive alterations of
the position of the orbit, and, therefore, of the globe of the eye with its
muscles, as we pass from Thyestes to man, is the natural consequence of the
growth of the frontal bone, _i.e._ of the brain.


THE TRIGEMINAL NERVES AND THE MUSCLES SUPPLIED BY THEM.

Turning now to the evidence as to the number of ventral segments, _i.e._
the motor and sensory supply to the prosomatic appendages afforded by the
trigeminal nerve, we must, I think, come to the same conclusion as Dohrn,
viz. that if there were originally seven dorsal or somatic segments in this
region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles
of the premandibular cavity, _sup. rectus_, _inf. rectus_, _int. rectus_,
_inf. oblique_, supplied by IIIrd nerve; 6, 7, muscles of the mandibular
cavity, _sup. oblique_, supplied by IVth nerve and muscle lost, there must
have been also seven corresponding ventral or splanchnic segments supplied
by the trigeminal. At present the evidence for such segments is nothing
like so strong as for the corresponding somatic ones; there are, however,
certain suggestive {280}facts which point distinctly in this direction in
connection with both the motor and sensory parts of the trigeminal. The
origin of the trigeminal motor fibres in the central nervous system is most
striking. We may take it for granted that a nucleus of cells giving origin
to one or more segmental motor nerves will possess a greater or less
longitudinal extension in the central nervous system, according to the
number of fused separate segmental centres it represents. Thus a nucleus
such as that of the IVth nerve or of the facial is small and compact in
comparison to the extensive conjoint nucleus of the vagus and cranial
accessory.

Upon examination of the motor nucleus of the trigeminal, we find a compact
or well-defined nucleus, the _nucl. masticatorius_, the nerves of which
supply the masseter, temporal, and other muscles, so that the anatomical
evidence at first sight appears to bear out van Wijhe's conclusion that the
motor trigeminal supplies at most two segments. Further examination,
however, shows that this is not all, for the extraordinary so-called
descending root of the Vth must be taken into consideration in any question
of the origin of the motor elements, just as the equally striking ascending
root enters into the consideration of the meaning of the sensory elements
of the Vth.

It is not necessary here to discuss the controversy as to whether this
descending root is motor or sensory. It is universally considered at
present to be motor, and is believed to supply, as Kölliker suggested,
among other muscles, the _m. tensor tympani_ and the _m. tensor veli
palati_. It is thus described by Obersteiner--

"From the region of the mid-brain the motor root receives an important
addition of thick fibres, which form the cerebral or descending root. The
large, round vesicular cells from which the fibres of the descending root
arise form no single compact group, but are partly single, partly arranged
like little bunches of grapes, as far as the region of the anterior corpora
quadrigemina. The further we go brainwards, the smaller is the number of
fibres. In the region of the anterior corpora quadrigemina, the few cells
of origin are found more and more median; so that the uppermost trigeminal
fibres descend in curves almost from the mid-line, as is shown by the
exceptional occurrence of one or more of the characteristic cells above the
aqueduct. At the height of the posterior commissure one finds the last of
these trigeminal cells."

{281}The anatomy of the Vth nerve reveals, then, three most striking
facts:--

1. The motor nucleus of the Vth extends from the very commencement of the
infra-infundibular region to nearly the commencement of the nucleus of the
VIIth; in other words, the motor nucleus of the Vth extends through the
whole prosomatic region, just as it must have done originally if its motor
nerves supplied the muscles of the prosomatic appendages. Such an extended
range of origin is indicative of the remains of an equally extended series
of segmental centres or ganglia.

2. Of these centres the caudalmost have alone remained large and vigorous,
constituting the _nucleus masticatorius_, which in the fish is divided into
an anterior and posterior group, thus indicating a double rather than a
single nucleus; while the foremost ones have dwindled away until they are
represented only by the cells of the descending root, the muscles of these
segments being still represented by possibly the _tensor veli palati_ and
the other muscles innervated from these cells.

3. The headmost of these cells takes up actually a position dorso-lateral
to the central canal, so that the groups on each side nearly come together
in the mid-dorsal line; a very unique and extraordinary position for a
motor cell-group, but not improbable when we recall to mind Brauer's
assertion as to the shifting of the foremost prosomatic ganglion-cells of
the scorpion from the ventral to the dorsal side of the alimentary canal.

On the sensory side the evidence is also suggestive, the question here
being not so much the distribution of the sensory nerves as the number of
ganglia belonging to each of the cranial nerves.

With respect to this question, morphologists have come to the conclusion
that there is a marked difference between spinal and cranial nerves, in
that whereas the posterior root-ganglia of the spinal nerves arise from the
central nervous system itself, _i.e._ from the neural crest, the ganglia of
the cranial nerves arise partly from the neural crest, partly from the
proliferation of cells on the surface of the animal; and because of the
situation of these proliferating epidermal patches over the gill-clefts in
the case of the vagus and glossopharyngeal nerves, they have been called by
Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia
into two sets, one connected with the neural ridges, called the neural
ganglia, {282}and the other connected with the surface-cells, which he
calls the lateral ganglia. This second set corresponds to Kupffer's
epibranchial ganglia. Now it is clear that in the case of the vagus nerve,
where, as is well shown in Ammocoetes, the nerve is not a single segmental
nerve, but is in reality made up of a number of nerves going to separate
branchial segments, the indication of such segments is not given by the
main vagus ganglion or neural ganglion, but by the series of lateral
ganglia. So also it is argued in the case of the trigeminal, that if in
addition to the ganglion-cells arising from the neural crest separate
ganglion-masses are found in the course of development, in connection with
proliferating patches of the surface (plakodes, Kupffer calls them), then
such isolated lateral ganglia are indications of separate segments, just as
in the case of the vagus, even though the separate segments do not show
themselves in the adult. So far the argument appears to me just, but the
further conclusion that the presence of such plakodes shows the previous
existence of _branchial_ sense-organs, and, therefore, that such ganglia
are _epibranchial_ ganglia, indicating the position of a lost gill-slit, is
not justified by the premises. If, as I suppose, the trigeminal nerve
supplied a series of non-branchial appendages serially homologous with the
branchial appendages supplied by the vagus, then it is highly probable that
the trigeminal should behave with respect to its sensory ganglia similarly
to the vagus nerve, without having anything to do with branchiæ.

Such plakodal ganglia, then, may give valuable indication of non-branchial
segments as well as of branchial segments. The researches of Kupffer on the
formation of the trigeminal ganglia in Ammocoetes are the chief attempt to
find out from the side of the sensory ganglia the number of segments
originally belonging to the trigeminal. The nature and result of these
researches is described in my previous paper (_Journal of Anatomy and
Physiology_, vol. xxxiv.), and it will suffice here to state that he
himself concludes that the trigeminal originally supplied five at least,
probably six, segments. As I have stated there, the evidence as given by
him seems to me to indicate even as many as seven segments.

In the full-grown Ammocoetes, as is well known, there are two distinct
ganglia belonging to the trigeminal, the one the ganglion of the _ramus
ophthalmicus_, the other the main ganglion.

According to Kupffer the larval Ammocoetes possesses three sets of ganglia,
not two, for between the foremost and hindmost ganglion {283}he describes a
nerve (_x._, Fig. 113), with four epibranchial ganglia, which do not
persist as separate ganglia, but either disappear or are absorbed into the
two main ganglia (Fig. 113). This discovery of Kupffer's is very
suggestive, for, as already stated, a transformation takes place when the
Ammocoetes is 5 mm. long, so that the arrangement of the parts before that
period is distinctly more indicative of the ancestral arrangement than any
later one.

If we use the name plakodal ganglia to represent that part of these ganglia
which was originally connected with the skin, then Kupffer's researches
assert that in the larval Ammocoetes there were seven such plakodal
ganglia, one in front belonging to the foremost trigeminal ganglion, two
behind, parts of the hindmost ganglion, and four in between, which do not
exist later as separate ganglia.

[Illustration: FIG. 113.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4
MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)

_A-B_, the line of epibranchial ganglia; _au._, 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.]

In accordance with the views put forward in this book, a possible
interpretation of these plakodal ganglia would be given as follows:--

Beard, who, after Froriep, drew attention to this relation of the cranial
ganglia to special skin-patches, has compared them with the parapodial
ganglia of annelids, _i.e_. ganglia in connection with annelidan
appendages; whether we are here obtaining a glimpse of the far-off
annelidan ancestry of both arthropods and vertebrates it would be premature
at present to say. It is natural enough to expect, on my view, to find
evidence of annelidan ancestry in {284}vertebrate embryology (as has been
so often asserted to be the case), seeing that undoubtedly the Arthropoda
are an advanced stage of Annelida; and, indeed, the way is not a long one
when we consider Beecher's evidence that the Trilobita belong to the
Phyllopoda, certainly a primitive crustacean group, which Bernard derives
directly from the annelid group Chætopoda. If, then, these plakodal ganglia
indicate the former presence of appendages, we obtain this result:--The
foremost ganglion on each side possesses one plakodal ganglion, and
therefore indicates an anterior pair of appendages, possibly the cheliceræ.
Then comes the peculiar nerve with four plakodal ganglia indicating on each
side four appendages close together, possibly the endognaths. Then,
finally, on each side, the second large ganglion with two plakodal ganglia,
indicating two pairs of appendages, possibly the ectognaths and the
metastoma.


SUMMARY.

  The consideration of the history of the cranial segmentation shows that
  whereas, from the commencement of that history, the evidence for two
  ventral segments supplied by the trigeminal nerve is clear and
  unmistakable, later observers have tended more and more to increase the
  number of these segments, until at the present time the evidence is in
  favour of at least six, probably seven, as the number of segments
  supplied by the motor part of the trigeminal.

  So, also, the original evidence for the number of dorsal or somatic
  segments limits the number to three, innervated respectively by the
  oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather
  two, since the last nerve belongs to the facial segment. The muscles
  which these three nerves supply are derived respectively from the walls
  of the premandibular, mandibular, and hyoid coelomic cavities.

  Later evidence points strongly to the conclusion that the oculomotor
  nerve and the premandibular cavity represent not one segment but the
  fusion of four, while the mandibular cavity represents two segments. In
  addition to these, Miss Platt has discovered a still more anterior
  head-cavity, which she has named the anterior cavity, so that the
  pro-otic segments on this reckoning are seven in number, viz.: (1) the
  anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the
  mandibular cavity. The somatic muscles belonging to these dorsal segments
  are the eye-muscles, which are all dorso-ventral in position, and are not
  the same as the longitudinal somatic muscles, but belong to a distinct
  dorso-ventral segmental group, the only representative of which at
  present known in the mesosomatic region is the external rectus innervated
  by the VIth nerve.

  These head-cavities, and these muscles of the vertebrate, resemble the
  corresponding cavities and muscles of the invertebrate to an
  extraordinary {285}degree, so that it becomes easy to see how the
  dorso-ventral muscles of the prosomatic segments of the latter have
  become converted into the eye-musculature of the former. The most
  powerful proof of all that such a conversion has taken place is that a
  natural and simple explanation is at once given of the extraordinary
  course taken by the IVth or trochlear nerve. Ever since neurology began,
  the course of this nerve has arrested the attention of anatomists. Why
  should just this one pair of nerve-roots of all those in the whole body
  be directed dorsalwards instead of ventralwards, and cross each other in
  the valve of Vieussens, each to supply a simple eye-muscle (the superior
  oblique) belonging to the other side? For generations anatomists have
  wondered and found no solution, and yet, without any straining of
  hypotheses, in consequence simply of the investigation of the anatomy of
  the corresponding pair of muscles in the scorpion group, the solution is
  immediately apparent.

  This pair of muscles alone, of all the musculature attached to the
  carapace, crosses the mid-dorsal line to be attached to the other side,
  thus carrying its nerve with it to the other side; by a continuation of
  the same process the relation of the trochlear to the superior oblique
  muscle can be explained.

  The comparison of the eye-muscles of the vertebrate with the
  dorso-ventral segmented muscles of the invertebrate makes the number and
  nature of the pro-otic segments much clearer.




{286}CHAPTER IX

_THE PROSOMATIC SEGMENTS OF AMMOCOETES_

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


In the last chapter it was seen not to be incompatible with both the
anatomical and morphological evidence to look upon the trigeminal nerves as
having originally supplied the seven prosomatic pairs of appendages of the
invertebrate ancestor, the foremost of which, the cheliceræ, and the four
pairs of endognaths dwindled away and became insignificant, leaving as
trace of their former presence the descending root of the Vth nerve; while
the two hindmost pairs, the ectognaths and the chilaria, or metastoma,
remained vigorous and developed, leaving as proof of their presence the
_nucleus masticatorius_. Evidence in favour of this suggestion and of the
nature of the dwindling process is afforded when we examine what the
trigeminus does supply in Ammocoetes. In all vertebrates this nerve
supplies the great muscles of mastication which, in all gnathostomatous
fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws;
they take in their food by attaching themselves to their prey and by means
of rasping teeth situated in serried rows within the circular mouth,
combined with a powerful suctorial apparatus, they suck the juices of the
fish they feed upon. Not possessing jaws, they feed by suction on the
living animal, a method of feeding which gives them no more claim to be
classed as parasitic animals than the whole group of spiders which feed in
a similar manner on living flies.


{287}THE ORIGIN OF THE SUCTORIAL APPARATUS OF PETROMYZON.

This powerful suctorial apparatus is innervated entirely by the trigeminal
nerve, so that here in its muscular arrangements any original segmental
arrangement of the muscles of mastication might be expected to be visible.
It consists of a large rod or piston, to which are attached powerful
longitudinal muscles; a large muscle, the basilar muscle, which assists the
piston in producing a vacuum, and annular muscles around the circular lip.

Turn now to the full-grown larval form, Ammocoetes, an animal in the case
of _Petromyzon Planeri_ as large as the full-grown Petromyzon, and seek for
this musculature. There is, apparently, no sign of it, no suctorial
apparatus whatever, only, as already mentioned, an oral chamber bounded by
the lower and upper lips and the remains of the septum between it and the
respiratory chamber--the velar folds. Attached to its walls a number of
tentacles are situated, which form a fringe around and within the mouth.
Most extraordinary is the contrast here between the larval and the adult
stages; in the former, no sign of the suctorial apparatus, but simply
tentacles and velar folds; in the latter, no sign of tentacles or of velar
folds, but a massive suctorial apparatus.

In order, then, to understand the origin of the muscles of mastication, it
is necessary to study the changes which occur at transformation, and thus
to find out how the suctorial apparatus of the adult arises. This most
important investigation has been undertaken by Miss Alcock, and owing to
the kindness of Mr. Millington, of Thetford, we have been able to obtain a
better series in the transformation process than has ever been obtained
before. Miss Alcock has not yet published her researches, but has allowed
me to make use of some of her facts.

An enormous proliferation of muscular tissue takes place with great
rapidity during this transformation, which causes the disappearance of the
tentacles, and gives origin to the suctorial apparatus. The starting point
of this proliferation can be traced back in all cases to little groups of
embryonic tissue found below the epithelial lining of the oral chamber in
Ammocoetes. Of these groups the most conspicuous one is situated at the
base of the large median ventral tentacles. Others are situated at the base
of the tentacular ridge. Further, although this extraordinary change takes
place in the {288}peripheral organ, no marked difference occurs in the
arrangement of the nerves issuing from the trigeminal motor centre, no new
nerves are formed to supply the new muscles, but every motor nerve-fibre
and the motor cell from which it arises increases enormously in size, and
these giant nerve-fibres thus formed split into innumerable filaments
corresponding with the proliferation of the muscular elements.

The clue, then, to the origin of the suctorial apparatus and of the nature
of the original organs supplied by the trigeminal is afforded in this case,
as in all other similar inquiries, by the central nervous system and its
outgoing nerves. Here is always the citadel, the fixed seat of government,
here is 'headquarters,' from which the answers to all our inquiries must
originate.

[Illustration: FIG. 114.--DISTRIBUTION OF TRIGEMINAL NERVE IN AMMOCOETES.

_ps. br._, pseudo-branchial groove; _met._, nerve to lower lip, or
metastomal nerve; _t._, nerve to tongue; _tent._, nerve to tentacles. The
mandibular and internal maxillary nerves are coloured red; the purely
sensory nerves to the external surface are coloured black.]


THE TRIGEMINAL NERVE OF AMMOCOETES.

Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution
of the trigeminal nerve as traced by her through a series of sections. It
arises, as is well known, from two separate ganglia, of which the foremost
gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the
hindmost to three nerves, the most posterior of which is purely cutaneous
and passes tailwards over the ventral branchial region, as shown in the
figure; the other two nerves, both {289}of which contain motor fibres, are
called by Hatschek the mandibular and maxillary nerves. Of these the
mandibular or velar nerve (_met._) is a large, conspicuous nerve, which
arises so separately from the rest of the trigeminal as almost to deserve
the title of a separate nerve. When it leaves the large posterior ganglion,
it passes into the anterior part of the velum, runs along with the tubular
muscles, which it supplies, to the ventral surface as far as the junction
of the lower lip with the thyroid plate, and has not been followed further
by Hatschek. Miss Alcock, however, by means of serial sections, has traced
it further, and shown that at this point it turns abruptly headwards to
terminate in the muscles of the lower lip. If, then, as suggested, the
lower lip represents the metastoma--the last pair of prosomatic
appendages--then this mandibular or velar nerve represents that segmental
nerve.

The other nerve--the maxillary nerve of Hatschek--which constitutes the
larger part of the trigeminal, passes forwards from the ganglion, and at a
point somewhere about the anterior region of the eyeball, divides into two,
an external (_black_ in Fig. 114) and an internal (_red_ in Fig. 114)
nerve. The external branch is apparently entirely sensory, and supplies the
external surfaces of the upper and lower lips. The internal branch is
mainly motor, and supplies the muscles of the upper lip; it contains also
the nerves of the tentacles.

The nerve to the median ventral tentacle (_t._) or tongue leaves the
internal division of the maxillary immediately after its separation from
the external; it runs ventralwards, and at the same time passes internally
until it reaches a position between the muco-cartilage and the epithelium
lining the cavity of the throat. It then turns, and passing posteriorly
(towards the tail) to the point where the median ventral tentacle is
attached to the lower lip, it supplies some very rudimentary-looking
muscles which run from the tentacle to the adjoining surface, and no doubt
serve to move the tentacle from side to side. A portion of the nerve still
continues to run along the side of the median ventral ridge, as far back as
the point where the muscles of the hyoid segment pass round to the ventral
side between the velum and the thyroid; in fact, this small nerve passes
along the whole length of the median ventral ridge.

This description shows that the trigeminal nerve divides itself into two
groups: the one represented black in the figure, which is purely cutaneous
and sensory, corresponding, in the main, according {290}to my theory, to
the epimeral nerves of Limulus; the other coloured red, which supplies
muscles belonging to the visceral or splanchnic muscle-group, and contains
also the nerves to the tentacles.

This latter group, which is formed by two distinct well-defined nerves,
viz. the mandibular and the internal branch of the maxillary, corresponds,
according to my theory, to the amalgamated nerves of the prosomatic
appendages, and is clearly divisible into three distinct nerves--

1. The lower lip-nerve or the metastomal nerve (_met._).

2. The tongue-nerve (_t._).

3. The nerve (_tent._) to the upper lip and tentacles.

Of these three pairs of nerves it is suggested that the first pair were
derived from the nerves to the metastomal appendage. The second pair of
nerves ought, on this theory, originally to have supplied the pair of
appendages immediately in front of the metastoma--that is, the pair of
ectognaths, and therefore the ventral pair of tentacles, known as the
tongue, would represent the last remnant of these ectognaths. Similarly,
the other tentacles would represent the endognaths, and therefore the third
pair of nerves would represent the fused nerves to these concentrated
endognaths, which, in the Eurypterids, stand aloof from the ectognaths.

Let us consider these three propositions separately. In the first place,
have we any right to attribute segmental value to the mandibular nerve?
What evidence is there of segments in this region in Ammocoetes?


THE SEGMENT OF THE LOWER LIP, OR METASTOMAL SEGMENT.

We have seen that in the branchial or mesosomatic region the segments
corresponding to the mesosomatic appendages were mapped out by means of
their supporting or skeletal structures, their segmental muscles, and their
nervous arrangements, as well as by the arrangement of the branchiæ.
Similarly, the segments in front of the branchial region, corresponding to
the prosomatic appendages, ought to be definable by the same means,
although, owing to the absence of branchiæ and the greater concentration in
this region, the separate segments would probably not be so conspicuous.

The last segment considered was the segment belonging to the VIIth nerve
corresponding to the opercular appendages of the {291}Eurypterid. The
segment immediately in front of this is the next for consideration, viz.
that corresponding to the chilarial appendages or metastoma; and as the
basal part of this pair of appendages was fused with the basal part of the
operculum, the one cannot be discussed without the other; therefore, the
segment to which the lower lip belongs must be considered in connection
with and not apart from the thyro-hyoid segments already dealt with.

In Chapter V., p. 188, I stated that the supporting bars of the foremost
mesosomatic segments, the thyro-hyoid segments, differed from the
cartilaginous bars of the branchial segments, in that they were composed of
muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a
ventral plate of muco-cartilage exists in Ammocoetes which covers over the
thyroid gland.

Similarly in the prosomatic segments the skeletal bars are composed of
muco-cartilage and the ventral plate of muco-cartilage continues forward as
the plate of the lower lip. It is of special interest, in connection with
the segments indicated by such supporting structures, to find that this
special tissue is entirely confined to the head-region, and disappears
absolutely at transformation, thus indicating the ancestral nature of the
segments marked out by its presence.

This muco-cartilaginous skeleton is the key to the whole position, and
requires, therefore, to be understood. It is of great importance, not only
because it demonstrates the position of the segments in Ammocoetes which
characterized its invertebrate ancestor, but also because it possesses a
structure remarkably similar to that found in the head-plates of the most
ancient fishes. For the present I will confine myself to the consideration
of this muco-cartilaginous skeleton as evidence of the relationship of
Ammocoetes to the Eurypterids, and in the next chapter will show how
absolutely the same skeleton corresponds to that of the Cephalaspidæ, so
that Ammocoetes is really a slightly modified Cephalaspid, the larval form
of which was Eurypterid in character.

{292}[Illustration: FIG. 115.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.

_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._,
median serrated flange of velar folds.] {293}[Illustration: FIG.
116.--HORIZONTAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES,
IMMEDIATELY VENTRALLY TO THE AUDITORY CAPSULE.

_sk_1_-_sk_5_, skeletal bars; _m_1_-_m_5_, striated visceral muscles;
_mt_1_-_mt_4_, tubular muscles; _br_1_-_br_3_, branchiæ; _tr._, trabeculæ;
_inf._, infundibulum; _ped._, pedicle; _V._, trigeminal nerve.
Muco-cartilage, _red_; soft cartilage, blue; hard cartilage, purple.]

{294}[Illustration: FIG. 117.--SAGITTAL LATERAL SECTION THROUGH THE
ANTERIOR PART OF AMMOCOETES.

Lettering and colouring same as in Fig. 116. _aud._, auditory capsule;
_j.v._, jugular vein.]

In Chapter IV., Figs. 63, 64, I have given a representation of the ventral
and dorsal views of an Ammocoetes cut in half horizontally. Such a section
shows with great clearness the series of branchial appendages with their
segmental muscles and cartilaginous bars which form the branchial segments
innervated by the IXth and Xth nerves, according to my view of the
branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid
or opercular appendage, which is clearly serially homologous with the other
branchial bars, is composed of muco-cartilage, and not of cartilage. If we
follow this series of horizontal sections nearer to the origin of the
cartilaginous bars from the sub-chordal cartilaginous rod on each side of
the notochord, we obtain a picture, as in Fig. 116, in which each branchial
segment is defined by the section of the branchial cartilaginous bar
(_sk_4_, _sk_5_), by the section of the separate branchiæ (_br_2_, _br_3_),
and by the separate segmental muscles arranged round each bar, these
muscles being partly ordinary striated (_m_4_, _m_5_), partly tubular
(_mt_3_, _mt_4_). The uppermost of these branchial segments shows the same
arrangement; (_sk_3_) is the branchial skeletal bar, which is now composed
of muco-cartilage, not cartilage; (_br_1_) is the branchiæ in the same
situation as the others, but here composed of glandular rather than of
respiratory epithelium, while the ordinary striated branchial muscles of
this segment are marked as (_m_3_), being separated from the tubular
muscles of the segment (_mt_2_), owing to the large size of the blood-space
in which these latter muscles are lying. In front of this segment so
defined we see again another well-marked skeletal bar (_sk_2_) of
muco-cartilage, evidently indicating a similar segment anterior to the
hyoid segment. In connection with this bar there are no branchiæ, but again
we see two sets of visceral muscles, the one ordinary striated, marked
(_m_2_), and the other tubular, marked (_mt_1_). Here, then, the section
indicates the existence of a segment of the same character as the
posteriorly situated branchial segments but belonging to a non-branchial
region--a segment which would represent a non-branchial appendage, the
last, therefore, of the prosomatic appendages. Let us, then, follow
{295}out these two segmental muco-cartilaginous bars and their attendant
muscles, and see to what sort of segments their investigation leads.

The bar which comes first for consideration (_sk_3_) arises immediately
behind the auditory capsule from the first branchial cartilage very soon
after it leaves the sub-chordal cartilaginous ligament; the soft cartilage
of the sub-chordal ligament ceases abruptly in its extension along the
notochord at the place where the hard cartilage of the parachordal joins
it, and in a sense it may be said to leave the notochord at this place and
pass into the basal part of the first branchial bar. The most anterior
continuation of this branchial system is this muco-cartilaginous bar
(_sk_3_), which passes forward and ventralwards, being separated from the
axial line by the auditory capsule (_cf._ Fig. 118, A, B, C). Its position
is well seen in a sagittal section, such as Fig. 117. It follows absolutely
the line of the pseudo-branchial groove (_ps. br._, Fig. 114), and
ventrally joins the plate of muco-cartilage which covers the thyroid gland.
It forms a thickened border to this plate anteriorly, just as the branchial
cartilaginous bars border it posteriorly. In fact, it behaves with respect
to the hyoid segment in a manner similar to the rest of the cartilaginous
bars with respect to their respective segments.

It represents, although composed of muco-cartilage, the cartilaginous bar
of the operculum in Limulus, which also forms the termination of the
branchial cartilaginous system, as fully explained in Chapter III.; it may
therefore be called the opercular bar.

The next bar (_sk_2_) is extremely interesting, as we are now out of the
branchial or mesosomatic region, and into the region corresponding to the
prosoma. It starts from a cartilaginous projection made of hard cartilage,
just in front of the auditory capsule, called by Parker the 'pedicle of the
pterygoid'--a projection (_ped._) which defines the posterior limit of the
trabeculæ on each side, where they join on to the parachordals,--and
winding round and below the auditory capsule, joins the opercular bar
(_cf._ Fig. 118), to pass thence into and form part of the
muco-cartilaginous plate of the lower lip. In the section figured (Fig.
116), this projection of hard cartilage is not directly continuous with
(_sk_2_), owing to a slight curvature in the bar; the next few sections
show clearly the connection between (_ped._) and (_sk_2_), and consequently
the complete separation by means of this bar of the hyoid segment from the
segment in front.

{296}[Illustration: FIG. 118.--SKELETON OF HEAD-REGION OF AMMOCOETES. A,
LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.

Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_.
_sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na.
cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._,
notochord.]

{297}In the figures, the hard cartilage is coloured purple, the soft
cartilage blue, and the muco-cartilage red, so that the position of this
bar is well shown. This bar may be looked upon as bearing the same relation
to the muco-cartilaginous plate of the lower lip as the opercular bar does
to the muco-cartilaginous plate over the thyroid; and seeing that these two
plates form one continuous ventral head-shield of muco-cartilage (Fig. 118,
B), and also that this bar fuses with the opercular bar, we may conclude
that the segment represented by the lower lip is closely connected with the
hyoid or opercular segments. In other words, if the lower lip arose from
the metastoma, then this pair of skeletal bars might be called the
metastomal bars, which formed the supporting skeleton of the last pair of
prosomatic appendages and, as is likely enough, arose in connection with
the posterior lateral horns of the plastron; these posterior lateral horns,
like the rest of the plastron, would give rise to hard cartilage, and so
form in Ammocoetes the two lateral so-called pterygoid projections.

In the branchial region the muscles which marked out each branchial segment
were of two kinds--ordinary striated visceral muscles and tubular muscles.
Of these the former represented the dorso-ventral muscles of the branchial
appendages, while the latter formed a separate group of dorso-ventral
muscles with a separate innervation which may have been originally the
segmental veno-pericardial muscles so characteristic of Limulus and the
scorpions. In Figs. 116, 117, the grouping of these muscles in each
branchial segment is well shown, and it is immediately seen that the hyoid
segment possesses its group of striated visceral muscles (_m_3_) supplied
by the VIIth nerve in the same manner as the posterior groups, as has
already been pointed out by Miss Alcock in her previous paper. Passing to
the segment in front, Fig. 116 shows that the group of visceral muscles
(_m_2_) corresponds in relative position with respect to the metastomal bar
to the hyoid muscles with respect to the opercular bar or to the branchial
visceral muscles with respect to each branchial bar. What, then, is this
muscular group? The series of sections show that these are the
dorso-ventral muscles belonging to the lower lip, which, as seen in Fig.
119 (_M._), form a well-marked muscular sheet, whose fibres interlace
across the mid-ventral line of the lower lip. This group of lower
lip-muscles is very suggestive, for these muscles arise, not from the
trabeculæ, but from the front dorsal region of the cranium, just in front
of the two lateral {298}eyes. In Fig. 117 the dorsal part is seen cut
across on its way to its dorsal attachment. Such an origin is reminiscent
of the tergo-coxal group of muscles, arising, as they do, from the
primordial cranium and the tergal carapace, and suggests at once that when
the chilarial appendages expanded to form a metastoma, their tergo-coxal
muscles formed a sheet of muscles similar to those of the lower lip of
Ammocoetes, by which the movements of the metastoma were effected. The
posterior limit of these muscles ventrally marks out the junction of the
segment of the lower lip with that of the thyroid; in other words,
indicates where the metastoma had fused ventrally with the operculum (Fig.
117).

[Illustration: FIG. 119.--VENTRAL VIEW OF HEAD-REGION OF AMMOCOETES.

_Th._, thyroid gland; _M._, lower lip, with its muscles.]

Besides the striated visceral muscles, each branchial segment possesses its
own tubular muscles, shown in Fig. 116 (_mt_3_) and (_mt_4_). As the
section shows, there is clearly a group of tubular muscle-fibres belonging
to the hyoid segment (_mt_2_), and also another group belonging to the
segment in front of the hyoid (_mt_1_); so that, judging from this section,
each of these segments possesses its own tubular musculature just as do the
branchial segments, the difference being that the tubular muscles are more
separated from the striated visceral group than in the true branchial
segments, owing to the size of the blood-spaces surrounding them. What,
then, are these two groups of muscles? Tracing them in the series of
sections, both groups are seen to belong to the system of velar muscles,
forming an anterior and a posterior group respectively; and we see,
further, that there is not the slightest trace of any tubular muscles
anterior to these muscles of the velum.

In the living Ammocoetes the velar folds on each side can be seen {299}to
move synchronously with the movements of respiration, contracting at each
expiration, and thus closing the slit by which the oral and respiratory
chambers communicate, and so forcing the waters of respiration through the
gill-slits, as described by Schneider. Such a fact is clear evidence that
these tubular muscles of the velar folds belong to the same series as the
tubular muscles of the branchial segments, so that if, as I have already
suggested, the latter muscles were originally the veno-pericardial muscles
of segments corresponding to the branchial appendages, then the former
would represent the veno-pericardial muscles of the segments corresponding
to the opercular and metastomal appendages. What, then, are these velar
folds, and how is it that the tubular muscles of these two segments become
the velar muscles? I will consider, in the first instance, the posterior
group of muscles (_mt_2_) in Fig. 116.

It has already been pointed out that the tubular muscles of the branchial
segments are dorso-ventral, but do not run with the ordinary constrictors,
having separate attachments and running part of their course internally to
and part externally to the ordinary constrictors. At first sight, as is
usually stated, the hyoid segment does not appear to possess tubular
muscles at all. If, however, we follow the posterior group of velar muscles
(_mt_2_), we see (Fig. 117) that they pass between the auditory capsule and
the opercular bar (_sk_3_) of muco-cartilage to reach the region of the
jugular vein (_j.v._) posteriorly to the auditory capsule, so that their
dorsal origin bears the same relation to the hyoid segment as the dorsal
attachment of the rest of the tubular muscles to their respective segments.
Further, these muscles run along the length of the velar fold, and are
attached ventrally on each side of the thyroid gland, so that their ventral
attachment also corresponds in position, as regards the hyoid segment, with
the ventral attachment of the rest of the tubular muscles as regards their
respective segments.

This ventral attachment is shown in Fig. 119 on each side of the thyroid,
and in Fig. 120 (_mt_2_); while in Fig. 117 the fibres are seen converging
to this ventral position. In other words, this large posterior muscle of
the velar folds is a dorso-ventral muscle, and would actually take the same
position in the hyoid segment as the dorso-ventral tubular muscles in the
other branchial segments, if the velum were put back into its original
position as the septum terminating the branchial chamber. Conversely, the
presence of these {300}hyoid tubular muscles in the velum gives evidence
that the opercular segment takes part in the formation of the septum, as
already suggested.

Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid
segment, which are attached to the muco-cartilage. Schaffer also speaks of
certain tubular muscles belonging to the velar group as piercing the
muco-cartilage (_h. r. s._) in his figures 24 and 25, _i.e._ the metastomal
bar, near its junction with the opercular bar. In my specimens there is a
distinct group of tubular muscles which pierce the opercular bar of
muco-cartilage at its junction with the metastomal bar, and pass into the
posterior group of velar muscles. They clearly belong to the hyoid segment,
as Miss Alcock supposed, but are not attached to the muco-cartilage. It is
possible that they represent a different group to those already considered,
and suggest the possibility that this opercular or thyro-hyoid segment is
double with respect to its original veno-pericardial muscles as well as in
other respects.

The anterior group of tubular muscles (_mt_1_, Figs. 116, 117) belonging to
the same segment as the metastomal bar must now be taken into
consideration. Very different is their origin to that of the posterior
group: they arise close up against the eye, and have given rise to
Kupffer's and Hatschek's misconception that the superior oblique muscle of
the eye arises from a part of the velar musculature. Naturally, as Neal has
pointed out, they have nothing to do with the eye-muscles; the superior
oblique muscle is plainly in its true place entirely apart from these velar
muscles, which form the foremost group of the segmental tubular muscles.
They pass into the anterior part of the velar folds and run round to the
ventral side just in the same way as does the posterior group. This
anterior group of tubular muscles represents the veno-pericardial muscle of
the segment immediately in front of the opercular, _i.e._ the metastomal
segment, and is the foremost of these veno-pericardial muscles. Its
presence shows that the velar folds, formed as they were by the breaking
down of the septum, are in reality part of two segments, viz. the opercular
and the metastomal, which have fused together in their basal parts, and by
such fusion have caused the inter-relationship between the VIIth and Vth
nerves, so apparent in the anatomy of the vertebrate cranial nerves.

A further piece of evidence that this anterior portion of the velum
{301}belongs to the same segment as the lower lip is the fact that in
addition to the tubular muscles a single ordinary striated muscle is found
in the velum which, like the muscles of the lower lip, is innervated by
this same mandibular nerve.

This muscle is attached laterally to the muco-cartilage of the metastomal
bar (_sk_2_) at its junction with the muco-cartilage of the lower lip, and
spreads out into a number of strands which are attached at intervals along
the whole length of the free anterior edge of the velum. It is the only
non-tubular muscle belonging to the velum, and by its contraction it draws
the anterior portions of the velar folds apart from each other, and so
opens the slit between them, through which the food and mud must pass.
Clearly from its position it does not belong to the original tergo-coxal
group of muscles as do those of the lower lip; it must have been one of the
intrinsic muscles of the metastoma itself.

This anterior portion of the velar folds affords yet another striking hint
of the correctness of my comparison of the lower lip segment of Ammocoetes
with the chilaria of Limulus or the metastoma of Eurypterus; for the most
dorsal anterior portion, which at its attachment possesses a wedge of
muco-cartilage, forms a separate, well-defined, rounded basal projection
marked _Ser._ in Fig. 115, and _B_ in the accompanying Fig. 120. This is
that part of the velar folds which comes together in the middle line and
closes the entrance into the respiratory chamber. The epithelial surface
here is most striking and suggestive, for it is markedly serrated, being
covered with a large number of closely-set projections or serræ. The
serration of the surface here is of so marked a character that Langerhans
considered this part of the velar folds to act as a masticating organ,
grinding and rasping the food and mud which passed through the narrow slit.
In fact, Langerhans supposed that this portion of the velum acted in a
manner closely resembling the action of the gnatho-bases of the prosomatic
appendages in Limulus or the Eurypteridæ.

This suggestion of Langerhans is surely most significant, considering that
this somewhat separate portion of the velum, to which he assigns such a
function, is in the very place where the gnathite portion of the metastomal
appendages would have been situated if it were true that the lower lip and
anterior portion of the velum of Ammocoetes were derived from the
metastoma.

In addition to this marked serrated edge the whole surface of {302}the
anterior portion of the velum is covered over with a scale-like or
tubercular pattern remarkably like the surface-ornamentation seen in many
of the members of the ancient group Eurypteridæ. In Fig. 121 I give a
picture of this surface-marking of the velum. It is striking to see that
just as in the case of the invertebrate this marking and these serræ are
formed simply by the cuticular surface of the epithelial cells; a surface
which, according to Wolff, possibly contains chitin. The interpretation
which I would give of the velar folds is therefore as follows:--

They represent the fused basal parts of the opercular and metastomal
appendages, the gnatho-bases of the latter still retaining in a reduced
degree their rasping surfaces, because, owing to their position on each
side of the opening into the respiratory chamber they were still able to
manipulate the food as it passed by them after the closure of the old
mouth.

[Illustration: FIG. 120.--AMMOCOETES CUT OPEN IN MID-VENTRAL LINE TO SHOW
POSITION OF VELUM; VELAR FOLDS REMOVED ON ONE SIDE.

_tr._, trabeculæ; vel., velum; _B._, anterior gnathic portion of velum;
_ps. br._, pseudo-branchial groove; _m_2_, muscles of lower lip segment;
_m_3_, muscles of thyro-hyoid segment; _mt_2_, insertion of tubular muscles
of velum near thyroid.]

[Illustration: FIG. 121.--SURFACE VIEW OF ANTERIOR SURFACE OF VELUM.]

The whole evidence points irresistibly to the conclusion that the
mandibular or velar nerve of the trigeminal does supply a splanchnic
{303}segment which is, in all respects, comparable with the segments
supplied by the facial, glossopharyngeal, and vagus nerves, except that it
does not possess branchiæ. This simply means that the appendages which
these nerves originally supplied were prosomatic, not mesosomatic, and
corresponded, therefore, to the chilarial or metastomal appendages.

A comparison of the ventral surface of Slimonia, as given in Fig. 8, p. 27,
with that of Ammocoetes (Fig. 119), when the thyroid gland and lower lip
muscles have been exposed to view, enables the reader to recognize at a
glance the correctness of this conclusion.


THE TENTACULAR SEGMENTS AND THE UPPER LIP.

Anterior to this metastomal segment, Fig. 116 shows a group of visceral
muscles, _m_1_, and yet again a muco-cartilaginous bar, _sk_1_, but, as
already stated, no tubular muscles. These visceral muscles indicate the
presence in front of the lower lip-segment of one or more segments of the
nature of appendages. The muscles in question (_m_1_) are the muscles of
the upper lip, the skeletal elements form a pair of large bars of
muco-cartilage (_sk_1_), which start from the termination of the trabeculæ,
and pass ventralwards to fuse with the muco-cartilaginous plate of the
lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on
each side, and gives small projections of muco-cartilage into each
tentacle. In addition to this tentacular bar, a special bar of
muco-cartilage exists for the fused pair of median tentacles, the so-called
tongue, which extends in the middle line along the whole length of the
lower lip, being separated from the muco-cartilaginous plate of the lower
lip by the muscles of the lower lip. This tongue bar of muco-cartilage
joins with the muco-cartilage of the lower lip at its junction with the
thyroid plate, and also with the tentacular bar just before the latter
joins the muco-cartilaginous plate of the lower lip. This arrangement of
the skeletal tissue suggests that the pair of tentacles known as the tongue
stand in a category apart from the rest of the tentacles; a suggestion
which is strongly confirmed by the separate character of its nerve-supply,
as already mentioned.

For three reasons, viz. the separateness both of their nerve-supply and of
their skeletal tissue, and the importance they assume at transformation,
this pair of ventral tentacles must, it seems to me, be put {304}into a
separate category from the rest of the tentacles. On the other hand, the
innervation of the rest of the tentacles by a single nerve which sends off
a branch as it passes each one, together with the concentration of their
skeletal elements into a single bar, with projections into each tentacle,
points directly to the conclusion that these tentacles must be considered
as a group, and not singly.

I suggest that these tentacles are the remains of the ectognaths and
endognaths; the tongue representing the two ectognaths, and the four
tentacles on each side the four pairs of endognaths.

As we see, this method of interpretation attributes segmental value to the
tentacles, a conclusion which is opposed to the general opinion of
morphologists, who regard them as having no special morphological
importance, and certainly no segmental value. On the other hand, the
importance of the pair of ventral tentacles, the 'tongue' of Rathke, which
lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor,
and others, all of whom are unanimous in asserting that at transformation
they are converted into that large and important organ the piston or tongue
of the adult Petromyzon. It is supposed that the rest of the tentacles
vanish at transformation, being absorbed; they appear to me rather to take
part in the formation of the sucking-disc, so that I am strongly inclined
to believe that the whole of the remarkable suctorial apparatus of
Petromyzon is derived from the tentacles of Ammocoetes. In other words, on
my view, a conversion of the prosomatic appendages into a suctorial
apparatus takes place at transformation, just as is frequently the case
among the Arthropoda.

It is to the arrangement of the muscles that we look for evidence of
segmental value. As long as it was possible to look upon these tentacles as
mere sensory feelers round the mouth entrance, it was natural to deny
segmental value to them. Matters are now, however, totally different since
Miss Alcock's discovery of the rudimentary muscles at the base of the
tentacles and their development at transformation. If these muscles
represent some of the appendage muscles belonging to the foremost
prosomatic segments just as the ocular muscles represent the dorso-ventral
somatic muscles of those same segments, then we may expect ultimately to be
able to give as good evidence of segmentation in their case as I have been
able to give in the case of these latter muscles; for the two sets of
muscles are curiously alike, seeing that the eye-muscles do not develop
until {305}transformation, but throughout the Ammocoetes stage remain in
almost as rudimentary a condition as the tentacular muscles.

Another difficulty with respect to the tentacles is the determination of
the number of them, owing to the fact that in addition to what may be
called well-defined tentacles a large number of smaller tactile projections
are found on the surface of the upper lip, as is seen in Fig. 115. In the
very young condition, 7 or 8 mm. in length, it is easier to make sure on
this point. At this stage they may be spoken of as arranged in two groups:
an anterior small group and a posterior larger group. The anterior group
consists of a pair of very small tentacles and a very small median
tentacle, all three situated quite dorsally in the front part of the upper
lip. The posterior group, which is separate from the anterior, consists of
five pairs of much larger tentacles, the most ventral pair in the mid-line
ventrally on the lower lip being fused together to form the large ventral
median tentacle or tongue already mentioned. This pair, according to
Shipley, is markedly larger than the others. There are, therefore, five
conspicuous tentacles on each side, and in front of them a smaller pair and
a small median dorsal one. In the very young condition the accessory
projections above-mentioned are not present, or at all events are not
conspicuous, and the tentacles are also markedly larger in comparison to
the size of the animal than in the older condition, where they have
distinctly dwindled.

This posterior group of five conspicuous tentacles is the one which I
suggest represents the four endognaths and one ectognath. What the
significance of the small anterior group is, I know not. It is possible
that the cheliceræ are represented here, for they are situated distinctly
anterior to the other group; I know, however, of no sign of a markedly
separate innervation to these most dorsal tentacles such as I should have
expected to find if they represented the cheliceræ.

The muscles of the upper lip, which distinctly belong to the visceral and
not to the somatic musculature, form part of the foremost segments, and in
these muscles the tentacular nerve reaches its final destination. From
their innervation, then, they must have belonged to the same appendages as
the tentacles supplied by the tentacular nerve, _i.e._ to the endognaths.
What conclusion can we form as to the probable origin of the upper lip of
Ammocoetes? Since the oral chamber was formed by the forward growth of the
metastoma, _i.e._ the lower lip of Ammocoetes, it follows that the upper
{306}lip is the continuation forwards of the original ventral surface of
such an animal as Limulus or a member of the scorpion group, where there is
no metastoma, and corresponds to the endostoma, as Holm calls it, of
Eurypterus. This termination of the ventral surface in all these animals is
made up of two parts: (1) Of sternites composing the true median ventral
surface of the body, called by Lankester the pro- and meso-sternites; and
(2) of the sterno-coxal processes of the foremost prosomatic appendages,
called in the case of Limulus gnathites, because they are the main agents
in triturating the food previously to its passage into the mouth. In
Limulus, a conjoined pro-mesosternite forms the median ventral wall to
which the sterno-coxal processes are attached on each side, and in Phrynus
and Mygale a well-marked pro-sternite and meso-sternite are present,
forming the posterior limit of the olfactory opening. In Buthus and the
true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th
prosomatic appendages take part in surrounding the olfactory tubular
passage; in Thelyphonus only the processes of the 2nd pair of prosomatic
appendages play such a part, the pro-sternite not being present (_cf._ Fig.
97).

Seeing, then, what a large share the sterno-coxal processes of one or more
of these prosomatic appendages plays in the formation of this endostoma,
and seeing also that the nerve which supplies the upper lip-muscles in
Ammocoetes is the same as that supplying the tentacles which are attached
to the upper lip, it appears to me more probable than not that the muscles
in question are the vestiges of the sterno-coxal muscles. These muscles
differ markedly in their attachments from the muscles of the lower lip, for
whereas the latter resemble the tergo-coxal group in their extreme dorsal
attachment, the former resemble the sterno-coxal group in their attachment
to what corresponds to the endostoma.

This interpretation of the meaning of the transformation process is in
accordance with all the previous evidence both from the side of the
palæostracan as from the side of the vertebrate, for it signifies that a
dwindling process has taken place in the foremost of the original
prosomatic appendages--the cheliceræ and the endognaths; while, on the
contrary, the ectognath and the metastoma have continued to increase in
importance right into the vertebrate stage. This process is simply a
continuation of what was already going on in the invertebrate stage, for
whereas in Eurypterus and other cases {307}the cheliceræ and endognaths had
dwindled down to mere tentacles, the ectognath was the large swimming
appendage, and the metastoma was on the upward grade from the two
insignificant chilaria of Limulus.

The transformation of these foremost appendages into a suctorial apparatus
is very common among the arthropods, as is seen in the transformation of
the caterpillar into the butterfly, and it is in accordance with the
evidence that the main mass of that suctorial apparatus should be formed
from appendages corresponding to the ectognath and metastoma rather than
from the four endognaths. In all probability the _nucleus masticatorius_ of
the trigeminal nerve with its innervation of the great muscles of
mastication is evidence of the continued development of the musculature of
these two last prosomatic appendages, just as the descending root of the
Vth demonstrates the further disappearance of all that belongs to the
foremost prosomatic appendages. As yet, however, as far as I know, the
musculature of the head-region of Petromyzon has not been brought into line
with that of other vertebrates, and until that comparative study has been
completed it is premature to discuss the exact position of the masticating
muscles of the higher vertebrates.

The analysis of these tentacular segments belonging to the trigeminal nerve
presents greater difficulties than that of any of the other cranial
segments, owing to the deficiency of our knowledge of what occurs at
transformation. Light is required not only on the origin of the new muscles
but also on the origin of the new and elaborate cartilages which are newly
formed at this time.

Miss Alcock has not yet worked out the origin of all these cartilages and
muscles, so that we are not yet in a position to analyze the trigeminal
supply in Petromyzon into its component appendage elements, an analysis
which ought ultimately to enable us to determine from which
appendage-muscles the masticating muscles in the higher vertebrates have
arisen. As far as the muscles are concerned, she gives me the following
information:--

The tongue-nerve supplies in Ammocoetes the rudimentary muscles which pass
laterally from the base of the large ventral tentacle to the wall of the
throat, and even in Ammocoetes must possess some power of moving that
tentacle.

At transformation these muscles proliferate and develop enormously, and
form the bulk of the large basilar muscle which {308}surrounds the throat
ventrally and laterally, and is the most bulky muscle in the suctorial
apparatus.

The velar or mandibular nerve supplies in Ammocoetes the muscles of the
lower lip. In Petromyzon it supplies also the longitudinal muscles of the
tongue. The tongue-cartilage first develops in the region of the median
ventral tentacle, and there the longitudinal tongue-muscles first begin to
develop, not from the rudimentary muscles in the tongue but from those in
the lower lip region.

In Ammocoetes the tentacular nerve supplies the rudimentary muscles in the
tentacles and the muscles of the upper lip. The latter disappear entirely
at transformation, and in Petromyzon the tentacular nerve supplies the
circular, pharyngeal, and annular muscles, which are derived from the
rudimentary tentacular muscles.

For the convenience of my reader I append here a table showing my
conception of the manner in which the endognathal and ectognathal segments
of the Palæostracan are represented in Ammocoetes. It shows well the
uniform manner in which all the individual segmental factors have been
fused together to represent the appearance of a single segment (van Wijhe's
first segment) in the case of the four endognathal segments, but have
retained their individuality in the case of the ectognathal segment.

  +----------+----------+---------------------------+----------+----------+
  |          |          |      Appendages.          |          |          |
  |V. Wijhe's|Eurypterid+-------------+-------------+ Appendage| Skeletal |
  |segments. |segments. | Eurypterid. | Ammocoetes. |  nerves. | elements.|
  |          |          |             |             |          |          |
  +----------+----------+-------------+-------------+----------+----------+
  |          |          |             |             |          |          |
  |          |     2}   |             |             |    1     |    1     |
  |     1    |     3}   |      4      |      4      |Tentacular|Tentacular|
  |          |     4}   |  Endognaths |  Tentacles  |   to 4   | bar to 4 |
  |          |     5}   |             |             | tentacles| tentacles|
  |          |          |             |             |          |          |
  +----------+----------+-------------+-------------+----------+----------+
  |          |          |             |             |          |          |
  |     2    |     6    |      1      |      1      | 1 Tongue | 1 Tongue |
  |          |          |  Ectognath  |   Tongue    |   nerve  |    bar   |
  |          |          |             |             |          |          |
  +----------+----------+-------------+-------------+----------+----------+

  +----------+----------+---------+---------+----------+---------+
  |          |          |         | Dorso-  |          |         |
  |V. Wijhe's|Eurypterid| Somatic | ventral |Coelomic|  Coxal  |
  |segments. |segments. |  motor  |segmental| cavities.| glands. |
  |          |          | nerves. | muscles.|          |         |
  +----------+----------+---------+---------+----------+---------+
  |          |          |         |         |          |    1    |
  |          |     2}   | 1 Oculo-|  Sup.   |  1 Pre-  |Pituitary|
  |     1    |     3}   |  motor  |inf. int.|mandibular|  body;  |
  |          |     4}   |supplying| rectus  |  fusion  |fusion of|
  |          |     5}   |    4    | and inf.|   of 4   | 4 coxal |
  |          |          | muscles | oblique |          |  glands |
  +----------+----------+---------+---------+----------+---------+
  |          |          | 1 Troch-|         |          |         |
  |     2    |     6    |  learis |   Sup.  |    1     |         |
  |          |          |supplying| oblique |Mandibular|         |
  |          |          | 1 muscle|         |          |         |
  +----------+----------+---------+---------+----------+---------+


{309}THE TUBULAR MUSCLES.

The only musculature innervated by the trigeminal nerve which remains for
further discussion, consists of those peculiar muscles found in the velum,
known by the name of striated tubular muscles. This group of muscles has
already been referred to in Chapter IV., dealing with respiration and the
origin of the heart.

It is a muscular group of extraordinary interest in seeking an answer to
the question of vertebrate ancestry, for, like the thyroid gland, it bears
all the characteristics of a survival from a prevertebrate form, which is
especially well marked in Ammocoetes. I have already suggested in this
chapter that the homologues of these muscles are represented in Limulus by
the veno-pericardial group of muscles. I will now proceed to deal with the
evidence for this suggestion.

The structure of the muscle-fibres is peculiar and very characteristic, so
that wherever they occur they are easily recognized. Each fibre consists of
a core of granular protoplasm, in the centre of which the nuclei are
arranged in a single row. This core is surrounded by a margin of striated
fibrillæ, as is seen in Fig. 122. Such a structure is characteristic of
various forms of striated muscle found in various invertebrates, such as
the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the
vertebrate kingdom, except in Ammocoetes. At transformation these muscles
entirely disappear, becoming fattily degenerated and then absorbed.

[Illustration: FIG. 122.--A TUBULAR MUSCLE-FIBRE OF AMMOCOETES.

A, portion of fibre seen longitudinally; B, transverse section of fibre
(osmic preparation); the black dots are fat-globules.]

For all these reasons they bear the stamp of a survival from a
prevertebrate form. This alone would not make this tissue of any great
importance, but when in addition these muscles are found to be arranged
absolutely segmentally throughout the whole of the branchial region, then
this tissue becomes a clue of the highest importance.

As mentioned in Chapter IV., the segmental muscles of respiration consist
of the adductor muscle and the two constrictor muscles--the {310}striated
constrictor and the tubular constrictor. Of these muscles, both the muscles
possessing ordinary striation are attached to the branchial cartilaginous
skeleton, whereas the tubular constrictors have nothing to do with the
cartilaginous basket-work, but are attached ventrally in the neighbourhood
of the ventral aorta.

These segmental tubular muscles are found also in the velar folds--the
remains of the septum or velum which originally separated the oral from the
respiratory chamber. In the branchial region they act with the other
constrictors as expiratory muscles, forcing the water out of the
respiratory chamber. In the living Ammocoetes, the velar folds on each side
can be seen to move synchronously with the movements of respiration,
contracting at each expiration; they thus close the slit by which the oral
and respiratory chambers communicate, and therefore, in conjunction with
the respiratory muscles, force the water of respiration to flow out through
the gill-slits, as described by Schneider.

These tubular muscles thus form a dorso-ventral system of muscles
essentially connected with respiration; they belong to each one of the
respiratory segments, and are also found in the velum; anterior to this
limit they are not to be found. What, then, are these tubular muscles in
the velar folds? Miss Alcock has worked out their topography by means of
serial sections, and, as already fully explained, has shown that they form
exactly similar dorso-ventral groups, which belong to the two segments
anterior to the purely branchial segments, _i.e._ to the facial or hyoid
segments and the lower lip-segment of the trigeminal nerve. If the velar
folds could be put back into their original position as a septum, then the
hyoid or facial group of tubular muscles would take up exactly the same
position as those belonging to each branchial segment.

The presence of these two so clearly segmental groups of muscles in the
velum--the one belonging to the region of the trigeminal, the other to the
region of the facial--is strong confirmation of my contention that this
septum between the oral and respiratory chambers was caused by the fusion
of the last prosomatic and the first mesosomatic appendages, represented in
Limulus by the chilaria and the operculum.

Yet another clue to the meaning of these muscles is to be found in their
innervation, which is very extraordinary and unexpected. Throughout the
branchial region the striated muscles of each segment {311}are strictly
supplied by the nerve of that segment, and, as already described, each
segment is as carefully mapped out in its innervation as it is in any
arthropod appendage. One exception occurs to this orderly, symmetrical
arrangement: a nerve arises in connection with the facial nerve, and passes
tailwards throughout the whole of the branchial region, giving off a branch
to each segment as it passes. This nerve (_Br. prof._, Fig. 123) is known
by the name of the _ramus branchialis profundus_ of the facial, and its
extraordinary course has always aroused great curiosity in the minds of
vertebrate anatomists. Miss Alcock, by the laborious method of following
its course throughout a complete series of sections, finds that each of the
segmental branches which is given off, passes into the tubular muscles of
that segment (Fig. 124). The tubular muscles which belong to the velum,
_i.e._ those belonging to the lower lip-segment and to the hyoid segments,
receive their innervation from the velar or mandibular nerve, and belong,
therefore, to the trigeminal, not to the facial, system.

[Illustration: FIG. 123.--DIAGRAM SHOWING THE DISTRIBUTION OF THE FACIAL
NERVE.

Motor branches, _red_; sensory branches, blue.]

The evidence presented by these muscles is as follows:--

In the ancestor of the vertebrate there must have existed a segmentally
arranged set of dorso-ventral muscles of peculiar structure, concerned with
respiration, and confined to the mesosomatic segments and to the last
prosomatic segment, yet differing from the other dorso-ventral muscles of
respiration in their innervation and their attachment.

Interpreting these facts with the aid of my theory of the origin of
vertebrates, and remembering that the homologue of the vertebrate ventral
aorta in such a palæostracan as Limulus is the longitudinal {312}venous
sinus, while the opercular and chilarial segments are respectively the
foremost mesosomatic and the last prosomatic segments; they signify that
the palæostracan ancestor must have possessed a separate set of segmental
dorso-ventral muscles confined to the branchial, opercular and chilarial or
metastomal segments, which, on the one hand, were respiratory in function,
and on the other were attached to the longitudinal venous sinus. Further,
these muscles must all have received a nerve-supply from the neuromeres
belonging to the chilarial and opercular segments, an unsymmetrical
arrangement of nerves, on the face of it, very unlikely to occur in an
arthropod.

[Illustration: FIG. 124.--DIAGRAM 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._, epibranchial ganglia of
vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial;
_J.v._, jugular vein; _Ep. pit._, epithelial pit.]

{313}Is this prophecy borne out by the examination of Limulus? In the first
place, these muscles were dorso-ventral and segmental, and, referring back
to Chapter VII., Lankester arranges the segmental dorso-ventral muscles in
three groups: (1) The dorso-ventral somatic muscles; (2) the dorso-ventral
appendage muscles; and (3) the veno-pericardial muscles. Of these the first
group is represented in the vertebrate by the muscles which move the eye,
the second group by the striated constrictor and adductor muscles and the
muscles for the lower lip. There is, then, the possibility of the third
group for this system of tubular muscles.

Looking first at the structure of these muscles as previously described, so
different are they in appearance from the ordinary muscles of Limulus, that
Milne-Edwards, as already stated, called them "brides transparentes," and
did not recognize their muscular character, while Blanchard called them in
the scorpion, "ligaments contractils."

Consider their attachment and their function. They are attached to the
longitudinal sinus, according to Lankester's observation, in such a way
that the muscle-fibres form a hollow cone filled with blood; when they
contract they force this blood towards the gills, and thus act as accessory
or branchial hearts. According to Blanchard, in the scorpion they contract
synchronously with the heart; according to Carlson, in Limulus they
contract with the respiratory muscles. In Ammocoetes, where the respiration
is effected after the fashion of Limulus, not of Scorpio, the tubular
muscles are respiratory in function.

Look at their limits. The veno-pericardial muscles in Limulus are limited
by the extent of the heart, they do not extend beyond the anterior limit of
the heart. In Fig. 70 (p. 176) two of these muscles are seen in front of
the branchial region also attached to the longitudinal venous sinus,
although in front of the gill-region. In Ammocoetes the upper limit of the
tubular muscles is the group found in the velum; this most anterior group
belongs to a region in front of the branchial region--that of the
trigeminal.

Moreover, the supposition that the segmental tubular muscles belong
throughout to the veno-pericardial group gives an adequate reason why they
do not occur in front of the velum; for, as their existence is dependent
upon the longitudinal collecting sinus in Limulus and Scorpio, which is
represented by the ventral aorta in {314}Ammocoetes, they cannot extend
beyond its limits. Now, Dohrn asserts that the ventral aorta terminates in
the spiracular artery, which exists only for a short time; and, in another
place, speaking of this same termination of the ventral aorta, he states:
"Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus
arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der
Mesodermfalte des späteren Velums in die Höhe steigt um in die Aorta der
betreffenden Seite einzumunden." These observations show that the vessel
which in Ammocoetes represents the longitudinal collecting sinus in the
Merostomata does not extend further forwards than the velum, and in
consequence the representatives of the veno-pericardial muscles cannot
extend into the segments anterior to the velum. One of the extraordinary
characteristics of these tubular muscles which distinguishes them from
other muscles, but brings them into close relationship with the
veno-pericardial group, is the manner in which the bundles of muscle-fibres
are always found lying freely in a blood-space; this is clearly seen in the
branchial region, but most strikingly in the velum, the interior of which,
apart from its muco-cartilage, is simply a large lacunar blood-space
traversed by these tubular muscles.

All these reasons point to the same conclusion: the tubular muscles in
Ammocoetes are the successors of the veno-pericardial system of muscles.

If this is so, then this homology ought to throw light on the extraordinary
innervation of these tubular muscles by the _branchialis profundus_ branch
of the facial nerve and the velar branch of the trigeminal. We ought, in
fact, to find in Limulus a nerve arising exclusively from the ganglia
belonging to the chilarial and opercular segments, which, instead of being
confined to those segments, traverses the whole branchial region on each
side, and gives off a branch to each branchial segment; this branch should
supply the veno-pericardial muscle of that side.

Patten and Redenbaugh have traced out the distribution of the peripheral
nerves in Limulus, and have found that from each mesosomatic ganglion a
segmental cardiac nerve arises which passes to the heart and there joins
the cardiac median nerve, or rather the median heart-ganglion, for this
so-called nerve is really a mass of ganglion-cells. In all the branchial
segments the same plan exists, each cardiac nerve belonging to that
neuromere is strictly segmental. {315}Upon reaching the opercular and
chilarial neuromeres an extraordinary exception is found; the cardiac
nerves of these two neuromeres are fused together, run dorsally, and then
form a single nerve called the pericardial nerve, which runs outside the
pericardium along the whole length of the mesosomatic region, and gives off
a branch to each of the cardiac nerves of the branchial neuromeres as it
passes them.

This observation of Patten and Redenbaugh shows that the pericardial nerve
of Limulus agrees with the very nerve postulated by the theory, as far as
concerns its origin from the chilarial and opercular neuromeres, its
remarkable course along the whole branchial region, and its segmental
branches to each branchial segment.

At present the comparison goes no further; there is no evidence available
to show what is the destination of these segmental branches of the
pericardial nerve, and so far all evidence of their having any connection
with the veno-pericardial muscles is wanting. Carlson, at my request,
endeavoured in the living Limulus to see whether stimulation of the
pericardial nerve caused contraction of the veno-pericardial muscles, but
was unable to find any such effect. On the contrary, his experimental work
indicated that each veno-pericardial muscle received its motor supply from
the corresponding mesosomatic ganglion. This is not absolutely conclusive,
for if, as Blanchard asserts in the case of the scorpion, a close
connection exists between the action of these muscles and of the heart, it
is highly probable that their innervation conforms to that of the heart.
Now Carlson has shown that this cardiac nerve from the opercular and
chilarial neuromeres is an inhibitory nerve to the heart, while the
segmental cardiac nerves belonging to the branchial ganglia are the
augmentor nerves of the heart.

His experiments, then, show that the motor nerves of the heart and of the
veno-pericardial muscles run together in the same nerves, but he says
nothing of the inhibitory nerves to the latter muscles. If they exist and
if they are in accordance with those to the heart, then they ought to run
in the pericardial nerve, and would naturally reach the veno-pericardial
muscles by the segmental branches of the pericardial nerve.

Moreover, inhibitory nerves are, in certain cases, curiously associated
with sensory fibres; so that the nerve which corresponds {316}to the
pericardial nerve, viz. the _branchialis profundus_ of the facial, may be
an inhibitory and sensory nerve, and not motor at all. Miss Alcock's
observations are purely histological; no physiological experiments have
been made.

At present, then, it does not seem to me possible to say that Carlson's
experiments have disproved _any_ connection of the pericardial nerve with
the veno-pericardial muscles. We do not know what is the destination of its
segmental branches; they may still supply the veno-pericardial muscles even
if they do not cause them to contract; they certainly do not appear to pass
directly into them, for they pass into the segmental cardiac nerves, and
can only reach the muscles in conjunction with their motor nerves. Such a
course would not be improbable when it is borne in mind how, in the frog,
the augmentor nerves run with the inhibitory along the whole length of the
vagus nerve.

Until further evidence is given both as to the function of the segmental
branches of the pericardial nerve in the Limulus, and of the _branchialis
profundus_ in Ammocoetes, it is impossible, I think, to consider that the
phylogenetic origin of these tubular muscles is as firmly established as is
that of most of the other organs already considered. I must say, my own
bias is strongly in favour of looking upon them as the last trace of the
veno-pericardial system of muscles, a view which is distinctly strengthened
by Carlson's statement that the latter system contracts synchronously with
the respiratory movements, for undoubtedly in Ammocoetes their function is
entirely respiratory. Then again, although at present there is no evidence
to connect the pericardial nerve in Limulus with this veno-pericardial
system of muscles, yet it is extraordinarily significant that in such
animals as Limulus and Ammocoetes, in both of which the mesosomatic or
respiratory region is so markedly segmental, an intrusive nerve should, in
each case, extend through the whole region, giving off branches to each
segment. Still more striking is it that this nerve should arise from the
foremost mesosomatic and the last prosomatic neuromeres in Limulus--the
opercular and chilarial segments--precisely the same neuromeres which give
origin to the corresponding nerve in Ammocoetes, for according to my theory
of the origin of vertebrates, the nerves which supplied the opercular and
metastomal appendages have become the facial nerve and the lower lip-branch
of the trigeminal nerve.

{317}With the formation of the vertebrate heart from the two longitudinal
venous sinuses and the abolition of the dorsal invertebrate heart, the
function of these tubular muscles as branchial hearts was no longer needed,
and their respiratory function alone remained. The last remnant of this is
seen in Ammocoetes, for the ordinary striated muscles were always more
efficient for the respiratory act, and so at transformation the inferior
tubular musculature was got rid of, there being no longer any need for its
continued existence.


THE PALÆOSTOMA, OR OLD MOUTH.

The arrangement of the oral chamber in Ammocoetes is peculiar among
vertebrates, and, upon my theory, is explicable by its comparison with the
accessory oral chamber which apparently existed in Eurypterus. According to
this explanation, the lower lip of the original vertebrate mouth was formed
by the coalescence of the most posterior pair of the prosomatic
appendages--the chilaria; from which it follows that the vertebrate mouth
was not the original mouth, but a new structure due to such a formation of
the lower lip.

It is very suggestive that the direct following out of the original working
hypothesis should lead to this conclusion, for it is universally agreed by
all morphologists that the present mouth is a new formation, and Dohrn has
argued strongly in favour of the mouth being formed by the coalescence of a
pair of gill-slits. Interpret this in the language of my theory, and
immediately we see, as already explained, gill-slits must mean in this
region the spaces between appendages which did not carry gills; the mouth,
therefore, was formed by the coalescence of a pair of appendages to form a
lower lip just as I have pointed out.

Where, then, must we look for the palæostoma, or original mouth? Clearly,
as already suggested, it was situated at the base of the olfactory passage,
and the olfactory passage or nasal tube of Ammocoetes was originally the
tube of the hypophysis, so that the following out of the theory points
directly to the tube of the hypophysis as the place where the palæostoma
must be looked for.

This conclusion is not only not at variance with the opinions of
morphologists, but gives a straightforward, simple explanation why the
palæostoma was situated in the very place where they are most inclined to
locate it. Thus, if we trace the history of the question, {318}we see that
Dohrn's original view of the comparison of the vertebrate and the annelid
led him to the conception that the vertebrate mouth was formed by the
coalescence of a pair of gill-slits, and that the original mouth was
situated somewhere on the dorsal surface and opened into the gut by way of
the infundibulum and the tube of the hypophysis. This, also, was
Cunningham's view as far as the tube of the hypophysis was concerned.
Beard, in 1888, holding the view that the vertebrates were derived from
annelids which had lost their supra-oesophageal ganglia, and that,
therefore, there was no question of an oesophageal tube piercing the
central nervous system of the vertebrate, explained the close connection of
the infundibulum with the hypophysis by the comparison of the tube of the
hypophysis with the annelidan mouth, so that the infundibular or so-called
nervous portion was a special nervous innervation for the original throat,
just as Kleinenberg had shown to be the case in many annelids. Beard
therefore called this opening of the hypophysial tube the old mouth, or
palæostoma. Recently, in 1893, Kupffer has also put forward the view that
the hypophysial opening is the palæostoma. basing this view largely upon
his observations on Ammocoetes and Acipenser.

[Illustration: FIG. 125.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN
SUCH A VERTEBRATE AS THE LAMPREY.

_Nc._, neural canal with its infundibular termination; _Nch._, notochord;
_Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial
or nasal tube; _Or._, oral chamber closed by septum.]

As is seen in Fig. 125, the position of this palæostoma is a very
suggestive one. At this single point in Ammocoetes, four separate tubes
terminate; here is the end of the notochordal tube, the termination of the
infundibulum, the blind end of the nasal tube or tube {319}of the
hypophysis, and the pre-oral elongation of the alimentary canal.

It is perfectly simple and easy for the olfactory tube to open into any one
of the other three. By opening into the infundibulum it reproduces the
condition of affairs seen in the scorpion; by opening into the gut it
produces the actual condition of things seen in Myxine and other
vertebrates; by opening into the notochordal tube it would produce a
transitional condition between the other two.

The view held by Kupffer is that this nasal tube (tube of the hypophysis)
opened into the anterior diverticulum of the vertebrate gut, and was for
this reason the original mouth-tube; then a new mouth was formed, and this
connection was closed, being subsequently reopened as in Myxine. My view is
that this tube originally opened into the infundibulum, in other words,
into the original gut of the palæostracan ancestor, and was for this reason
the original mouth-tube, in the same sense as the olfactory passage of the
scorpion may be, and often is, called the mouth-tube. When, with the
breaking through of the septum between the oral and respiratory chambers,
the external opening of the oral chamber became a new mouth, the old mouth
was closed but the olfactory tube still remained, owing to the importance
of the sense of smell. Subsequently, as in Myxine and the higher
vertebrates, it opened into the pharynx, and so formed the nose of the
higher vertebrates.

It is not, to my mind, at all improbable that during the transition stage,
between its connection with the old alimentary canal, as in Eurypterus or
the scorpions, and its blind ending, as in Ammocoetes, the nasal tube
opened into the tube of the notochord. This question will be discussed
later on when the probable significance of the notochord is considered.


THE PITUITARY GLAND.

Turning back to the comparison of Fig. 106, B, and Fig. 106, C, which
represent respectively an imaginary sagittal section through an
Eurypterus-like animal and through Ammocoetes at a larval stage, all the
points for comparison mentioned on p. 244 have now been discussed with the
exception of the suggested homology between the coxal glands of the one
animal and the pituitary body of the other.

{320}This latter gland undoubtedly arises posteriorly to the hypophysial
tube, or Rathke's pouch (as it is sometimes called), and, as already
mentioned, is supposed by Kupffer to be formed from the posterior wall of
this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who
has investigated this matter, finds that the glandular hypophysis is not
formed from the walls of Rathke's pouch, but from the tissue of the
rudimentary connection or stalk between the two premandibular cavities,
which becomes closely connected with the posterior wall of Rathke's pouch,
and becoming cut off from the rest of the premandibular cavity on each
side, becomes permanently a part of the 'Hypophysis Anlage.'

The importance of Nusbaum's investigation consists in this, that he derives
the glandular hypophysis from the connecting stalk between the two
premandibular cavities, and therefore from the walls of the ventral
continuation of this cavity on each side.

This may be expressed as follows:--

The coelomic cavity, known as the premandibular cavity, divides into a
dorsal and a ventral part; the walls of the dorsal part give origin to the
somatic muscles belonging to the oculomotor nerve, while the walls of the
ventral part on each side form the connecting stalk between the two
cavities, and give origin to the glandular hypophysis.

Now, as already pointed out, the premandibular cavity is homologous with
the 2nd prosomatic coelomic cavity of Limulus, and this 2nd prosomatic
coelomic cavity divides, according to Kishinouye, into a dorsal and a
ventral part; and, further, the walls of this ventral part form the coxal
gland. Both in the vertebrate, then, and in Limulus, we find a marked
glandular tissue in a corresponding position, and the conclusion is forced
upon us that the glandular hypophysis was originally the coxal gland of the
invertebrate ancestor. As in all other cases already considered, when the
facts of topographical anatomy, of morphology and of embryology, all
combine to the same conclusion as to the derivation of the vertebrate organ
from that of the invertebrate, then there must be also a structural
similarity between the two. What, then, is the nature of the coxal gland in
the scorpions and Limulus? Lankester's paper gives us full information on
this point as far as the scorpion and Limulus are concerned, and he shows
that the coxal gland of Limulus differs markedly from that of Scorpio in
the size of the cells and in the {321}arrangement of the tubes. In Fig.
126, A, I give a picture of a piece of the coxal gland of Limulus taken
from Lankester's paper.

Turning now to the vertebrate, Bela Haller's paper gives us a number of
pictures of the glandular hypophysis from various vertebrates, and he
especially points out the tubular nature of the gland and its
solidification in the course of development in some cases. In Fig. 126, B,
I give his picture of the gland in Ammocoetes.

The striking likeness between Haller's picture and Lankester's picture is
apparent on the face of it, and shows clearly that the histological
structure of the glands in the two cases confirms the deductions drawn from
their anatomical and morphological positions.

[Illustration: FIG. 126.--A, SECTION OF COXAL GLAND OF LIMULUS (from
LANKESTER); B, SECTION OF PITUITARY BODY OF AMMOCOETES (from BELA HALLER).

_n.a._, termination of nasal passage.]

The sequence of events which gave rise to the pituitary body of the
vertebrate was in all probability somewhat as follows:--

Starting with the excretory glands of the Phyllopoda, known as
shell-glands, which existed almost certainly in the phyllopod Trilobite, we
pass to the coxal gland of the Merostomata. Judging from Limulus, these
were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor
appendages. When these appendages became reduced in size and purely tactile
they were compressed and concentrated round the mouth region, forming the
endognaths of the Merostomata; as a necessary consequence of the
concentration of the coxæ of the endognaths, the coxal gland also became
concentrated, {322}and took up a situation close against the pharynx, as
represented in Fig. 106, B. When, then, the old mouth closed, and the
pharynx became the _saccus vasculosus_, the coxal gland remained in close
contact with the _saccus vasculosus_, and became the pituitary body, thus
giving the reason why there is always so close a connection between the
pituitary body and the infundibular region.

Whatever was the condition of the digestive tracts at the transition stage
between the arthropod and the vertebrate, the original mouth-opening at the
base of the olfactory tube was ultimately closed. The method of its closure
was exceedingly simple and evident. The membranous cranium was in process
of formation by the extension of the plastron laterally and dorsally; a
slight growth of the same tissue in the region of the mouth would suffice
to close it and thus separate the infundibulum from the olfactory tube. As
evidence that such was the method of closure, it is instructive to see how
in Ammocoetes the glandular tissue of the pituitary body is embedded in and
mixed up with the tissue of this cranial wall; how the termination of the
nasal tube is embedded in this same thickened mass of the cranial
wall--how, in fact, both coxal gland and olfactory tube have become
involved in the growth of the tissue of the plastron, by means of which the
mouth was closed.

I have now passed in review the nature of the evidence which justifies a
comparison between the segments supplied by the cranial nerves of the
vertebrate and the prosomatic and mesosomatic segments of the palæostracan.
For the convenience of my readers I have put these conclusions into tabular
form (see p. 323), for all the segments as far as that supplied by the
glossopharyngeal nerves. In both vertebrate and invertebrate this is a
fixed position, for in the former, however variable may be the number of
branchial segments which the vagus supplies, the second branchial segment
is always supplied by a separate nerve, the glossopharyngeal, and in the
latter, though the number of segments bearing branchiæ varies, the minimum
number of such segments (as seen in the Pedipalpi) is never less than two.

{323}TABLE OF COMPARISON OF CORRESPONDING SEGMENTS IN THE EURYPTERIDS AND
IN AMMOCOETES (_i.e._ IN CEPHALASPIDS).

  Key:
  So. = Supra-oesophageal.
  Si. = Supra-infundibular.
  Io. = Infra-oesophageal.
  Ii. = Infra-infundibular.
  Ps. = Prosomatic.
  Ms. = Mesosomatic.

  +---+--------------------------------------------------------+
  |   |                      Median Eyes.                      |
  |   +--------------------------------------------------------+
  |So.|                      Lateral Eyes.                     |
  |   +--------------------------------------------------------+
  |   |                      Camerostome.                      |
  +---+--------------------------------------------------------+
  |   |         Invertebrate (Limulus or Eurypterid).          |
  +---+---+---------+-----------------------------+------------+
  |   |   |         |        Appendages.          |  Coelomic  |
  |   |   |Segments.+-------------+---------------+  Cavities. |
  |   |   |         |  Limulus.   | Eurypterid.   |            |
  |   |   +---------+-------------+---------------+------------+
  |   |   |    1    |  Cheliceræ  | Cheliceræ     |     1      |
  |   |   +---------+-------------+---------------+------------+
  |   |   |    2    |1st Locomotor|}              |            |
  |   |   +---------+-------------+}              |     2      |
  |   |   |    3    | 2nd    "    |}              |Ventral part|
  |   |Ps.+---------+-------------+}4 Endognaths  | forms coxal|
  |   |   |    4    | 3rd    "    |}              |   gland.   |
  |   |   +---------+-------------+}              |            |
  |   |   |    5    | 4th    "    |}              |            |
  |   |   +---------+-------------+---------------+------------+
  |Io.|   |    6    | 5th    "    |   Ectognath   |     3      |
  |   |   |         |             |               |            |
  |   |   +---------+-------------+---------------+------------+
  |   |   |    7    | Chilaria    |   Metastoma   |     4      |
  |   |   |         |             |               |            |
  |   +---+---------+-------------+---------------+------------+
  |   |   |    8    |  Operculum  |Genital  }     |     5      |
  |   |   |---------+-------------+         }Oper-+------------+
  |   |   |    9    |1st Branchial|         }culum|     6      |
  |   |   |         |             |Branchial}     |            |
  |   |Ms.+---------+-------------+---------------+------------+
  |   |   |   10    | 2nd    "    | 2nd Branchial |     7      |
  +---+---+---------+-------------+---------------+------------+

  +-----------------------------------------------------+------+
  |                Pineal Eyes.                         |      |
  +-----------------------------------------------------+      |
  |                Lateral Eyes.                        |  Si. |
  +-----------------------------------------------------+      |
  |               Olfactory Organ.                      |      |
  +-----------------------------------------------------+------+
  |        Vertebrate (Ammocoetes or Cephalaspid).      |      |
  +-------------+---------------+-------------+---------+------+
  |             |  Splanchnic   |   Somatic   | Somatic |      |
  | Appendages. |    Nerves.    |  Segmental  | Nerves. |      |
  |             |               |   Muscles.  |         |      |
  +-------------+---------------+-------------+---------+      |
  |      ...    |      ...      |     ...     |   ...   |      |
  +-------------+---------------+-------------+---------+      |
  |             |               |             |         |      |
  |             |      V        |   Muscles   |         |      |
  |4 tentacles  |  Tentacular   | supplied by |   III   |      |
  | and upper   |  and upper    |  oculomotor |         |      |
  |    lip.     |  lip nerve.   |   nerve.    |         |      |
  |             |               |             |         |      |
  |             |               |             |         |      |
  +-------------+---------------+-------------+---------+      |
  |   Tongue    |       V       | Sup.oblique |   IV    |  Ii. |
  |             | Tongue nerve  |             |         |      |
  +-------------+---------------+-------------+---------+      |
  |  Lower lip  |       V       |             |         |      |
  |             |Lower lip nerve|             |         |      |
  +-------------+---------------+-------------+---------+      |
  |   Thyroid   |}              |             |         |      |
  +-------------+}     VII      | Ext. rectus |    VI   |      |
  |    Hyoid or |}              |Retract oculi|         |      |
  |1st Branchial|}              |             |         |      |
  +-------------+---------------+-------------+---------+      |
  |2nd Branchial|       IX      |     ...     |   ...   |      |
  +-------------+---------------+-------------+---------+------+

  +-------------------------+------+
  |        Pineal Nerve.    |      |
  +-------------------------+      |
  |          II             |  Si. |
  +-------------------------+      |
  |           I             |      |
  +-------------------------+------+
  |        Vertebrate.      |      |
  +-------------+-----------+------+
  |   Coelomic  |V. Wijhe's |      |
  |   Cavities. | Segments. |      |
  |             |           |      |
  +-------------+-----------+      |
  |  Anterior.  |    ...    |      |
  +-------------+-----------+      |
  |  Premandi-  |           |      |
  |    bular    |           |      |
  |   Ventral   |           |      |
  |  part forms |     1     |      |
  |   pituitary |           |      |
  |   body.     |           |      |
  |   pituitary |           |      |
  +-------------+-----------+      |
  |  Mandibular |}          |  Ii. |
  |             |}          |      |
  +-------------+}    2     |      |
  |  Mandibular |}          |      |
  |             |}          |      |
  +-------------+-----------+      |
  |   Hyoid_1   |     3     |      |
  +-------------+-----------+      |
  |   Hyoid_2   |     4     |      |
  |             |           |      |
  +-------------+-----------+      |
  |2nd Branchial|     5     |      |
  +-------------+-----------+------+

{324}SUMMARY.

  The general consideration of the evidence of the number of segments, and
  their nature in the pro-otic region of the vertebrate, as given in the
  last chapter, is not incompatible with the view that the trigeminal nerve
  originally supplied seven appendages, which appendages did not carry
  branchiæ, but were originally used for purposes of locomotion as well as
  of mastication.

  Such appendages clearly no longer exist in the higher vertebrates, the
  muscles of mastication only remaining; but in the earliest fish-forms
  they must have existed, as, indeed, is seen in Pterichthys and
  Bothriolepis. Judging from all the previous evidence some signs of their
  existence may reasonably be expected still to remain in Ammocoetes. Such
  is indeed the case.

  In the adult Petromyzon the trigeminal nerve innervates specially a
  massive suctorial apparatus, by means of which it holds on to other
  fishes, or to stones in the bottom of the stream. There is here no
  apparent sign of appendages. Very great, however, is the difference in
  the oral chamber of Ammocoetes; here there is no sign of any suctorial
  apparatus, but instead, a system of tentacles, together with the remains
  of the septum or velum, which originally closed off the oral from the
  respiratory chamber. These tentacles are the last remnants of the
  original foremost prosomatic appendages of the palæostracan ancestor.
  Like the lateral eyes they do not develop until the transformation comes,
  but during the whole larval condition their musculature remains in an
  embryonic condition, and then from these embryonic muscles the whole
  massive musculature of the suctorial apparatus develops; a sucking
  apparatus derived from the modification of appendages, as so frequently
  occurs in the arthropods.

  The study of Ammocoetes indicates that the velum and lower lip correspond
  to the metastoma of the Eurypterid, _i.e._ the chilaria of Limulus, while
  the large ventral pair of tentacles, called the tongue, correspond to the
  ectognaths of the Eurypterids, and probably to the oar-like appendages of
  Pterichthys and Bothriolepis. From these two splanchnic segments the
  suctorial apparatus in the main arises; the motor supply of these two
  segments forms the mass of the trigeminal nerve-supply, and the nerves
  supplying them, the velar nerve and the tongue-nerve, are markedly
  separate from the rest of the trigeminal nerve.

  The rest of the tentacles present much less the sign of independent
  segments. In their nerves, their muco-cartilaginous skeleton, and their
  rudimentary muscles, they indicate a concentration and amalgamation, such
  as might be expected from the concentrated endognaths. The continuation
  of the dwindling process, already initiated in the Eurypterid, would
  easily result in the tentacles of Ammocoetes.

  The nasal tube of Ammocoetes, which originates in the hypophysial tube,
  corresponds absolutely in position and in its original structure, to the
  olfactory tube of a scorpion-like animal. From this homology two
  conclusions of importance follow: (1) the old mouth, or palæostoma, of
  the vertebrate was situated at the end of this tube, therefore, at the
  termination of the infundibulum; (2) the upper lip, which by its growth,
  brings the olfactory tube from a ventral to a dorsal position, was
  originally formed by the foremost sternites or endostoma, or else by the
  sterno-coxal processes of the second pair of prosomatic appendages of the
  palæostracan ancestor.

  In strict accordance with the rest of the comparisons made in this
  region, the pituitary body shows by similarity of structure, as well as
  of position, that it arose from the coxal glands, which were situated at
  the base of the four endognaths.

  {325}One after another, when once the clue has been found, all these
  mysterious organs of the vertebrate, such as the pituitary and thyroid
  glands, fall harmoniously into their place as the remnants of
  corresponding important organs in the palæostraca.

  Yet another clue is afforded by the tubular muscles of Ammocoetes, that
  strange set of non-vertebrate striated muscles, which are so markedly
  arranged in a segmental manner, which disappear at transformation, and
  are never found in any of the higher vertebrates, for the limits of their
  distribution correspond to the veno-pericardial muscles of Limulus.

  Their nerve-supply in Ammocoetes is most extraordinary; for, although
  they are segmentally arranged throughout the whole respiratory region,
  which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are
  found in front of this region only in one segment, that of the lower lip,
  which is supplied by the velar branch of the Vth nerve, yet they are not
  supplied segmentally, but only by the velar nerve and a branch of the
  VIIth, the _ramus branchialis profundus_. This latter nerve extends
  throughout the respiratory region, and gives off segmental branches to
  supply these muscles.

  It is also a curious coincidence that in such a markedly segmented animal
  as Limulus, a nerve--the pericardial nerve--which arises from the nerves
  of the chilarial and opercular segments, should pass along the whole
  respiratory region and give off branches to each mesosomatic segment. It
  is strange, to say the least of it, that the chilarial or metastomal and
  the opercular segments of Limulus should, on the theory advocated in this
  book, correspond to the lower lip and hyoid segments of the vertebrate.
  At present the homology suggested is not complete, for there is no
  evidence as yet that the veno-pericardial muscles have anything to do
  with the pericardial nerve.




{326}CHAPTER X

_THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE
OSTRACODERMATA_

  The nose of the Osteostraci.--Comparison of head-shield of Ammocoetes and
  of Cephalaspis.--Ammocoetes the 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.


The shifting of the orifice of the olfactory passage, which led to the old
mouth, from the ventral to the dorsal side, as seen in the transformation
of the ventrally situated hypophysial tube of the young Ammocoetes, to the
dorsally situated nasal tube of the full-grown Ammocoetes, affords one of
the most important clues in the whole of this story of the origin of
vertebrates; for, if Ammocoetes is the nearest living representative of the
first-formed fishes, then we ought to expect to find that the dorsal
head-shield of such fishes is differentiated from that of the contemporary
Palæostraca by the presence of a median frontal opening anterior to the
eyes. Conversely, if such median nasal orifice is found to be a marked
characteristic of the group, in combination with lateral and median eyes,
as in Ammocoetes, then we have strong reasons for interpreting these
head-shields by reference to the head of Ammocoetes.

The oldest known fishes belong to a large group of strange forms which
inhabited the Silurian and Devonian seas, classed together by Smith
Woodward under the name of Ostracodermi. These are divided into three
orders: (1) the Heterostraci, including one family, the Pteraspidæ, to
which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into
two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis,
Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the
Antiarcha, with one family, the Astrolepidæ, including Astrolepis,
Pterichthys, and Bothriolepis. {327}Of these, the first two orders belong
to the Upper Silurian, while the third is Devonian.


THE DORSAL HEAD-SHIELD OF THE OSTEOSTRACI.

Of the three orders above-named, the Heterostraci and Osteostraci are the
oldest, and among them the Cephalaspidæ have afforded the most numerous and
best worked-out specimens. At Rootziküll, in the island of Oesel, the form
known as _Thyestes (Auchenaspis) verrucosus_ is especially plentiful, being
found thickly present in among the masses of Eurypterid remains, which give
the name to the deposit. Of late years this species has been especially
worked at by Rohon, and many beautiful specimens have been figured by him,
so that a considerable advance has been made in our knowledge since Pander,
Eichwald, Huxley, Lankester, and Schmidt studied these most interesting
primitive forms.

All observers agree that the head-region of these fishes was covered by a
dorsal and ventral head-shield, while the body-region was in most cases
unknown, or, as in Eichwald's specimens, and in the specimens figured in
Lankester and Smith Woodward's memoirs, was made up of segments which were
not vertebral in character, but formed an aponeurotic skeleton, being the
hardened aponeuroses between the body-muscles. This body-skeleton, which
possesses its exact counterpart in Ammocoetes, will be considered more
fully when I discuss the origin of the spinal region of the vertebrate.

Of the two head-shields, ventral and dorsal, the latter is best known and
characterizes the group. It consists of a dorsal plate, with characteristic
horns, which in _Thyestes verrucosus_ (Fig. 128), as described by Rohon, is
composed of two parts, a frontal part and an occipital part (_occ._), the
occipital part being composed of segments, and possessing a median
ridge--the _crista occipitalis_. In Lankester's memoir and in Smith
Woodward's catalogue, a large number of known forms are described and
delineated, and we may perhaps say that in some of the forms, such as
_Eukeraspis pustuliferus_ (Fig. 127, B), the frontal part of the shield
only is capable of preservation as a fossil, while in Cephalaspis (Fig.
127, A) not only the frontal part but a portion of the occipital region is
preserved, the latter being small in extent when compared with the
occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and
Didymaspis, the whole of both frontal {328}and occipital region is capable
of preservation, the line of demarcation between these two regions being
well marked in the latter species.

[Illustration: FIG. 127.--A, DORSAL HEAD-SHIELD OF CEPHALASPIS (from
LANKESTER); B, DORSAL HEAD-SHIELD OF KERASPIS (from LANKESTER).]

In the best preserved specimens of all this group of fishes a frontal
median orifice is always present; it appears in some specimens obscurely
partially divided into two parts. Perhaps the best specimen of all was
obtained by Rohon at Rootziküll, and is thus described by him:--

The frontal part of the dorsal head-plate carried (Fig. 128) the two orbits
for the lateral eyes (_l.e._), a marked frontal organ (_fro._), and a
median depression (_gl._), to which he gives the name parietal organ. The
occipital part (_occ._) was clearly segmented, and carried, he thinks, the
branchiæ. I reproduce Rohon's figure of the frontal organ in Thyestes (Fig.
129); he describes it as a deeply sunk pit, divided in the middle by a
slit, which leads deeper in, he supposes, towards the central nervous
system.

[Illustration: FIG. 128.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis)
verrucosus_. (From ROHON.)

_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate
over brain; _Occ._, occipital region.]

{329}A similar organ was described by Schmidt in Tremataspis, and
considered by him to be a median nose. Such also is the view of Jaekel, who
points out that a median pineal eye exists between the two lateral eyes in
this animal, as in all other of these ancient fishes, so that this frontal
organ does not, as Patten thinks, represent the pineal eye. The whole of
this group of fishes, then, is characterized by the following striking
characteristics:--

1.  Two well-marked lateral eyes near the middle line.

2.  Between the lateral eyes, well-marked median eyes, very small.

3.  In front of the eye-region a median orifice, single.

In addition, behind the eye-region a median plate is always found,
frequently different in structure to the rest of the head-shield, being
harder in texture--the so-called post-orbital plate.

[Illustration: FIG. 129.--NARIAL OPENING AND LATERAL ORBITS OF _Thyestes
Verrucosus_. (From ROHON.)]


STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT OF AMMOCOETES.

What is the structure of this head-shield? It has been spoken of as formed
of bone because it possesses cells, being thus unlike the layers of chitin,
which are formed by underlying cells but are not themselves cellular. At
the same time, it is recognized on all sides that it has no resemblance to
bone-structure as seen in fossil remains of higher vertebrates. The latest
and best figure of the structure of this so-called bone is given in Rohon's
paper already referred to. It is, so he describes, clearly composed of
fibrillæ and star-shaped cells, arranged more or less in regular layers,
with other sets of similar cells and fibrillæ arranged at right angles to
the first set, or at varying angles. The groundwork of this tissue, in
which these cells and fibrils are embedded, contained calcium salts, and so
the whole tissue was preserved. In places, spaces are found in it, in the
deepest layer large medullary spaces; more superficially, ramifying spaces
which he considers to be vascular, and calls Haversian canals; the
{330}star-like cells, however, are not arranged concentrically around these
spaces, as in true Haversian canals.

This structure is therefore a calcareous infiltration of a tissue with
cells in it. Where is there anything like it?

As soon as I saw Rohon's picture (Fig. 130), I was astounded at its
startling resemblance to the structure of muco-cartilage as is seen in Fig.
131, taken from Ammocoetes. If such muco-cartilage were infiltrated with
lime salts, then the muco-cartilaginous skeleton of Ammocoetes would be
preserved in the fossil condition, and be comparable with that of
Cephalaspis, etc.

[Illustration: FIG. 130.--SECTION OF A HEAD-PLATE OF A CEPHALASPID. (From
ROHON.)]

[Illustration: FIG. 131.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE
OF AMMOCOETES.]

The whole structure is clearly remarkably like Rohon's picture of a section
of the head-plate of a Cephalaspid (Fig. 130). In the latter case the
matrix contains calcium salts, in the former it is composed of the peculiar
homogeneous mucoid tissue which stains so characteristically with thionin.
With respect to this calcification, it is instructive to recall the
calcification in the interior of the branchial cartilages of Limulus, as
described in Chapter III., for this example shows how easy it is to obtain
a calcification in this chondro-mucoid material. With respect to the
medullary spaces and smaller spaces in this tissue, as described by Rohon,
I would venture to suggest that they need not all necessarily indicate
blood-vessels, for similar spaces would appear in the head-shield of
Ammocoetes if its muco-cartilage alone {331}were preserved. Of these, some
would indicate the position of blood-vessels, such, for instance, as of the
external carotid which traverses this structure; but the largest and most
internal spaces, resembling Rohon's medullary spaces, would represent
muscles, being filled up with bundles of the upper lip-muscles.


THE MUCO-CARTILAGINOUS HEAD-SHIELD OF AMMOCOETES.

The resemblance between the structure of the head-shield of Thyestes and
the muco-cartilage of Ammocoetes, is most valuable, for muco-cartilage is
unique, occurs in no other vertebrate, and every trace of it vanishes at
transformation; it is essentially a characteristic of the larval form, and
must, therefore, in accordance with all that has gone before, be the
remnant of an ancestral skeletal tissue. The whole story deduced from the
study of Ammocoetes would be incomplete without some idea of the meaning of
this tissue. So also, as already mentioned, the skeleton of Ammocoetes is
incomplete without taking this tissue into account. It is confined entirely
to the head-region; no trace of it exists posteriorly to the branchial
basket-work. It consists essentially of dorsal and ventral head-shields,
connected together by the tentacular, metastomal, and thyroid bars, as
already described. The ventral shield forms the muco-cartilaginous plate of
the lower lip and the plate over the thyroid gland, so that the skeleton
ventrally is represented by Fig. 118, B, which shows how the cartilaginous
bars of the branchial basket-work are separated from each other by this
thyroid plate. At transformation, with the disappearance of this
muco-cartilaginous plate, the bars come together in the middle line, as in
the more posterior portion of the branchial basket-work.

The dorsal head-shield of muco-cartilage covers over the upper lip, sends a
median prolongation over the median pineal eyes and a lateral prolongation
on each side as far as the auditory capsules, giving the shape of the
head-shield of muco-cartilage, as in Fig. 118, C.

Not only then is the structure of the head-shield of a Cephalaspid
remarkably like the muco-cartilage of Ammocoetes, but also its general
distribution strangely resembles that of the Ammocoetes muco-cartilage.

Now, these head-shields in the Cephalaspidæ and Tremataspidæ {332}vary very
much in shape, as is seen by the comparison of Tremataspis and Auchenaspis
with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms
belong to a single group, the Osteostraci.

The conception that Ammocoetes is the solitary living form allied to this
group affords a clue to the meaning of this variation of shape, which
appears to me to be possible, if not indeed probable. There is a certain
amount of evidence given in the development of Ammocoetes which indicates
that the branchial region of its ancestors was covered with plates of
muco-cartilage as well as the prosomatic region.

The evidence is as follows:--

The somatic muscles of Ammocoetes form a continuous longitudinal sheet of
muscles along the length of the body, which are divided up by connective
tissue bands into a series of imperfect segments or myotomes. This simple
muscular sheet can be dissected off along the whole of the head-region of
the animal, with the exception of the most anterior part, without
interfering with the attachments or arrangements of the splanchnic muscular
system in the least. The reason why this separation can be so easily
effected is to be found in the fact that the two sets of muscles are not
attached to the same fascia. The sheet of fascia to which the somatic
muscles are attached is separated from the fascia which encloses the
branchial cavity by a space (_cf._ Figs. 63 and 64) filled with
blood-spaces and cells containing fat, in which space is also situated the
cartilaginous branchial basket-work. These branchial bars are closely
connected with the branchial sheet of fascia, and have no connection with
the somatic fascia, their perichondrium forming part of the former sheet.
Upon examination, this space is seen to be mainly vascular, the
blood-spaces being large and frequently marked with pigment; but it also
possesses a tissue of its own, recognized as fat-tissue by all observers.
The peculiarity of the cells of this tissue is their arrangement; they are
elongated cells arranged at right angles to the plates of fascia, just as
the fibres of the muco-cartilage are largely arranged at right angles to
their limiting plates of perichondrium. These cells do not necessarily
contain fat; and when they do, the fat is found in the centre of each cell,
and does not push the protoplasm of the cell to the periphery, as in
ordinary fat cells.

{333}In Fig. 132, B, I give a specimen of this tissue stained by osmic
acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage taken
from the plate of the lower lip; and in Fig. 133, A, a modification of the
muco-cartilage taken from the velum, which shows the formation of a tissue
intermediate between ordinary muco-cartilage and this branchial fat-tissue.

Further, in fully-grown specimens of Ammocoetes, in the region of undoubted
muco-cartilage, a fatty degeneration of the cells frequently appears,
together with an increase in the blood spaces,--the precursor, in fact, of
the great change which overtakes this tissue soon afterwards, at the time
of transformation, when it is invaded by blood, and swept away, except in
those places where new cartilage is formed. I conclude, then, that the
tissue of this vascular space was originally muco-cartilage, which has
degenerated during the life of the Ammocoetes. The fact that in most cases
undoubted muco-cartilage is to be found here and there in this space, is
strong confirmation of the truth of this conclusion.

[Illustration: FIG. 132.--A, MUCO-CARTILAGE OF LOWER LIP (_Mc._); _m.ph._,
muscle of lower lip; _m.sm._, somatic muscle; _Cor._, laminated layer of
skin. B, DEGENERATED MUCO-CARTILAGE OF BRANCHIAL REGION. _F._, fat layer;
_P._, pigment; _Bl._, blood-space; _N._, somatic nerve; _m.br._, branchial
muscle; _m.sm._, somatic muscle.]

If this conclusion is correct, we may expect that it would be confirmed by
the embryological history of the tissue, and we ought to find that in much
younger stages a homogeneous tissue of the same nature as muco-cartilage
fills up the spaces in the branchial {334}region, where in the Ammocoetes
only blood and fat-containing cells are present. For this purpose Shipley
kindly allowed me to examine his series of sections through the embryo at
various ages. These specimens are very instructive, especially those
stained by osmic acid, which preserves the natural thickness of this space
better than other staining methods. At an age when the branchial cartilages
are seen to be formed, when no fat-cells are present, a distinctive tissue
(Fig. 133, B) is plainly visible in the velum and at the base of the
tentacles, in the very position where in the more advanced Ammocoetes
muco-cartilage exists. Taking, then, this tissue as our guide, the
specimens show that the space between the skin and the visceral muscles in
which the cartilaginous basket-work lies is filled with a similar material.
At this stage a sheet of embryonic tissue occupies the position where,
later on, blood-spaces and fat-cells are found, and this tissue resembles
that seen in the velum and other places where muco-cartilage is afterwards
found.

[Illustration: FIG. 133.--A, MUCO-CARTILAGE OF VELUM; B, EMBRYONIC
MUCO-CARTILAGE OF TENTACULAR BAR.]

I conclude, therefore, that originally the branchial or mesosomatic region
was covered with a dorsal plate of muco-cartilage, which carried on its
under surface the dorsal part of the branchial basket-work, and sprang from
the central core of skeletogenous tissue around the notochord; this plate
was separated from the plate which covered this region ventrally by the
lateral grove in which the gill-slits are situated. The ventral plate
carried on its under surface the ventral part of the branchial basket-work,
and was originally continuous with the plate over the thyroid gland.

{335}[Illustration: FIG. 134.--SKELETON OF HEAD-REGION OF AMMOCOETES. A,
LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.

Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_.
_sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na.
cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._,
notochord.]

{336}In Fig. 134, A, B, C, the cranial skeleton of Ammocoetes is
represented from the dorsal, ventral, and lateral aspects. The
muco-cartilage is coloured red, the branchial or soft cartilage blue, and
the hard cartilage purple. The degenerated muco-cartilage of the branchial
region is represented as an uncoloured plate, on which the branchial
basket-work stands in relief. If it were restored to its original condition
of muco-cartilage, it would represent a uniform plate, on the _under_
surface of which the basket-work would be situated; and if it were
calcified and made solid, the branchial basket-work would not show at all
in these figures.

Is it possible to find the reason why this skeletal covering has
degenerated so early before transformation, and why the thyroid plate
remains intact until transformation? We see that all that part which has
degenerated is covered over by the somatic muscles,--by, in fact, muscles
which, being innervated by the foremost spinal nerves, belong naturally to
the region immediately following the branchial. I suggest, therefore, that
the original skeletal covering of muco-cartilage has remained intact only
where it has not been invaded and covered over by somatic muscles, but has
been invaded by blood and undergone the same kind of degenerative change as
overtakes the great mass of this tissue at transformation wherever the
somatic muscles have overgrown it.

The covering somatic muscles in the branchial region form a dorsal and
ventral group, of which the latter is formed in the embryo much later than
the former, the line of separation between the two groups being the lateral
groove, with its row of branchial openings. This groove ends at the first
branchial opening, but the ventral and dorsal somatic muscles continue
further headwards. It is instructive to see that, although the lateral
groove terminates, the separation between the two groups of muscles is
still marked out by a ridge of muco-cartilage, represented in Fig. 134, A,
which terminates anteriorly in the opercular bar.

Passing now to the prosomatic region, we find that here, too, the
muco-cartilaginous external covering is divisible into a dorsal and a
ventral head-plate, the ventral head-plate being the plate of the lower
lip, and the dorsal head-plate the plate of muco-cartilage over the front
part of the head. The staining reaction with thionin maps out this dorsal
head-plate in a most beautiful manner, and shows that the whole of the
upper lip-region in front of the nasal orifice is one large plate of
muco-cartilage, obscured largely by the invasion of the crossing muscles of
the upper lip, but left pure and uninvaded all around the nasal orifice,
and where the upper and lower lips come together. In addition to this
foremost plate, a median tongue of muco-cartilage covers over the pineal
eye and fills up the {337}median depression between the two median dorsal
somatic muscles. Also, two lateral cornua pass caudalwards from the main
frontal mass of muco-cartilage over the lateral eyes, forming the
well-known wedge which separates the dorsal and lateral portions of the
dorso-lateral somatic muscle. In fact, similarly to what we find in the
branchial region, the muco-cartilaginous covering can be traced with
greater or less completeness only in those parts which are not covered by
somatic muscles.

In Fig. 134, A, B, C, this striking muco-cartilaginous head-shield, both
dorsal and ventral, is shown. Seeing that the upper lip wraps round the
lower one on each side, and that this most ventral edge of the upper lip
contains muco-cartilage, as is seen in Fig. 117, the dorsal head-shield of
muco-cartilage ought, strictly speaking, to extend more ventrally in the
drawings. I have curtailed it in order not to interfere with the
representation of the lower lip and tentacular muco-cartilages.

From what has been said, it follows that the past history of the skeletal
covering of the whole head-region of Ammocoetes, both frontal and
occipital, can be conjectured by means of the ontogenetic history of the
foremost myomeres.

Dohrn and all other observers are agreed that during the development of
this animal a striking forward growth of the foremost somatic myomeres
takes place, so that, as Dohrn puts it, the body-musculature has extended
forwards over the gill-region, and at the same time the gill-region has
extended backwards. It is therefore probable that in the ancestral form the
myotomes, innervated by the first spinal nerves, immediately succeeded the
branchial region. Judging from Ammocoetes, the forward growth was at first
confined to the dorsal region, and therefore invaded the dorsal head-plate,
the ventral musculature being distinctly a later growth. With respect to
this dorsal part of the myotomes, the first myotome is originally situated
some distance behind the auditory capsule, and then grows forward towards
the nasal opening; the lateral part, according to Hatschek, grows forward
more quickly than the dorsal part, and splits itself above and below the
eye into a dorso-lateral part, which extends up to the olfactory capsule,
and a ventro-lateral part (_m. lateralis capitis_ anterior, superior, and
inferior), thus giving rise to the characteristic appearance of the
muco-cartilaginous head-shield of Ammocoetes.

According, then, to the extent of the growth of these somatic {338}muscles,
the shape of the muco-cartilaginous head-shield will vary, and if it were
calcified and then fossilized we should obtain fossil head-shields of
widely differing configuration, although such fossils might be closely
allied to each other. This is just what is found in this group. Let the
muco-cartilage extend over the whole of the branchial region of Ammocoetes,
the resulting head-shield would be as in Fig. 135, A; the branchial bars
below the muco-cartilaginous shield might or might not be evident, and the
line between the branchial and the trigeminal region might or might not be
indicated. Such a head-shield would closely resemble those of Didymaspis
and Tremataspis respectively. Now suppose the somatic musculature to
encroach slightly on the branchial region and also laterally to the end of
the anterior branchial region, then we should obtain a shape resembling
that of Thyestes (Fig. 135, B). Continue the same process further, the
lateral muscle always encroaching further than the median masses, until the
whole or nearly the whole branchial region is invested, and we get the
head-shield of Cephalaspis (Fig. 135, C); further still, that of Keraspis,
and yet still further, that of Ammocoetes (Fig. 135, D).

[Illustration: FIG. 135.--DIAGRAMS TO SHOW THE DIFFERENT SHAPES OF
HEAD-SHIELDS DUE TO THE FORWARD GROWTH OF THE SOMATIC MUSCULATURE.

A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocoetes.]

So close is this similarity, from the comparative point of view, between
the dorsal head-shield of the Osteostraci and the dorsal cephalic region of
Ammocoetes that it justifies us in taking Ammocoetes as the nearest living
representative of such types; it is justifiable, therefore, to interpret by
means of Ammocoetes the position of other organs in these forms. First and
foremost is the hard plate {339}known as the post-orbital plate, so
invariably found. In Fig. 134, C, I have inserted (_cr._) the position of
the membranous cranium of Ammocoetes, and it is immediately evident that
the primordial cranium of the Osteostraci must occupy the exact position
indicated by this median hard plate. For this very reason this median plate
would be harder than the rest in order to afford a better protection to the
brain underneath. This plate, because of its position, may well receive the
same name as the similar plate in the trilobite and various palæostracans
and be called the glabellum.


EVIDENCE OF SEGMENTATION IN THE HEAD-SHIELD--FORMATION OF CRANIUM.

We may thus conceive the position of the nose, lateral eyes, median eyes,
and cranium in these old fishes. In addition, other indications of a
segmentation in this head-region have been found. The most striking of all
the specimens hitherto discovered are some of _Thyestes verrucosus_,
discovered by Rohon, in which the dorsal shield has been removed, and so we
are able to see what that dorsal shield covered.

In Fig. 136, I reproduce his drawing of one of his specimens from the
dorsal and lateral aspects. These drawings show that the frontal part of
the shield covered a markedly segmented part of the animal; five distinct
segments are visible apart from the median most anterior region. This
segmented region is entirely confined to the prosomatic region, _i.e._ to
the region innervated by the trigeminal nerve. An indication of similar
markings is given in Lankester's figure of _Eukeraspis pustuliferus_ (see
Fig. 127, B), and, indeed, evidence of a segmentation under the
antero-lateral border of the head-shield is recognized at the present time,
not only in the Cephalaspidæ, but also in the Pteraspidæ, as was pointed
out to me by Smith Woodward in the specimens at the British Museum. Also,
in _Cyathaspis_, Jaekel has drawn attention to markings of a similar
segmental nature (Fig. 137).

There seems, then, little doubt but that these primitive fishes possessed
something in this region which was of a segmental character, and indicated
at least five segments, probably more.

Rohon entitles his discovery 'the segmentation of the primordial cranium.'
It would, I think, be better to call it the segmentation of {340}the
anterior region of the head, for that is in reality what his figures show,
not the segmentation of the primordial cranium, which, to judge from
Ammocoetes, was confined to the region of the glabellum.

What is the interpretation of this appearance?

[Illustration: FIG. 136.--LATERAL AND DORSAL VIEWS OF THE FRONTAL AND
OCCIPITAL REGIONS OF THE HEAD-SHIELD OF THYESTES, AFTER REMOVAL OF THE
OUTER SURFACE. (From ROHON.)]

[Illustration: FIG. 137.--UNDER SURFACE OF HEAD-SHIELD OF CYATHASPIS. (From
JAEKEL.)

_A._, lateral eyes; _Ep._, median eyes.]

Any segmentation in the head-region must be indicative of segments
belonging to the trigeminal or prosomatic region, or of segments belonging
to the vagus or mesosomatic region. Many palæontologists, looking upon
segmentation as indicative of gills and gill-slits, have attempted to
interpret such markings as branchial segments, regardless of their
position. As the figures show, they extend in front of the eyes and reach
round to the front middle line, a position which is simply impossible for
gills, but points directly to a segmentation connected with the trigeminal
nerve. Comparison with Ammocoetes makes it plain enough that the markings
in question are prosomatic in position, and that the gill-region must be
sought for in the place {341}where Schmidt and Rohon located it in
Thyestes, viz. the so-called occipital region.

This discovery of Rohon's is, in my opinion, of immense importance, for it
indicates that, in these early fishes, the prosomatic segmentation,
associated with the trigeminal nerve, was much more well-marked than in any
fishes living in the present day. Why should it be more well-marked?
Turning to the palæostracan, it is very suggestive to compare the markings
on their prosomatic carapace with these markings. Again and again we find
indications of segmentation in these fossils similar to those seen in the
ancient fishes. Thus in Fig. 138 I have put side by side the palæostracan
_Bunodes_ and the fish _Thyestes_, both life size. In the latter I have
indicated Rohon's segments; in the former the markings usually seen.

From the evidence of Phrynus, Mygale, etc., as already pointed out, such
markings in the palæostracan fossils would indicate the position of the
tergo-coxal muscles of the prosomatic appendages, even though such
appendages have not yet been discovered, and it is significant that in all
these cases there is a distinct indication of a median plate or glabellum
in addition to the segmental markings. Especially instructive is the
evidence of Phrynus, as is seen by a comparison of Figs. 107 and 108, which
shows clearly that this median plate (_glab._) covered the brain-region, a
brain-region which is isolated and protected from the tergo-coxal muscles
by the growth dorsalwards of the flanges of the plastron. In this way an
incipient cranium of a membranous character is formed, which helps to give
attachment to these tergo-coxal muscles. As such cranium is derived
directly from the plastron, it is natural that it should ultimately become
cartilaginous, just as occurs when Ammocoetes becomes Petromyzon and the
cartilaginous cranium of the latter arises from the membranous cranium of
the former. In Galeodes also the growth dorsalwards of the lateral flanges
of the plastron to form an incipient cranium in which the brain lies is
very apparent.

[Illustration: FIG. 138.--A, OUTLINE OF _Thyestes Verrucosus_ WITH ROHON'S
SEGMENTS INDICATED; B, OUTLINE OF _Bunodes Lunula_ WITH LATERAL EYES
INSERTED.

Both figures natural size.]

{342}I venture, then, to suggest that in the Osteostraci the median hard
plate or glabellum protected a brain which was enclosed in a membranous
cranium, very probably not yet complete in the dorsal region--certainly not
complete if the median pineal eyes so universally found in these ancient
fishes were functional--a cranium derived from the basal trabeculæ, in
precisely the same manner as we see it already in its commencement in
Phrynus and other scorpions. With the completion of this cranium and its
conversion into cartilage, and subsequently into bone, an efficient
protection was afforded to the most vital part of the animal, and thus the
hard head-shield of the Palæostraca and of the earliest fishes was
gradually supplanted by the protecting bony cranium of the higher
vertebrates.

Step by step it is easy to follow in the mind's eye the evolution of the
vertebrate cranium, and because it was evolved direct from the plastron,
the impossibility of resolving it into segments is at once manifest; for
although the plastron was probably originally segmented, as Schimkéwitsch
thinks, all sign of such segmentation had in all probability ceased, before
ever the vertebrates first made their appearance on the earth.

It follows further, from the comparison here made, that those
antero-lateral markings indicative of segments, found so frequently in
these primitive fishes, must be interpreted as due not to gills but to
aponeuroses, due to the presence of muscles which moved prosomatic
appendages, muscles which arose from the dorsal region in very much the
same position as do the muscles of the lower lip in Ammocoetes; the latter,
as already argued, represent the tergo-coxal muscles of the last pair of
prosomatic appendages--the chilaria or metastoma. Such an interpretation of
these markings signifies that the first-formed fishes must have possessed
prosomatic appendages of a more definite character than the tentacles of
Ammocoetes, something intermediate between those of the palæostracan and
Ammocoetes.

For my part I should not be in the least surprised were I to hear that
something of the nature of appendages in this region had been found,
especially in view of the well-known existence of the pair of appendages in
the members of the Asterolepidæ--large, oar-like appendages which may well
represent the ectognaths.


{343}THE RELATIONSHIP OF THE OSTRACODERMS.

Of the three groups of fishes--the Heterostraci, the Osteostraci, and the
Antiarcha--the last is Devonian, and therefore the latest in time of the
three, while the earliest is the first group, as both Pteraspis and
Cyathaspis have been found in lower levels of the Silurian age than any of
the Osteostraci, and, indeed, Cyathaspis has been discovered in Sweden in
the lower Silurian. This, the earliest of all groups of fishes, is confined
to two forms only--Pteraspis and Cyathaspis,--for Scaphaspis is now
recognized to be the ventral shield of Pteraspis.

Hitherto a strong tendency has existed in the minds both of the comparative
anatomist and the palæontologist to look on the elasmobranchs as the
earliest fishes, and to force, therefore, these strange forms of fish into
the elasmobranch ranks. For this purpose the same device is often used as
has been utilized in order to account for the existence of the
Cyclostomata, viz. that of degeneration. The evidence I have put forward is
very strongly in favour of a connection between the cyclostomes and the
cephalaspids, and agrees therefore with all the rest of the evidence that
the jawless fishes are more ancient than those which bore jaws--the
Gnathostomata.

This is no new view. It was urged by Cope, who classified the Heterostraci,
Osteostraci, and Antiarcha under one big group--the Agnatha--from which
subsequently the Gnathostomata arose. Cope's arguments have not prevailed
up to the present time, as is seen in the writings of Traquair, one of the
chief authorities on the subject in Great Britain. He is still an advocate
of the elasmobranch origin of all these earliest fishes, and claims that
the latest discoveries of the Silurian deposits (_Thelodus Pagei_) and
other members of the Coelolepidæ confirm this view of the question.

This view may be summed up somewhat as follows:--

Cartilaginous jaws would not fossilize, and the Ostracoderms may have
possessed them.

They may have degenerated from elasmobranchs just as the cyclostomes are
supposed to have degenerated.

Seeing that bone succeeds cartilage, the presence of bony shields in
Cephalaspis, etc., indicates that their precursors were cartilaginous,
presumably elasmobranch fishes.

Of these arguments the strongest is based on the supposed bony
{344}covering of the Osteostraci, with the consequent supposition that
their ancestors possessed a cartilaginous covering. This argument is
entirely upset, if, as I have pointed out, the structure of the cephalaspid
shield is that of muco-cartilage and not of bone. If these plates are a
calcified muco-cartilage, then the whole argument for their ancestry from
animals with a cartilaginous skeleton falls to the ground, for
muco-cartilage is the precursor not only of bone, but also of cartilage
itself.

The evidence, then, points strongly in favour of Cope's view that the most
primitive fishes were Agnatha, after the fashion of cyclostomes, as is also
believed by Smith Woodward, Bashford Dean, and Jaekel.

Among living animals, as I have shown, the Limulus is the sole survivor of
the palæostracan type, and Ammocoetes alone gives a clue to the nature of
the cephalaspid, _i.e._ the osteostracan fish. Older than the latter is the
heterostracan, Pteraspis, and Cyathaspis. Is it possible from their
structure to obtain any clue as to the actual passage from the palæostracan
to the vertebrate?

Here again, as in the case of the Osteostraci, a relationship to the
elasmobranch has been supposed, for the following reasons:--

The latest discoveries in the Silurian and Devonian deposits have brought
to light strange forms such as Thelodus and Drepanaspis, of which the
latter from the Devonian must, according to Traquair, be included in the
Heterostraci. It possessed, as seen in Fig. 139, large plates, after the
fashion of Pteraspis, and also many smaller ones.

The former, from the upper Silurian, belongs to the Coelolepidæ, and was
covered over with shagreen composed of small scutes, after the fashion of
an elasmobranch. Traquair suggests that Thelodus arose from the original
elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis
occurred, and, with still further fusion, Pteraspis.

There are always two ways of looking at a question, and it seems to me
possible and more probable to turn the matter round and to argue that the
original condition of the surface-covering was that of large plates, as in
Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was
formed, and later on, by further splitting, the elasmobranch, Thelodus
being a stage on the way to the formation of an elasmobranch, and not a
backward stage from the elasmobranch towards Pteraspis.

{345}This method of looking at the problem seems to me to be more in
consonance with the facts than the reverse; for, as pointed out by Jaekel,
the fishes with large plates are the oldest, and in Cyathaspis, the very
oldest of all, the size of the plates is most conspicuous; he considers,
therefore, this preconceived view that large plates are formed by the
fusion of small ones must give way to the opposite belief.

[Illustration: FIG. 139.--DREPANASPIS. VENTRAL AND DORSAL ASPECTS. (After
LANKESTER.)

_A._, anus; _E._, lateral eyes.]

So also Rohon, as quoted by Traquair, who, in his first paper accepted
Lankester's view that the ridges of the pteraspidian shield were formed by
the fusion of a linear arrangement of numbers of placoid scales, suggests
in his second paper that these ridges may have been the most primitive
condition of the dermal skeleton of the vertebrate, out of which, by
differentiation, the dermal denticles (placoid scales) of the selachian, as
well as their modifications in the ganoids, teleosteans, and amphibians,
have arisen.

One thing is agreed upon on all sides; no sign of bone-corpuscles is to be
found in this dermal covering of Pteraspis. In the deeper layers are large
spaces, the so-called pulp-cavities leading into narrow canaliculi, the
so-called dentine canals. The structure is {346}looked upon as similar to
that of the pulp and dentine canals of many fish-scales.

On the other hand, this dermal covering of Pteraspis has been compared by
Patten with the arrangement of the chitinous structure of certain parts of
the external covering of Limulus, a comparison which to my mind presents a
great difficulty. The chitin-layers in Limulus are _external_ to the
epidermal cells, being formed by them; the layers in Pteraspis which look
like chitin must have been _internal_ to the epidermal layer; for each
vascular canal which passes from a pulp-cavity on its way to be distributed
into the dentine canals of the ridge gives off short side branches, which
open directly into the groove between the ridges. If these canals were
filled with blood they could not possibly open directly into the open
grooves between the ridges; these openings must, therefore, have been
covered over with an epithelial layer which covered over the surface of the
animal, and consequently the chitin-like structure must have been internal
to the epidermis, and not external, as on Patten's view. The comparison of
this structure with the dentine of fish-scales signifies the same thing,
for in the latter the epidermis is external to the dentine-plates, the hard
skeletal structure is in the position of the cutis, not of the cuticle.

The position appears to me to be this: the dermal cranial skeleton of
vertebrates, whether it takes the form of a bony skull or of the dorsal
plates of a cephalaspid or a pteraspid is, in all cases, not cuticular,
_i.e._ is not an external formation of the epidermal cells, but is formed
in tissue of the nature of connective tissue underlying the epidermis. On
the contrary, the hard part of the head-carapace of the palæostracan is an
external formation of the epidermal cells.

If, then, this tissue of Pteraspis is not to be looked upon as chitin, how
can we imagine its formation? It is certainly not bone, for there are no
bone-corpuscles; it is a very regular laminated structure resembling in
appearance chitin rather than anything else.

As in all cases of difficulty, turn to Ammocoetes and let us see what clue
there is to be found there. The skin of Ammocoetes is peculiar among
vertebrates in many respects. It consists of a number of epidermal cells,
as in Fig. 140, the varying function of which need not be considered here,
covered over with a cuticular layer which is extraordinarily thick for the
cuticle of a vertebrate skin; this cuticular layer is perforated with fine
canaliculi, through which the {347}secretion of the underlying cells
passes, as is seen in Fig. 140, A and B. This cuticle corresponds to the
chitinous covering of the arthropod, and like it is perforated with
canaliculi, and, according to Lwoff, possibly contains chitin. The
epidermal cells rest on a thick layer of most striking appearance (Fig.
141), for it resembles, in an extraordinary degree, when examined
superficially, a layer of chitin; it is called the laminated layer, and is
characterized by the extreme regularity of the laminæ. This appearance is
due, as the observations of Miss Alcock show, to alternate layers of
connective tissue fibres arranged at right angles to each other, each fibre
running a straight course and possessing its own nucleus. Although the
fibres in each layer are packed close together, they are sufficiently apart
to form with the fibres of the alternate layers a meshwork rather than a
homogeneous structure, and thus the surface view of this layer shows a
regular network of very fine spaces through which nerve-fibres and fluid
pass. This layer is easily dissolved in a solution of hypochlorite of soda,
a fluid which dissolves chitin. Any one looking at Ammocoetes would say
that the only part of its skin which resembles chitin is this laminated
layer, and therefore the only part of its skin which would afford an
indication of the nature of the skeleton of Pteraspis is this laminated
layer, which belongs to the cutis, and not to the cuticle. Yet another
significant peculiarity of this layer is its entire disappearance at
transformation. Miss Alcock, in a research not yet published, has shown
that this layer is completely broken up and absorbed at transformation; the
cutis of Petromyzon is formed entirely anew, and no longer presents any
regular laminated character, but resembles rather the sub-epidermal
connective tissue layer of the skin of higher vertebrates. This laminated
layer, then, just like the muco-cartilage, shows, by its complete
disappearance at transformation, its ancestral character.

[Illustration: FIG. 140.--EPITHELIAL CELLS OF AMMOCOETES TO SHOW THE
CANALICULI IN THE THICK CUTICLE (B). A, TRANSVERSE SECTION THROUGH THE
CUTICLE.]

Very suggestive is the arrangement of the different skeletal {348}tissues
in the head-region of Ammocoetes. Fig. 141 represents a section through the
head near the pineal eye. Most internally is _a_, a section of the
membranous cranium, then comes _b_, the muco-cartilaginous skeleton, then
_c_, the laminated layer, and finally _d_, the external cuticle. If in
Ammocoetes we possess an epitome of the history of the vertebrate, how
would these layers be represented in the past ages, supposing they could be
fossilized?

[Illustration: FIG. 141.--SECTION OF SKIN AND UNDERLYING TISSUES IN THE
HEAD-REGION OF AMMOCOETES.

_a_, cranial wall; _b_, muco-cartilage; _c_, laminated layer; _d_, external
cuticular layer.]

The most internal layer _a_, by the formation of cartilage and then bone,
represents the great mass of vertebrate fossils; the next layer _b_, by a
process of calcification, as previously argued, represents the head-shield
of the Osteostracan fishes; while the cuticular layer _d_, no longer thin,
is the remnant of the Palæostracan head-carapace. Between these two layers,
_b_ and _d_, lies the laminated layer _c_. Intermediate to the Palæostracan
and the Osteostracan comes the Heterostracan, with its peculiar
head-shield--a head-shield whose origin is more easily conceivable as
arising from something of the nature of the laminated layer than from any
other structure represented in Ammocoetes.

My present suggestion, then, is this: the transition from the skeletal
covering of the Palæostracan to that of the highest vertebrates was brought
about by the calcification of successive layers from without inwards, all
of which still remain in Ammocoetes and show how the external chitinous
covering of the arthropod was gradually replaced by the deep-lying internal
bony cranium of the higher vertebrates.

In Ammocoetes the layer which represents the covering of the
{349}Palæostracan has already almost disappeared. At transformation the
layers representing the stage arrived at by the Heterostracan and the
Osteostracan disappear; but the stage representing the higher vertebrates,
far from disappearing, by the formation of cartilage reaches a higher stage
and prepares the way for the ultimate stage of all--the formation of the
bony cranium.

So much for the evidence as to the nature of the structure of the
head-shield of the Pteraspidæ.

It suggests that these fishes were covered anteriorly with armoured plates
derived from the cutis layer of the skin, a layer which was specially
thickened and very vascular, apparently, to enable respiration to be very
largely, if not entirely, effected by the surface of the body. It is
difficult to understand how the sea-scorpions breathed, and it is easy to
see how the formation of ventral and dorsal plates enclosing the
mesosomatic appendages may at the outset have hindered the action of the
branchiæ. The respiratory chamber, according to my view, had at first the
double function of respiration and digestion. A new digestive apparatus was
the pressing need at the time; it would, therefore, be of distinct
advantage to remove, as much as possible, the burden of respiration from
this incipient alimentary canal.

What can be said as to the shape of these ancient forms of fishes? Certain
parts of them are absolutely known, other parts are guesswork. They are
known to have possessed a dorsal shield, a ventral shield formerly looked
upon as belonging to a separate species, called Scaphaspis, and a spine
attached to the dorsal shield. The rest of their configuration, as given in
Smith Woodward's restoration (Fig. 142) is guesswork; the fish-like body
with its scales, the heterocercal tail, is based on the most insufficient
evidence of something of the nature of scales having being found near the
head-plates.

The dorsal shield is characterized by a pair of lateral eyes situated on
the edge of the shield, not as in Cephalaspis near the middle line. In the
middle line, where the rostrum meets the large dorsal plate, median eyes
were situated. But the slightest sign of any median single nasal opening,
such as is so characteristic of the head-shield of the Osteostraci and of
Ammocoetes has never been discovered. The olfactory organ must have been
situated on the ventral side as in the larval stage of Ammocoetes, or in
the Palæostraca. Many of these head-shields are remarkably well preserved,
{350}and it is difficult to believe that an olfactory opening would not be
seen if any such had existed, as it does in Thyestes.

[Illustration: FIG. 142.--RESTORATION OF PTERASPIS. (After SMITH
WOODWARD.)]

The difficulty of interpreting these types is the difficulty of
understanding their method of locomotion; that is largely the reason why
the spine has been placed as if projecting from the back, and a fish-like
body with a heterocercal tail-fin added. If, on the contrary, the spine is
a terminal tail-spine, then, as far as the fossilized remains indicate, the
animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to
which must be added two apparently lateral pieces and a few scales. If the
animal did not possess a flexible body with a tail-fin, but terminated in a
rigid spike after the fashion of a Limulus-like animal, then it must have
moved by means of {351}appendages. At present we have not sufficient
evidence to decide this question.

That the animal crawled about in the mud by means of free appendages is by
no means an impossible view, seeing how difficult it is to find the remains
of appendages in the fossils of this far-back time, even when we are sure
that they existed. Thus, for many generations, the appendages of
trilobites, which occur in such countless numbers, and in such great
variety of form, were absolutely unknown, until at last, in consequence of
a fortunate infiltration by pyrites, they were found by Beecher preserved
down to the minutest detail. Even to this day no trace of appendages has
been found in such forms as Hemiaspis, Bunodes, Belinurus, Prestwichia.

The whole question of the evidence of any prosomatic appendages in these
ancient fishes is one of very great interest, and of late years has been
investigated by Patten. It has long been known that forms such as
Pterichthys and Bothriolepis possessed two large, jointed locomotor
appendages, and Patten has lately obtained better specimens of Bothriolepis
than have ever been found before, which show not only the general
configuration of the fish, but also the presence of mandibles or gnathites
in the mouth-region resembling those of an arthropod. These mandibles had
been seen before (Smith Woodward), but Patten's specimens are more perfect
than any previously described, and cause him to conclude that these ancient
fish were of the nature of arthropods rather than of vertebrates.

Patten has also been able to obtain some excellent specimens of the under
surface of the head of Tremataspis, which, as evident in Fig. 143, show the
presence of a series of holes, ranging on each side from the mouth-opening,
in a semicircular fashion towards the middle line. He considers that these
openings indicate the attachments of appendages, in opposition to other
observers, such as Jaekel, who look upon them as gill-slits. To my mind,
they are not in the right position for gill-slits; they are certainly in a
prosomatic rather than in a mesosomatic position, and I should not be at
all surprised if further research justified Patten's position. So convinced
is he of the presence of appendages in all these old forms, that he
considers them to be arthropods rather than vertebrates, although, at the
same time, he looks upon them as indicating the origin of vertebrates from
arthropods. Here, perhaps, it is advisable to say a few words on Patten's
attitude towards this question.

{352}Two years after I had put forward my theory of the derivation of
vertebrates from arthropods, Patten published, in the _Quarterly Journal of
Microscopical Science_, simultaneously with my paper in that journal, a
paper entitled "The Origin of Vertebrates from Arachnids." In this paper he
made no reference to my former publications, but he made it clear that
there was an absolutely fundamental difference between our treatment of the
problem; for he took the old view that of necessity there must be a
reversal of surfaces in order that the internal organs should be in the
same relative positions in the vertebrate and in the invertebrate. He
simply, therefore, substituted Arachnid for Annelid in the old theory.
Because of this necessity for the reversal of surfaces he discarded the
terms dorsal and ventral as indicative of the surfaces of an animal, and
substituted hæmal and neural, thereby hopelessly confusing the issue and
making it often very difficult to understand his meaning.

[Illustration: FIG. 143.--UNDER-SURFACE OF HEAD-REGION IN TREMATASPIS.
(After PATTEN.)]

He still holds to his original opinion, and I am still waiting to find out
when the reversal of surfaces took place, for his investigations lead him,
as must naturally be the case, to compare the dorsal (or, as he would call
it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and of the
Pteraspidæ with the dorsal surface of the Palæostraca.

All these ancient fishes are, according to him, still in the arthropod
stage, have not yet turned over, though in a peculiarly unscientific manner
he argues elaborately that they must have swum on their back rather than on
their front, and so indicated the coming reversal. Because they were
arthropods they cannot have had a {353}frontal nose-organ; therefore,
Patten looks upon the nose and the two lateral eyes of the Osteostraci as a
complex median eye, regardless of the fact that the median eyes already
existed.

Every atom of evidence Patten has brought forward, every new fact he has
discovered, confirms my position and makes his still more hopelessly
confused. Keep the animal the right side uppermost, and the evidence of the
rocks confirms the transition from the Palæostracan to the Cyclostome;
reverse the surfaces, and the attempt to derive the vertebrate from the
palæostracan becomes so confused and hopelessly muddled as to throw
discredit on any theory of the origin of vertebrates from arthropods. For
my own part, I fully expect that appendages will be found not only in the
Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue
his researches with increasing success. I feel sure, however, his task will
be much simplified if he abandons his present position and views the
question from my standpoint.


SUMMARY.

  The shifting of the nasal tube from a ventral to a dorsal position, as
  seen in Ammocoetes, is, perhaps, the most important of all clues in
  connection with the comparison of Ammocoetes to the Palæostracan on the
  one hand, and to the Cephalaspid on the other; for, whereas the exact
  counterpart of the opening of such a tube is always found on the dorsal
  head-shield in all members of the latter group, nothing of the kind is
  ever found on the dorsal carapace of the former group.

  The reason for this difference is made immediately evident in the
  development of Ammocoetes itself, for the olfactory tube originates as a
  ventral tube--the tube of the hypophysis--in exactly the same position as
  the olfactory tube of the Palæostracan, and later on in its development
  takes up a dorsal position.

  In fact, Ammocoetes in its development indicates how the Palæostracan
  head-shield became transformed into that of the Cephalaspid.

  In another most important character Ammocoetes indicates its relationship
  to the Cephalaspidæ, for it possesses an external skeleton or head-shield
  composed of muco-cartilage, which is the exact counterpart of the
  so-called bony head-shield of the latter group; and still more strikingly
  the structure of the cephalaspidian head-shield is remarkably like that
  of muco-cartilage. In the one case, by the deposition of calcium salts, a
  hard external skeleton, capable of being preserved as a fossil, has been
  formed; in the other, by the absence of the calcium salts, a soft
  chondro-mucoid matrix, in which the characteristic cells and fibrils are
  embedded, distinguishes the tissue.

  The recognition that the head-shields of these most primitive fishes were
  not composed of bone, but of muco-cartilage, the precursor of both
  cartilage and bone, immediately clears up in the most satisfactory manner
  the whole {354}question of their derivation from elasmobranch fishes; for
  the main argument in favour of the latter derivation is the exceedingly
  strong one that bone succeeds cartilage--not _vice versâ_--therefore,
  these forms, since their head-shield is bony, must have arisen from some
  other fishes with a cartilaginous skeleton, most probably of an
  elasmobranch nature. Seeing, however, that the structure of their shields
  resembles muco-cartilage much more closely than bone, and that Ammocoetes
  forms a head-shield of muco-cartilage closely resembling theirs, there is
  no longer any necessity to derive the jawless fishes from the
  gnathostomatous; but, on the contrary, we may look with certainty upon
  the Agnatha as the most primitive group from which the others have been
  derived.

  The history of the rocks shows that the group of fishes, Pteraspis and
  Cyathaspis, are older than the Cephalaspidæ--come, therefore,
  phylogenetically between the Palæostraca and the latter group. In this
  group the head-shields are of a very different character, without any
  sign of any structure comparable with that of bone, and although they
  possessed both lateral and median eyes, there is never in any case any
  trace of a dorsal nasal orifice. Their olfactory passage, like that of
  the Palæostraca, must have been ventral.

  The remarkable comparison which exists between the head-shields of
  Ammocoetes and Cephalaspis, enables us to locate the position of the
  brain and cranium of the latter with considerable accuracy, and so to
  compare the segmental markings found in many of these fossils with the
  corresponding markings, found either in fossil Palæostraca or on the
  head-carapaces of living scorpions and spiders, such as Phrynus and
  Mygale. In all cases the cranial region was covered with a median plate,
  often especially hard, which corresponded to the glabellum of the
  trilobite; the growth of the cranium can be traced from its beginnings as
  the upturned lateral flanges of the plastron to the membranous cranium of
  Ammocoetes.

  From such a comparison it follows that the segments, found in the
  antero-lateral region of the head-shield, were not segments of the
  cranium, but of parts beyond the region of the cranium, and from their
  position must have been segments supplied by the trigeminal nerve, and
  not by the vagus group; segments, therefore, which did not indicate gills
  and gill-slits, but muscles, innervated by the trigeminal nerve; muscles
  which, as indicated by the corresponding markings on the carapace of
  Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic
  appendages.

  The discovery of the nature of these appendages in the Pteraspidæ and
  Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and
  Bothriolepis), is a problem of the future, though in the latter, not only
  have the well-known oar-like appendages been long since discovered, but
  Patten has recently found specimens of Bothriolepis which throw light on
  the anterior masticating gnathite-like appendages which these ancient
  forms possessed.




{355}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 VIIth nerve passes in and out of capsule.--Evidence
  of Ammocoetes.--Intrusion of glandular mass round brain into auditory
  capsule.--Intrusion of generative and hepatic mass round brain into base
  of flabellum.--Summary.


When speaking of the tripartite arrangement of the cranial nerves, an
arrangement which gave the clue to the meaning of the cranial segments, I
spoke of the trigeminal as supplying the sensory nerves to the skin in the
head-region, and I compared this dorsal system of afferent nerves to the
system of epimeral nerves in Limulus which supply the prosomatic and
mesosomatic carapaces of Limulus with sensory fibres. I compared the
ventral system of eye-muscle nerves with the system of nerves supplying the
segmental dorso-ventral somatic muscles of the prosomatic region, and I
compared the lateral system of mixed nerves with the nerves supplying the
prosomatic and mesosomatic appendages of Limulus. I compared, also, the
optic nerves and the olfactory nerves with the corresponding nerves in the
same invertebrate group. My readers will see at once that one well-marked
group of nerves--the auditory and lateral line system--has been entirely
omitted up to the present, it has not even been mentioned in the scheme of
the cranial segments; I have purposely reserved its consideration until
now, because the organs these nerves supply, though situated in the skin,
are of such a special character {356}as to form a category by themselves.
These nerves cannot be classed among the afferent nerves of the skin any
more than the nerves of the optic and olfactory apparatus; they require
separate consideration. A very extensive literature has grown up on the
subject of this system of lateral line sense-organs and their innervation,
the outcome of which is decisively in favour of this system being classed
with the sense-organs supplied by the auditory nerve, so that in
endeavouring to understand the position of the auditory nerve, we must
always bear in mind that any theory as to its origin must apply to the
system of lateral line nerves as well.

Now, although the auditory apparatus is common to all vertebrates, the
lateral line system is not found in any land-dwelling animals; it belongs
essentially to the fishes, and is, therefore, an old system so far as
concerns the vertebrate group. Its sense-organs are arranged along the
lateral line of the fish, and, in addition, on the head-region in three
well-marked lines known as the supra-orbital, infra-orbital, and mandibular
line systems. These sense-organs lie in the skin in a system of canals, and
are innervated by a special nervous system different to that innervating
adjacent skin-areas. The great peculiarity of their innervation consists in
the fact that their nerves all belong to the branchial system of nerves; no
fibres arise in connection with the trigeminal, but all of them in
connection with the facial, glossopharyngeal and vagus nerves. In other
words, although organs in the skin, their nerve-supply belongs to the
lateral nervous system which supplies splanchnic and not somatic segments,
a system which, according to the theory advanced in this book, originated
in the nerves supplying appendages. The conclusion, therefore, is that in
order to obtain some clue as to the origin of the sense-organs of this
system in the assumed palæostracan ancestor, we must examine the
mesosomatic appendages and see whether they possess any special
sense-organs of similar function.

Further, considering that the auditory organ is to be regarded as a
specially developed member of this system, we must especially look for an
exceptionally developed organ in the region supplied by the auditory nerve.

The question of the origin of this system of lateral line sense-organs
possesses a special interest for all those who attempt to obtain a solution
of the origin of vertebrates, for the upholders of the view that the
vertebrates have descended from annelids have always {357}found its
strongest support in the similarity of two sets of segmental organs found
in annelids and vertebrates. On the one hand, great stress was laid upon
the similarity of the segmental excretory organs in the two groups of
animals, as will be discussed later; on the other, of the similarity of the
segmentally arranged lateral sense-organs.

These lateral sense-organs of the annelids have been specially described by
Eisig in the Capitellidæ, and, according to Lang, "there are many reasons
for considering these lateral organs to be homologous with the dorsal cirri
of the ventral parapodia of other Polychæta, and in the family of the
Glyceridæ we can follow, almost step by step, the transformation of the
cirri into lateral organs." Eisig describes them in the thoracic
prebranchial region as slightly different from those in the abdominal
branchial region; in the latter region, the ventral parapodia are
gill-bearing, so that these lateral organs are in the branchial region
closely connected with the branchiæ, just as is also the case in the
vertebrates. It is but a small step from the gill-bearing ventral parapodia
of the annelid to the gill-bearing appendages of the phyllopod-like
protostracan; so that if we assume that this is the correct line along
which to search for the origin of the vertebrate auditory apparatus, then,
on my theory of the origin of the vertebrates from a group resembling the
Protostraca, it follows that special sense-organs must have existed either
on or in close connection with the branchial and prebranchial appendages of
the protostracan ancestor of the vertebrates, which would form an
intermediate link between the lateral organs of the annelids and the
lateral and auditory organs of the vertebrates.

Further, these special sense-organs could not have been mere tactile hairs,
but must have possessed some special function, and their structure must
have been compatible with that function. Can we obtain any clear conception
of the original function of this whole system of sense-organs?

A large amount of experimental work has been done to determine the function
of the lateral line organs in fishes, and they have been thought at one
time or another to be supplementary organs for equilibration, organs for
estimating pressure, etc. The latest experimental work done by Parker
points directly to their being organs for estimating slow vibrations in
water in contradistinction to the quicker vibrations constituting sound. He
concludes that surface wave-movements, whether produced by air moving on
the water or {358}solid bodies falling into the water, are accompanied by
disturbances which are stimuli for the lateral line organs.

One of these segmental organs has become especially important and exists
throughout the whole vertebrate group, whether the animal lives on land or
in water--this is the auditory organ. Throughout, the auditory organ has a
double function--the function of hearing and the function of equilibration.
If, then, this is, as is generally supposed, a specialized member of the
group, it follows that the less specialized members must possess the
commencement of both these functions, just as the experimental evidence
suggests.

In our search, then, for the origin of the auditory organ of vertebrates,
we must look for special organs for the estimation of vibrations and for
the maintenance of the equilibrium of the animal, situated on the
appendages, especially the branchial or mesosomatic appendages; and,
further, we must specially look for an exceptional development of such
segmental organs at the junction of the prosomatic and mesosomatic regions.

Throughout this book the evidence which I have put forward has in all cases
pointed to the same conclusion, viz. that the vertebrate arose by way of
the Cephalaspidæ from some arthropod, either belonging to, or closely
allied to, the group called Palæostraca, of which the only living
representative is Limulus. If, then, my argument so far is sound, the
appendages of Limulus, both prosomatic and mesosomatic, ought to possess
special sense-organs which are concerned in equilibration or the
appreciation of the depth of the water, or in some modification of such
function, and among these we might expect to find that somewhere at the
junction of the prosoma and mesosoma such sense-organs were specially
developed to form the beginning of the auditory organ.

Now, it is a striking fact that the appendages of Limulus do possess
special sense-organs of a remarkable character, which are clearly not
simply tactile. Thus Gegenbaur, as already stated, has drawn attention to
the remarkable branchial sense-organs of Limulus; and Patten has pointed
out that special organs, which he considers to be gustatory in function,
are present on the mandibles of the prosomatic appendages. I myself, as
mentioned in my address to the British Association at Liverpool in 1896,
searched for some special sense-organ at the junction of the prosoma and
mesosoma, and was rewarded by finding that that extraordinary adjunct to
the {359}last locomotor appendage, known as the flabellum, was an elaborate
sense-organ. I now propose to show that all these special sense-organs are
constructed on a somewhat similar plan; that the structure of the branchial
sense-organs suggests that they are organs for the estimation of water
pressures; that among air-breathing arthropods sense-organs, built up on a
somewhat similar plan, are universally found, and are considered to be of
the nature of auditory and equilibration organs; and, what is especially of
importance, in view of the fact that the most prominent members of the
Palæostraca were the sea-scorpions, that the remarkable sense-organs of the
scorpions known as the pectens belong apparently to the same group.


THE PORIFEROUS SENSE-ORGANS OF THE APPENDAGES IN LIMULUS.

On all the branchial appendages in Limulus, special sense-organs are found
of a most conspicuous character. They form in the living animal bluish
convex circular patches, the situation of which on the appendages is shown
in Fig. 58. These organs are not found on the non-branchial operculum.
Gegenbaur, who was the first to describe them, has pointed out how the
surface of the organ is closely set with chitinous goblets shaped as seen
in Fig. 144, A, which do not necessarily project free on the surface, but
are extruded on the slightest pressure. Each goblet fits into a socket in
the chitinous covering, and is apparently easily protruded by variations of
pressure from within. The whole surface of the organ on the appendage is
slightly bulged in the living condition, and the chitin is markedly softer
here than in the surrounding part of the limb. Each of these organs is
surrounded by a thick protection of strongly branching spines. On the
surface of the organ itself no spines are found, only these goblets, so
that the surface-view presents an appearance as in Fig. 144, B. Each goblet
possesses a central pore, which is the termination of a very fine, very
tortuous, very brittle chitinous tubule (_ch.t._), which passes from the
goblet through the layers of the chitin into the subjacent tissue. The
goblets vary considerably in size, a few very large ones being scattered
here and there. The fine chitinous tubule is especially conspicuous in
connection with these largest goblets. In the smaller ones there is the
same appearance of a pore and a commencing tube, but I have not been able
to trace the tube through the chitinous layers, as in the case of the
larger goblets.

{360}[Illustration: FIG. 144.--A, A GOBLET FROM ONE OF THE BRANCHIAL
SENSE-ORGANS OF LIMULUS (_ch.t._, chitinous tubule); B, SURFACE VIEW OF A
PORTION OF A BRANCHIAL SENSE-ORGAN.]

[Illustration: FIG. 145.--THE ENDOGNATHS OF LIMULUS PUSHED OUT OF THE WAY
ON ONE SIDE IN ORDER TO SHOW THE POSITION OF THE FLABELLUM (_fl._)
PROJECTING TOWARDS THE CRACK BETWEEN THE PROSOMATIC AND MESOSOMATIC
CARAPACES.]

Gegenbaur, in his picture, draws a straight tubule passing from every
goblet among the fine canaliculi of the chitin. He says they are difficult
to see, except in the case of the larger goblets. The tubule from the
larger goblets is most conspicuous, and is in my sections always tortuous,
never straight, as represented by Gegenbaur. A special branch of the
appendage-nerve passes to these organs, and upon the fine branches of this
nerve groups of ganglion-cells are seen, very similar in appearance to the
groups described by Patten on the terminal branches of the nerves which
supply the mandibular organs. At present I can see no mechanism by which
the goblets are extruded or returned into place. In the case of the
Capitellidæ, Eisig describes retractor muscles by means of which the
lateral sense-organs are {361}brought below the level of the surface, and
he imagines that the protrusion is effected by hydraulic means, by the aid
of the vascular system. In the branchial sense-organs of Limulus there are
no retractor muscles, and it seems to me that both retraction and
protrusion must be brought about by alterations of pressure in the vascular
fluids. Certainly the cavity of the organ is very vascular. If this be so,
it seems likely enough that such an organ should be a very delicate organ
for estimating changes in the pressure of the external medium, for the
position of the goblets would depend on the relation between the pressure
of the fluid inside the organ and that on the surface of the appendage.
Whether the chitinous tubule contains a nerve-terminal or not I am unable
to decide from my specimens, but, judging from Patten's description of the
similar chitinous tubules belonging to the mandibular organs, it is most
highly probable that these tubules also contain a fine terminal
nerve-fibre.

These organs, then, represent segmental branchial sense-organs, of which it
can be said their structure suggests that they may be pressure-organs; but
the experimental evidence is at present wanting.

Passing now from the branchial to the prosomatic region, the first thing
that struck me was the presence of that most conspicuous projection at the
base of the last locomotor appendage, which is usually called the
flabellum, and has been described by Lankester as an exopodite of this
appendage. It is jointed on to the most basal portion of the limb (_cf._
Fig. 155), and projects dorsally from the limb into the open slit between
the prosomatic and mesosomatic carapace, as is seen in Fig. 145 (_fl._). Of
its two surfaces, the undermost is very convex and the uppermost nearly
flat from side to side, the whole organ being bent, so that when the animal
is lying half buried in sand, entirely covered over by the prosomatic and
mesosomatic carapaces except along this slit between the two, the upper
flat or slightly convex surface of the flabellum is exposed to any movement
of water through this slit, and owing to its possessing a joint, the
direction of the whole organ can be altered to a limited extent. The whole
of this flat upper surface is one large sense-organ of a striking
character, thus forming a great contrast to the convex under surface, which
is remarkably free from tactile spines or special sense-organs.

The nerve going to the flabellum is very large, almost as large as the
nerve to the rest of the appendage, and the very large majority {362}of the
nerve-fibres turn towards the flat, uppermost side, where the sense-organ
is situated. Between the nerve-fibres (_n._) and the chitinous surface
containing the special sense-tubes masses of cells (_gl._) are seen, as in
Fig. 146, apparently nerve-cells, which form a broad border between the
nerve-fibres and the pigmented chitinogenous layer (_p._). On the opposite
side, nothing of the sort intervenes between the pigmented layer and the
blood-spaces and nerve-fibres which constitute the central mass of the
flabellum.

[Illustration: FIG. 146.--SECTION THROUGH FLABELLUM.

_ch._, chitinous layers; _s.o._, sense-organs; _sp._, spike-organ; _p._,
pigment layer; _gl._, ganglion cell layer; _bl._ and _n._, blood-spaces and
nerves.]

[Illustration: FIG. 147.--SECTION PARALLEL TO THE SURFACE OF FLABELLUM,
SHOWING THE POROUS TERMINATIONS OF THE SENSE-ORGANS AND THE ARRANGEMENT OF
THE CANALICULI ROUND THEM.]

At present I am inclined to look upon this mass of cells as constituting a
large ganglion, which extends over the whole length and breadth of the
upper surface of the flabellum. At the same {363}time, my preparations are
not sufficiently clear to enable me to trace out the connections of these
cells, especially their connections with the special sense-organs.

[Illustration: FIG. 148.--SECTION THROUGH THE THREE SENSE-ORGANS OF
FLABELLUM.

_bl._, blood-spaces; _n._, nerve; _gl._, layer of ganglion-cells; _p._,
pigment layer; _ch._, 1, 2, 3, the three layers of chitin; _ch.t._,
chitinous tubule in large tube of sense-organ; _cap._, capitellum or
swollen extremity of large tube; _can._, very fine porous canals or
canaliculi of chitin.]

In Fig. 148 I give a magnified representation of a section through three of
these flabellar sense-organs. As is seen, the section divides itself into
four zones: (1) the chitinous layer (_ch._); (2) the layer of pigment
(_p._) and hypodermal cells; (3) the layer of ganglion-cells (_gl._); and
(4) the layer of nerve-fibres (_n._) and blood-spaces (_bl._). The
chitinous layer is composed of the usual three zones of the Limulus
surface--externally (Fig. 148), a thin homogeneous layer, followed by a
thick layer of chitin (3), in which the fine vertical tubules or canaliculi
are well marked; the external portion (2) of this layer is differentiated
from the rest by the presence of well-marked horizontal layers in addition
to the canaliculi.

In this chitinous layer the special sense-organs are found. They consist of
a large tube which passes through all the layers of the chitin except the
thin homogeneous most external layer. {364}This tube is conical in shape,
its base, which rests on the pigmented layer, being so large and the organs
so crowded together that a section of the chitin across the base of the
tubes gives the appearance of a honeycomb, the septa of which is all that
remains of the chitin. This large tube narrows down to a thin elongated
neck as it passes through the chitin, and then, at its termination, bulges
out again into an oval swelling (_cap._) situated always beneath the
homogeneous most external layer of chitin. Within this tube a fine
chitinous tubule (_ch. t._) is situated similar to that seen in the
branchial sense-organs; it lies apparently free in the tube, not straight,
but sinuous, and it passes right through all the chitinous layers to open
at the surface as a pore; in the last part of its course, where it passes
through the most external layer (1) of chitin, it lies always at right
angles to the surface.

If the flabellum be stained with methylene blue and acid fuchsin, then all
the canaliculi in the chitin show up as fine red lines, and present the
appearance given in Fig. 148, and it is seen that each of the terminations
of the tubules is surrounded in the homogeneous layer of chitin by a
thick-set circular patch of canaliculi which pass to the very surface of
the chitin, while the canaliculi in other parts terminate at the
commencement of the homogeneous layer and do not reach the surface.
Further, the contents of the oval swelling, and, indeed, of the tube as a
whole, are stained blue, the chitinous tubule being either unstained or
slightly pink in colour. We see, then, that the chitinous tubule alone
reaches the surface, while the large tube, which contains the tubule,
terminates in an oval swelling, which often presents a folded or wrinkled
appearance, as in Fig. 149 (see also Patten's Fig. 1, Plate I.). This
terminal bulging of the tube is reminiscent of the bulging in the chitinous
tubes of the lyriform organs of the Arachnida, as described by Gaubert, and
of the poriferous chordotonal organs in insects, as described by Graber
(see Fig. 150). This terminal swelling is filled with a homogeneous
refringent mass staining blue with methylene blue, in which I have seen no
trace of a nucleus; through this the chitinous tubule makes its way without
any sign of bulging on its part. Patten, in his description of the
sense-organs on the mandibles of Limulus, which are evidently the same in
structure as those on the flabellum, refers to this homogeneous mass as a
coagulum. I doubt whether this is an adequate description; it appears to me
to stain rather more {365}readily than a blood-coagulum, yet in the sense
of being structureless it resembles a coagulum.

The enormous number of these organs crowded together over the whole flat
surface of the flabellum produces a very striking appearance when viewed on
the surface. Such a view presents an appearance resembling that of the
surface-view of the branchial sense-organs; in both cases the surface is
covered with a great number of closely set circular plaques, in the centre
of each of which is seen a well-marked pore. The circular plaques in the
case of the flabellum are much smaller than those of the branchial
sense-organs, and clearly are not protrusible as in the latter organs, the
appearance as of a plaque being due to the ring of thickly-set canaliculi
round the central tubule, as already described. When stained with methylene
blue, the surface view of the flabellum under a low power presents an
appearance of innumerable circular blue masses, from each of which springs
a fine bent hair, terminating in a pore at the surface. The blue masses are
the homogeneous substance (_cap._) of the bulgings seen through the
transparent external layer of chitin, and the hairs are the terminal part
of the chitinous tubules. Patten has represented their appearance in the
mandibles in his Fig. 2, Plate I.

The large tubes in the chitin alter in shape according to their position.
Those in the middle of the sensory surface of the flabellum, in their
course through the chitinous layers, are hardly bent at all; as they
approach the two lateral edges of this surface, their long thin neck
becomes bent more and more, the bending always being directed towards the
middle of the surface (see Fig. 146); in this way the chitinous tubules
increase more or less regularly in length from the centre of the organ to
the periphery. The large basal part of the conical tube contains, besides
the chitinous tubule, a number of nuclei which are confined to this part of
the tube; some of these nuclei look like those belonging to nerve-fibres,
others are apparently the nuclei of the chitinogenous membrane lining the
tube. I have never seen any sign of nerve-cells in the tube itself.

The only other kind of sense-organ I have found in connection with these
sense-organs are a few spike-like projections, the appearance of which is
given in Fig. 149. I have always seen these in the position given in Fig.
146 (_sp._), _i.e._ at the junction of the surface which contains the
sense-organs and the surface which is free from them. They are, so far as I
have seen, not very numerous; I have {366}not, however, attempted to
examine the whole sense-organ for the purpose of estimating their number
and arrangement.

As is seen in Fig. 149, they possess a fine tubule of the same character as
that of the neighbouring sense-organs, which apparently terminates at the
apex of the projecting spike. They appear to belong to the same group as
the other poriferous sense-organs, and are of special interest, because in
their appearance they form a link between the latter and the poriferous
sense-organs which characterize the pecten of the scorpion (_cf._ Fig. 152,
C).

[Illustration: FIG. 149.--SPIKE-ORGAN OF FLABELLUM.

_ch.t._, chitinous tubule.]

Such, then, is the structure of this remarkable sense-organ of the
flabellum, as far as I have been able to work it out with the materials at
my disposal. It is evident that the flabellar organs, apart from the
spike-organs, are of the same kind as those described by Patten on the
mandibles and chelæ of Limulus, and therefore it is most probable that the
nerve-terminals in the chitinous tubules, and the origin of the latter, are
similar in the two sets of organs.

These organs, as Patten has described them, are situated in lines on the
spines of the mandibles of the prosomatic locomotor appendages, and are
grouped closely together to form a compact sense-organ on the surface of
the inner mandible (Lankester's epicoxite) (_i.m._ in Fig. 155), so that a
surface-view of the organ here gives the characteristic appearance of these
poriferous sense-patches. Precisely similar organs are found on the
chilaria, which are, in function at all events, simply isolated mandibles,
to use Patten's terminology.

On the digging appendage (ectognath), as the comparison of Fig. 155, A and
C, shows, the mandibular spines are almost non-existent, and the inner
mandible or epicoxite is not present, so that {367}the special sense-organ
of this appendage is represented solely by the flabellum.

This sketch of the special sense-organs of Limulus shows that all the
appendages of Limulus possess special sense-organs, with the exception of
the operculum. All these sense-organs are formed on the same plan, in that
they possess a fine chitinous tubule passing through the layers of chitin
into the underlying hypodermal and nervous tissues, which terminates on the
surface in a pore. The surface of the chitin where these pores are situated
is perfectly smooth, although, in the case of the branchial sense-organs,
the goblet-shaped masses of chitin, each of which contains a pore, are able
to be pressed out beyond the level of the surface.

As to their functions, we unfortunately do not know much that is definite.
Patten considers that he has evidence of a gustatory function in the case
of the mandibular organs, and suggests also a temperature-sense in the case
of some of these organs. The large organ of the flabellum and the branchial
organs he has not taken into consideration. The situation of these organs
puts the suggestion of any gustatory function, as far as they are
concerned, out of the question; and I do not think it probable that such
large specialized organs would exist only for the estimation of
temperature, when one sees how, in the higher animals, the
temperature-nerves and the nerves of common sensation are universally
distributed over the body. As already stated, the structure of the
branchial organs seems to me to point to organs for estimating varying
pressures more than anything else, and I am strongly inclined to look upon
the whole set of organs as the derivatives of the lateral sense-organs of
annelids, such as are described by Eisig in the Capitellidæ. This is
Patten's opinion with respect to the mandibular organs; and from what I
have shown, these organs cannot be separated in type of structure from
those of the flabellum and the branchial sense-organs.

In our search, then, for the origin of the vertebrate auditory organ in
Limulus and its allies, we see so far the following indications:--

1.  The auditory organ of the vertebrate is regarded as a special organ
belonging to a segmentally arranged set of lateral sense-organs, whose
original function was co-ordination and equilibration.

2.  Such a set of segmentally arranged lateral sense-organs is found in
annelids in connection with the dorsal cirri of the ventral parapodia.

{368}3.  If, as has been supposed, there is a genetic connection between
(1) and (2) and if, as I suppose, the vertebrates did not arise from the
annelids directly, but from a protostracan group, then it follows that the
lateral sense-organs, one of which gave rise to the auditory organ, must
have been situated on the protostracan appendages.

4.  In Limulus, which is the sole surviving representative of the
palæostracan group, such special sense-organs are found on both the
prosomatic and mesosomatic appendages, and therefore may be expected to
give a direct clue to the origin of the vertebrate auditory organ.

5.  Both from its position, its size, and its specialization, the
flabellum, _i.e._ an organ corresponding to the flabellum, must be looked
upon as more likely to give a direct clue to the origin of the auditory
organ than the sense-organs of the branchial appendages, or the so-called
gustatory organs of the mandibles.


THE AUDITORY ORGANS OF ARACHNIDS AND INSECTS.

The difficulty of the investigating these organs consists in the fact that
so little is known about them in those Arthropoda which live in the water;
the only instance of any organ apparently of the nature of an auditory
organ, is the pair of so-called auditory sacs at the base of the antennæ in
various decapods. We are in a slightly better position when we turn to the
land-living arthropods; here the presence of stridulating organs in so many
instances carries with it the necessity of an organ for appreciating sound.
It has now been shown that such stridulating organs are not confined to the
Insecta, but are present also in the scorpion group, and I myself have
added to their number by the discovery of a distinct stridulating apparatus
in various members of the Phrynidæ. We may then take it for granted that
arachnids as well as insects hear. Where is the auditory organ?

Many observers believe that certain surface-organs found universally among
the spiders, to which Gaubert has given the name of lyriform organs, are
auditory in function. His investigations show that they are universally
present on the limbs and pro-meso-sternite of all spiders; that they are
present singly, not in groups, on the limbs of Thelyphonus, and that a
group of them exists on the second segment of each limb in the members of
the Phrynus tribe. In the latter case this organ is the most elaborate of
all described by him.

{369}It is especially noticeable that they do not exist in Galeodes or in
the scorpions, but in the former special sense-organs are found in the
shape of the so-called 'racquet-organs,' on the basal segments of the most
posterior pair of appendages, and also, according to Gaubert, on the
extremity of the palps and the first pair of feet, while in the latter they
occur in the shape of the pectens.

This observation of Gaubert suggests that the place of the lyriform organs
in other arachnids is taken in Galeodes by the racquet-organs, and in the
scorpions by the pectens. Bertkau, Schimkéwitsch, and Wagner, as quoted by
Gaubert, all suggest that the lyriform organs of the arachnids belong to
the same group of sense-organs as the porous chordotonal organs of the
Insecta, sense-organs which have been found in every group of Insecta, and
are generally regarded as auditory organs. Gaubert does not agree with
this, and considers the lyriform organs to be concerned with the
temperature-sense rather than with audition.

The chordotonal organs of insects have been specially studied by Graber. He
divides them into two groups, the poriferous and the non-poriferous, the
former being characterized by the presence of pores on the surface arranged
in groups or lines. These poriferous chordotonal organs are remarkably
constant in position, being found only at the base of the wings on the
subcostal ridge, in marked contrast to the other group of chordotonal
organs which are found chiefly on the appendages in various regions. The
striking character of this fixity of position of these organs and the
universality of their presence in the whole group, led Graber to the
conclusion that in these poriferous chordotonal organs we are studying a
form of auditory apparatus which characterized the ancestor of the
insect-group. These organs are always well developed on the hind wings, and
in the large group of Diptera the auditory apparatus has usurped the whole
of the function of the wing; for the balancers or 'halteres,' as they are
called, are the sole representatives of the hind wings, and they are
usually considered to be of the nature of auditory organs. It is
instructive to find that such an auditory organ serves not only for the
purpose of audition, but also as an organ of equilibration; thus Lowne
gives the evidence of various observers, and confirms it himself, that
removal of the balancers destroys the power of orderly flight in the
animal.

A striking peculiarity of these organs in the Insecta, as described {370}by
Graber, is the bulging of the porous canal near its termination (Fig. 150,
C). This bulging is filled with a homogeneous, highly refractive material,
from which, according to Lowne, a chordotonal thread passes, to be
connected with a ganglion-cell and nerve. This sphere of refractive
material he calls the 'capitellum' of the chordotonal thread. The presence
of this material produces in a surface view an appearance as of a halo
around the terminal plaque with its central pore; Graber has attempted to
represent this by the white area round the central area (in Fig. 150, B). A
very similar appearance is presented by the surface view of the flabellum
in those parts where the tube runs straight to the surface, so that the
refractive material which fills the oval bulging shines through the
overlying chitin and appears to surround the terminal plaque with a
translucent halo.

[Illustration: FIG. 150 (from GRABER).--A, SECTION OF SUBCOSTAL NERVURE OF
HIND WING OF DYTISCUS TO SHOW PATCH OF PORIFEROUS ORGANS (_s.o._). B,
SURFACE VIEW OF PORIFEROUS ORGANS; THE WHITE SPACE ROUND EACH ORGAN
INDICATES THE DEEPER LYING REFRINGENT BODY WHICH FILLS THE BULGING OF THE
CANAL SEEN IN TRANSVERSE SECTION IN C.]

Such a peculiarity must have a very definite meaning, and suggests that the
canals in the flabellum of Limulus and in the hind wings of insects belong
to the same class of organ, the chitinous tubule with its nerve-terminal in
the former corresponding to the chordotonal thread in the latter. One
wonders whether this sphere of refractive material or 'capitellum' (to use
Lowne's phraseology) is so universally present in order to act as a damper
upon the vibrations of the chordotonal thread in the one case and of the
{371}chitinous tubule in the other, just as the _membrana tectoria_ and the
otoliths act in the case of the vertebrate ear.

Patten says that the only organs which seem to him to be comparable with
the gustatory porous organs of Limulus are the sense-organs in the
extremities of the palps and of the first pair of legs of Galeodes, as
described by Gaubert. I imagine that he was thinking only of arachnids, for
the comparison of his drawings with those of Graber show what a strong
family resemblance exists between the poriferous sense-organs of Limulus
and those of the insects. On the course of the terminal nerve-fibres,
between the nerve-cell and their entrance into the porous chitinous canal,
Graber describes the existence of rods or scolophores. On the course of the
terminal fibres in the Limulus organ, between the nerve-cells and their
entrance into the porous chitinous canal, Patten describes a spindle-shaped
swelling, containing a number of rod-like thickenings among the fibrils in
the spindle, which present an appearance reminiscent of the rods described
by Graber.

It appears as though a type of sense-organ, characterized by the presence
of pores on the surface and a fine chitinous canal which opens at these
pores, was largely distributed among the Arthropoda. According to Graber,
this kind of organ represents a primitive type of sense-organ, which was
probably concerned with audition and equilibration, and he expresses
surprise that similar organs have not been discovered among the Crustacea.
It is, therefore, a matter of great interest to find that so ancient a type
of animal as Limulus, closely allied to the primitive crustacean stock,
_does_ possess poriferous sense-organs upon its appendages which are
directly comparable with these poriferous chordotonal organs of the
Insecta.


THE PECTENS OF SCORPIONS.

Among special sense-organs such as those with which I am now dealing, the
pectens of scorpions and the 'racquet-organs' of Galeodes must, in all
probability, be classed. I have given my reasons for this conclusion in my
former paper.[2] At present such reasons are based entirely upon the
structure of the organs; experimental {372}evidence as to their function is
entirely wanting. With respect to the pectens of the scorpion (Fig. 151),
it has been suggested that they are of the nature of copulatory organs, a
suggestion which may be dismissed without hesitation, for they are not
constructed after the fashion of claspers, but are simply elaborate
sense-organs, and, as such, are found equally in male or female. The only
observer who has hitherto specially studied the structure of the
sense-organs in the pecten is, as far as I know, Gaubert, and he describes
their structure together with that of the sense-organs of the racquets of
Galeodes, in connection with the lyriform organs of arachnids, as though he
recognized a family resemblance between the three sets of organs.

[Illustration: FIG. 151.--UNDER SURFACE OF SCORPION (ANDROCTONUS).

The operculum is marked out with dots, and on each side of it is seen one
of the pectens.]

The pecten of the scorpions is an elaborate sense-organ, or rather group of
sense-organs, the special organ being developed on each tooth of the comb;
its surface, which is frequently flattened, being directed backwards and
inwards, when the axis of the pecten is horizontal at right angles to the
length of the body. The surface view of this part of the tooth resembles
that of the branchial organs or of the flabellum in Limulus, in that it is
thickly covered with circular patches, in the centre of which an
ill-defined appearance as of a fine pore is seen. In Fig. 152, B, I give a
sketch of the surface view of a part of the organ.

Transverse sections of a tooth of the comb of _Scorpio Europæus_ present
the appearance given in Fig. 152, A, and show that each of these circular
patches is the surface-view of a goblet-shaped chitinous organ, Fig. 152,
C, from the centre of which a short, somewhat cylindrical chitinous spike
projects. Within this spike, and running through the goblet into the
subjacent tissue, is a fine tubule. The series of goblets gives rise to the
appearance of the circular plaques on the surface-view, while the spike
with its tubule {373}is the cause of the ill-defined appearance of the
central pore, just as the terminal pore is much less conspicuous on
surface-view in the spike-organs of the flabellum than in the purely
poriferous organs, no part of which projects beyond the level of the
chitinous surface.

[Illustration: FIG. 152.--A, SECTION THROUGH TOOTH OF PECTEN OF SCORPION;
B, SURFACE VIEW OF SENSE-ORGANS; C, GOBLET OF SENSE-ORGAN MORE HIGHLY
MAGNIFIED.

_bl._ and _n._, region of blood-spaces and nerves; _gl._, ganglion-cell
layer; _ch._, modified chitinous layer; _s.o._, sense-organ.]

The fine tubule is soon lost in the thickened but soft modification of the
chitinous layer (_ch._) which is characteristic of the sense-organ; at all
events, I have not succeeded in tracing it through this layer with any more
success than in the corresponding case of the tubules belonging to the
smaller goblets of the branchial sense-organ of Limulus already described.

At the base of the modified chitinous layer a series of cells is seen,
many, if not all, of which belong to the chitinogenous layer. Next to these
is the marked layer of ganglion-cells (_gl._), similar to those seen in the
flabellum of Limulus. The rest of the space in the section of the tooth is
filled up with nerves (_n._) and blood-spaces (_bl._) just as in the
section, Fig. 146, of the flabellum of Limulus.

Gaubert does not appear to have seen the goblets at all clearly; {374}he
describes them simply as conical eminences, and states that they
"recouvrent un pore analogue a celui des poils mais plus petit; il est
rempli par le protoplasma de la couche hypodermique." From the ganglion,
according to him, nervous prolongations pass, which traverse the
chitinogenous layer and terminate at the base of the conical eminences.
Each of these prolongations "présente sur son trajet, mais un peu plus près
du ganglion que de sa terminaison périphérique, une cellule nerveuse
fusiforme (_g._) offrant, comme celles du ganglion, un gros noyau." He
illustrates his description with the following, Fig. 153, taken from his
paper.

[Illustration: FIG. 153 (from GAUBERT).--SECTION OF A TOOTH OF PECTEN OF
SCORPION.

_n._, nerve; _gl._, ganglion.]

I have not been able to obtain any evidence of a fusiform nerve-cell on the
course of the terminal nerve-fibres as depicted by him; fusiform cells
there are in plenty, as depicted in my drawing, but none with a large
nucleus resembling those of the main ganglion. In no case, either in the
flabellum or in the branchial organs of Limulus, or in the pecten-organs,
have I ever seen a ganglion-cell within the chitin-layer; all the nuclei
seen there resemble those of the cells of the hypodermis or else the
elongated nuclei characteristic of the presence of nerve-fibres. Gaubert's
drawing is a striking one, and I have looked through my specimens to see
whether there was anything similar, but have hitherto failed to obtain any
definite evidence of anything of the kind.

I feel, myself, that an exhaustive examination of the structure and
function of the pecten of scorpions ought to be undertaken. At present I
can only draw the attention of my readers to the similarity of the
arrangement of parts, and of the nature of the end-organs, in the
sense-organs of the flabellum of Limulus and of the pecten of the scorpion.
In both cases the special nerve-fibres terminate in a massive ganglion,
situated just below the chitinogenous layer. In both cases the terminal
fibres from these ganglion-cells pass through the modified chitinous layer
to supply end-organs of a striking character; and although the end-organ of
the pecten of the scorpion does {375}not closely resemble the majority of
the end-organs of the flabellum, yet it does resemble, on the one hand, the
isolated poriferous spikes found on the flabellum (Fig. 149) and, on the
other, the poriferous goblets found on the sense-patches of the branchial
appendages of Limulus (Fig. 144, A), so that a combination of these two
end-organs would give an appearance very closely resembling that of the
pecten of the scorpion.

Finally, the special so-called 'racquet-organs' of Galeodes, which are
found on the most basal segments of the last pair of prosomatic appendages,
ought also to be considered here. Gaubert has described their structure,
and shown how the nerve-trunk in the handle of the racquet splits up into a
great number of separate bundles, which spread out fan-shaped to the free
edge of the racquet; each of these separate bundles supplies a special
sense-organ, which terminates as a conical eminence on the floor of a deep
groove, running round the whole free edge of the racquet. This groove is
almost converted into a canal, owing to the projection of its two sides.
Gaubert imagines that the sense-organs are pushed forward out of the groove
to the exterior by the turgescence of the whole organ; each of the
nerve-fibres forming a bundle is, according to Gaubert, connected with a
nerve-cell before it reaches its termination.

This sketch of the special sense-organs on the appendages of Limulus, of
the scorpions, of Galeodes, and other arachnids, and their comparison with
the porous chordotonal organs of insects, affords reason for the belief
that we are dealing here with a common group of organs, which, although
their nature is not definitely known, have largely been accredited with the
functions of equilibration and audition, a group of organs among which the
origin of the auditory organ of vertebrates must be sought for, upon any
theory of the origin of vertebrates from arthropods.

Whenever in any animal these organs are concentrated together to form a
special organ, it is invariably found that the nerve going to this organ is
very large, out of all proportion to the size of the organ, and also that
the nerve possesses, close to its termination in the organ, large masses of
nerve-cells. Thus, although the whole hind wing in the blow-fly has been
reduced to the insignificant balancers or 'halteres,' yet, as Lowne states,
the nerves to them are the largest in the body.

The pectinal nerve in the scorpion is remarkable for its size, and {376}so,
also, is the nerve to the flabellum in Limulus, while the large size of the
auditory nerve in the vertebrate, in distinction to the size of the
auditory apparatus, has always aroused the attention of anatomists.

Throughout this book my attention has been especially directed to both
Limulus and the scorpion group in endeavouring to picture to myself the
ancestor of the earliest vertebrates, because the Eurypteridæ possessed
such marked scorpion-like characteristics; so that in considering the
origin of a special sense-organ, such as the vertebrate auditory organ near
the junction of the prosoma and mesosoma, it seems to me that the presence
of such marked special sense-organs as the flabellum on the one hand and
the pecten on the other, must both be taken into account, even although the
former is an adjunct to a prosomatic appendage, while the latter
represents, according to present ideas, the whole of a mesosomatic
appendage.

From the point of view that the VIIIth nerve represents a segment
immediately posterior to that of the VIIth, it is evident that an organ in
the situation of the pecten, immediately posterior to the operculum, _i.e._
according to my view, posterior to the segment originally represented by
the VIIth nerve, is more correctly situated than an organ like the
flabellum, which belongs to a segment anterior to the operculum.

On the other hand, from the point of view of the relationship between the
scorpions and the king-crabs, it is a possibly debatable question whether
the pecten really belongs to a segment posterior to the operculum. The
position of any nerve in a series depends upon its position of origin in
the central nervous system, rather than upon the position of its peripheral
organ. Now, Patten gives two figures of the brain of the scorpion built up
from serial sections. In both he shows that the main portion of the
pectinal nerve arises from a swelling, to which he gives the name _ganglion
nodosum_. This swelling arises on each side in close connection with the
origin of the most posterior prosomatic appendage-nerve, according to his
drawings, and posteriorly to such origin he figures a small nerve which he
says supplies the distal parts of the sexual organs. This nerve is the only
nerve which can be called the opercular nerve, and apparently arises
posteriorly to the main part of the pectinal nerve. If this is so, it would
indicate that the pectens arose from sense-organs which were originally,
like the flabella, pre-opercular in position, but have shifted to a
post-opercular position.


{377}THE ORIGIN OF THE PARACHORDALS AND AUDITORY CARTILAGINOUS CAPSULE.

In addition to what I have already said, there is another reason why a
special sense-organ such as the pecten is suggestive of the origin of the
vertebrate auditory organ, in that such a suggestion gives a clue to the
possible origin of the parachordals and auditory cartilaginous capsules.

In the lower vertebrates the auditory organ is characterized by being
surrounded with a cartilaginous capsule which springs from a special part
of the axial cartilaginous skeleton on each side, known as the pair of
parachordals. The latter, in Ammocoetes, form a pair of cartilaginous bars,
which unite the trabecular bars with the branchial cartilaginous
basket-work. They are recognized throughout the Vertebrata as distinct from
the trabecular bars, thus forming a separate paired cartilaginous element
between the trabeculæ and the branchial cartilaginous system, which of
itself indicates a position for the auditory capsule between the prosomatic
trabeculæ and the mesosomatic branchial cartilaginous system.

The auditory capsule and parachordals when formed are made of the same kind
of cartilage as the trabeculæ, _i.e._ of hard cartilage, and are therefore
formed from a gelatin-containing tissue, and not from muco-cartilage.
Judging from the origin already ascribed to the trabeculæ, viz. their
formation from the great prosomatic entochondrite or plastron, this would
indicate that a second entochondrite existed in the ancestor of the
vertebrate in the region of the junction of the prosoma and mesosoma, which
was especially connected with the sense-organ to which the auditory organ
owes its origin. This pair of entochondrites becoming cartilaginous would
give origin to the parachordals, and subsequently to the auditory capsules,
their position being such that the nerve to the operculum would be
surrounded at its origin by the growth of cartilage.

On this line of argument it is very significant to find that the scorpions
do possess a second pair of entochondrites, viz. the supra-pectinal
entochondrites, situated between the nerve-cord and the pectens, so that if
the ancestor of the Cephalaspid was sufficiently scorpion-like to have
possessed a second pair of entochondrites and at the same time a pair of
special sense-organs of the nature either of {378}the pectens or flabella,
then the origin of the auditory apparatus would present no difficulty.

It is also easy to see that the formation of the parachordals from
entochondrites homologous with the supra-pectinal entochondrites, would
give a reason why the VIIth or opercular nerve is involved with the VIIIth
in the formation of the auditory capsule, especially if the special
sense-organ which gave origin to the auditory organ was originally a
pre-opercular sense-organ such as the flabellum, which subsequently took up
a post-opercular position like that of the pecten.


THE EVIDENCE OF AMMOCOETES.

As to the auditory apparatus itself, we see that the elaborate organ for
hearing--the cochlea--has been evolved in the vertebrate phylum itself. In
the lowest vertebrates the auditory apparatus tends more and more to
resolve itself into a simple epithelial sac, the walls of which in places
bear auditory hairs projecting into the sac, and in part form otoliths.
Such a simple sac forms the early stage of the auditory vesicle in
Ammocoetes, according to Shipley; subsequently, by a series of foldings and
growings together, the chambers of the ear of the adult Petromyzon, as
figured and described by Retzius, are formed. Further, we see that
throughout the Vertebrata this sac was originally open to the exterior, the
auditory vesicle being first an open pit, which forms a vesicle by the
approximating of its sides, the last part to close being known as the
_recessus labyrinthicus_; in many cases, as in elasmobranchs, this part
remains open, or communicates with the exterior by means of the _ductus
endolymphaticus_.

Judging, therefore, from the embryological evidence, it would appear that
the auditory organ originated as a special sense-organ, formed by modified
epithelial cells of the surface, which epithelial surface becoming
invaginated, came to line a closed auditory vesicle under the surface. This
special sense-organ was innervated from a large ganglionic mass of
nerve-cells, situated close against the peripheral sense-cells, the
axis-cylinder processes of which formed the sensory roots of the nerve.

Yet another peculiarity of striking significance is seen in connection with
the auditory organ of Ammocoetes. The opening of the cartilaginous capsule
towards the brain is a large one (Fig. 154), and {379}admits the passage
not only of the auditory and facial nerves, but also of a portion of the
peculiar tissue which surrounds the brain. The large cells of this tissue,
with their feebly staining nuclei and the pigment between them, make them
quite unmistakable; and, as I have already stated, nowhere else in the
whole of Ammocoetes is such a tissue found. When I first noticed these
cells within the auditory capsule, it seemed to me almost impossible that
my interpretation of them as the remnant of the generative and hepatic
tissue which surrounds the brain of animals such as Limulus could be true,
for it seemed too unlikely that a part of the generative system could ever
have become included in the auditory capsule. Still, they are undoubtedly
there; and, as already argued with respect to the substance round the
brain, they must represent some pre-existing tissue which was functional in
the ancestor of Ammocoetes. If my interpretation is right, this tissue must
be generative and hepatic tissue, and its presence in the auditory capsule
immediately becomes a most important piece of evidence, for it proves that
the auditory organ must have been originally so situated that a portion of
the generative and hepatic mass surrounding the cephalic region of the
nervous system followed the auditory nerve to the peripheral sense-organ.

[Illustration: FIG. 154.--TRANSVERSE SECTION THROUGH AUDITORY CAPSULES AND
BRAIN OF AMMOCOETES.

_Au._, auditory organ; _VIII_, auditory nerve; _gl._, ganglion cells of
VIIIth nerve; _Au. cart._, cartilaginous auditory capsule; _gen._, cells of
old generative tissue round brain and in auditory capsule; _bl._,
blood-vessels]

{380}Here there was a test of the truth of my theory ranking second only to
the test of the median eyes; the strongest possible evidence of the truth
of any theory is given when by its aid new and unexpected facts are brought
to light. The theory said that in the group of animals from which the
vertebrates arose, a special sense-organ of the nature of an auditory organ
must have existed on the base of one of the appendages situated at the
junction of the prosoma and mesosoma, and that into this basal part of the
appendage a portion of the cephalic mass of generative and hepatic material
must have made its way in close contiguity to the nerve of the special
organ.

The only living example which nearly approaches the ancient extinct forms
from which, according to the theory, the vertebrates arose, is Limulus,
and, as has already been shown, in this animal, in the very position
postulated by the theory, a large special sense-organ--the
flabellum--exists, which, as already stated, may well have given rise to a
sense-organ concerned with equilibration and audition. If, further, it be
found that a diverticulum of the generative and hepatic material does
accompany the nerve of the flabellum in the basal part of the appendage,
then the evidence becomes very strong that the auditory organ of
Ammocoetes, _i.e._ of the ancient Cephalaspids, was derived from an organ
homologous with the flabellum; that, therefore, the material round the
brain of Ammocoetes was originally generative and hepatic material; that,
in fact, the whole theory is true, for all the parts of it hang together so
closely that, if one portion is accepted, all the rest must follow. As
pointed out in my address at Liverpool, and at the meeting of the
Philosophical Society at Cambridge, it is a most striking fact that a mass
of the generative and hepatic tissue does accompany the flabellar nerve
into the basal part of this appendage. Into no other appendage of Limulus
is there the slightest sign of any intrusion of the generative and hepatic
masses; nowhere, except in the auditory capsule, is there any sign of the
peculiar large-celled tissue which surrounds the brain and upper part of
the spinal cord of Ammocoetes. The actual position of the flabellum on the
basal part of the ectognath is shown in Fig. 155, A, and in Fig. 155, B, I
have removed the chitin, to show the generative and hepatic tissue (_gen._)
lying beneath.

The reason why, to all appearance, the generative and hepatic mass
penetrates into the basal part of this appendage only is apparent {381}when
we see (as Patten and Redenbaugh have pointed out) to what part of the
appendage the flabellum in reality belongs.

[Illustration: FIG. 155.--A, THE DIGGING APPENDAGE OR ECTOGNATH OF LIMULUS;
B, THE MIDDLE PROTUBERANCE (2) OF THE ENTOCOXITE OPENED, TO SHOW THE
GENERATIVE AND HEPATIC TISSUE (_gen._) WITHIN IT; C, ONE OF THE PROSOMATIC
LOCOMOTOR APPENDAGES OR ENDOGNATHS OF LIMULUS, FOR COMPARISON WITH A.

_fl._, flabellum; _cox._, coxopodite; _ent._, entocoxite; _m._, mandible;
_i.m._, inner mandible or epicoxite.]

Patten and Redenbaugh, in their description of the prosomatic appendages of
Limulus, describe the segments of the limbs as (1) the dactylopodite, (2)
the propodite, (3) the mero- and carpo-podites, (4) the ischiopodite, (5)
the basipodite, and (6) the coxopodite (_cox._ in Fig. 155). Still more
basal than the coxopodite is situated the entocoxite (_ent._ in Fig. 155),
which is composed of three sclerites {382}or sensory knobs, to use Patten's
description. The middle one of these three sclerites enlarges greatly in
the digging appendage, and grows over the coxopodite to form the base from
which the flabellum springs. Thus, as they have pointed out, the flabellum
does not belong to the coxopodite of the appendage, but to the middle
sensory knob of the entocoxite. Upon opening the prosomatic carapace, it is
seen that the cephalic generative and hepatic masses press closely against
the internal surface of the prosomatic carapace and also of the entocoxite,
so that any enlargement of one of the sensory knobs of the entocoxite would
necessarily be filled with a protrusion of the generative and hepatic
masses. This is the reason why the generative and hepatic material
apparently passes into the basal segment of the ectognath, and not into
that of the endognaths; it does not really pass into the coxopodite of the
appendage, but into an enlarged portion of the entocoxite, which can hardly
be considered as truly belonging to the appendage. Kishinouye has stated
that a knob arises in the embryo at the base of each of the prosomatic
locomotor appendages, but that this knob develops only in the last or
digging appendage (ectognath) forming the flabellum. Doubtless the median
sclerites of the entocoxites of the endognaths represent Kishinouye's
undeveloped knobs.

I conclude, therefore, that the flabellum, together with its basal part, is
an adjunct to the appendage rather than a part of it, and might, therefore,
easily remain as a separate and well-developed entity, even although the
appendage itself dwindled down to a mere tentacle.

The evidence appears to me very strong that the flabellum of Limulus and
the pecten of scorpions are the most likely organs to give a clue to the
origin of the auditory apparatus of vertebrates. At present both the
Eurypterids and Cephalaspids have left us in the lurch; in the former there
is no sign of either flabellum or pecten; in the latter, no sign of any
auditory capsule beyond Rohon's discovery of two small apertures situated
dorsally on each side of the middle line in Tremataspis, which he considers
to be the termination of the _ductus endolymphaticus_ on each side. In both
cases it is probable, one might almost say certain, that any such special
sense-organ, if present, was not situated externally, but was sunk below
the surface as in Ammocoetes.

The method by which such a sense-organ, situated externally on {383}the
surface of the animal, comes phylogenetically to form the lining wall of an
internally situated membranous capsule is given by the ontogeny of this
capsule, which shows step by step how the sense-organ sinks in and forms a
capsule, and finally is entirely removed from the surface except as regards
the _ductus endolymphaticus_.


SUMMARY.

  The special apparatus for hearing is of a very different character from
  that for vision or for smell, for its nerve belongs to the
  infra-infundibular group of nerves, and not to the supra-infundibular, as
  do those of the other two special senses. Of the five special senses the
  nerves for touch, taste, and hearing, all belong to the
  infra-infundibular segmental nerve-groups. The invertebrate origin, then,
  of the vertebrate auditory nerve must be sought for in the
  infra-oesophageal segmental group of nerves, and not in the
  supra-oesophageal.

  The organs supplied by the auditory nerve are only partly for the purpose
  of hearing; there is always present also an apparatus--the semicircular
  canals--concerned with equilibration and co-ordination of movements. Such
  equilibration organs are not confined to the auditory nerve, but in the
  water-living vertebrates are arranged segmentally along the body, forming
  the organs of the lateral line in fishes; the auditory organ is but one
  of these lateral line organs, which has been specially developed.

  These lateral line organs have been compared to similar segmental organs
  found in connection with the appendages in worms, especially the
  respiratory appendages. In accordance with this suggestion we see that
  they are all innervated from the region of the respiratory nerves--the
  vagus, glosso-pharyngeal, and facial--nerves which originally supplied
  the respiratory appendages of the palæostracan ancestor.

  The logical conclusion is that the appendages of the Palæostraca
  possessed special sense-organs concerned with the perception of special
  vibrations, especially in the mesosomatic or respiratory region, and that
  somewhere at the junction of the prosoma and mesosoma, one of these
  sense-organs was specially developed to form the origin of the vertebrate
  auditory apparatus.

  Impressed by this reasoning I made search for some specially striking
  sense-organ at the base of one of the appendages of Limulus, at the
  junction of the prosoma and mesosoma, and was immediately rewarded by the
  discovery of the extraordinary nature of the flabellum, which revealed
  itself as an elaborate sense-organ supplied with a nerve out of all
  proportion to its size. Up to this time no one had the slightest
  conception that this flabellum was a special sense-organ; the discovery
  of its nature was entirely due to the logical following out of the theory
  of the origin of vertebrates described in this book.

  The structure of this large sense-organ is comparable with that of the
  sense-organs of the pectens of the scorpion, and of many other organs
  found on the appendages of various members of the scorpion group, of
  arachnids and {384}other air-breathing arthropods. Many of these organs,
  such as the lyriform organs of arachnids, and the 'halteres' or balancers
  of the Diptera, are usually regarded as auditory and equilibration
  organs.

  On all the mesosomatic appendages of Limulus very remarkable sense-organs
  are found, apparently for estimating pressures, which, when the
  appendages sank into the body to form with their basal parts the
  branchial diaphragms of Ammocoetes, could easily be conceived as
  remaining at the surface, and so giving rise to the lateral line organs.

  Further confirmation of the view that an organ, such as the flabellum,
  must be looked upon as the originator of the vertebrate auditory organ,
  is afforded by the extraordinary coincidence that in Limulus a
  diverticulum of the generative and hepatic mass accompanies the flabellar
  nerve into the basal part of the digging appendage, while in Ammocoetes,
  accompanying the auditory nerve into the auditory capsule, there is seen
  a mass of cells belonging to that peculiar tissue which fills up the
  space between the brain and the cranial walls, and has already, on other
  grounds, been homologized with the generative and hepatic masses which
  fill up the encephalic region of Limulus.

  For all these reasons special sense-organs, such as are found in the
  flabellum of Limulus and in the pectens of scorpions, may be looked upon
  as giving origin to the vertebrate auditory apparatus. In such case it is
  highly probable that the parachordals, with the auditory capsules
  attached, arose from a second entochondrite of the same nature as the
  plastron; a probability which is increased by the fact that the scorpion
  does possess a second entochondrite, which, owing to its special
  relations to the pecten, is known as the supra-pectinal entochondrite.




{385}CHAPTER XII

_THE REGION OF THE SPINAL CORD_

  Difference between cranial and spinal regions.--Absence of lateral
  root.--Meristic variation.--Segmentation of coelom.--Segmental excretory
  organs.--Development of nephric organs; pronephric, mesonephric,
  metanephric.--Excretory organs of Amphioxus.--Solenocytes.--Excretory
  organs of Branchipus and of Peripatus, appendicular and
  somatic.--Comparison of coelom of Peripatus and of
  vertebrate.--Pronephric organs compared to coxal glands.--Origin of
  vertebrate body-cavity (metacoele).--Segmental duct.--Summary of
  formation of excretory organs.--Origin of somatic
  trunk-musculature.--Atrial cavity of Amphioxus.--Pleural folds.--Ventral
  growth of pleural folds and somatic musculature.--Pleural folds of
  Cephalaspidæ and of Trilobita.--Significance of the ductless
  glands.--Alteration in structure of excretory organs which have lost
  their duct in vertebrates and in invertebrates.--Formation of lymphatic
  glands.--Segmental coxal glands of arthropods and of vertebrates.--Origin
  of adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless
  glands.--Summary.


The consideration of the auditory nerve and the auditory apparatus
terminates the comparison between the cranial nerves of the vertebrate and
the prosomatic and mesosomatic nerves of the arthropod, and leaves us now
free to pass on to the consideration of the vertebrate spinal nerves and
the organs they supply. Before doing so, it is advisable to pass in review
the conclusions already attained.

Starting with the working hypothesis that the central nervous system of the
vertebrate has arisen from the central nervous system of the arthropod, but
has involved and enclosed the alimentary canal of the latter in the
process, so that there has been no reversal of surfaces in the derivation
of the one form from the other, we have been enabled to compare closely all
the organs of the head-region in the two groups of animals, and in no
single case have we been compelled to make any startling or improbable
assumptions. The simple following out of this clue has led in every case in
the most natural {386}manner to the interpretation of all the organs in the
head-region of the vertebrate from the corresponding organs of the
arthropod.

That it is possible to bring together all the striking resemblances between
organs in the two classes of animals, such as I have done in preceding
chapters, has been ascribed to a perverted ingenuity on my part--a
suggestion which is flattering to my imaginative powers, but has no
foundation of fact. There has been absolutely no ingenuity on my part; all
I have done is to compare organs and their nerve-supply, as they actually
exist in the two groups of animals, on the supposition that there has been
no turning over on to the back, no reversal of dorsal and ventral surfaces.
The comparison is there for all to read; it is all so simple, so
self-evident that, given the one clue, the only ingenuity required is on
the part of those who fail to see it.

The great distinction that has arisen between the two head-regions is the
disappearance of appendages as such, never, however, of important organs on
those appendages. If the olfactory organs of the one group were originally
situated on antennules, the olfactory organs still remain, although the
antennules as such have disappeared. The coxal excretory organs at the base
of the endognaths remain and become the pituitary body. A special
sense-organ, such as the flabellum of Limulus or the pecten of scorpion,
remains and gives rise to the auditory organ. A special glandular organ,
the uterus in the base of the operculum, remains, and gives rise to the
thyroid gland. The branchiæ and sense-organs on the mesosomatic appendages
remain, and even the very muscles to a large extent. As will be seen later,
the excretory organs at the base of the metasomatic appendages remain. It
is merely the appendage as such which vanishes either by dwindling away, or
by so great an alteration as no longer to be recognizable as an appendage.

This dwindling process was already in full swing before the vertebrate
stage; it is only a continuation of a previous tendency, as is seen in the
dwindling of the prosomatic appendages in the Merostomata and the inclusion
of the branchiæ within the body of the scorpion. Already among the
Palæostraca, swimming had largely taken the place of crawling. The whole
gradual transformation from the arthropod to the vertebrate is associated
with a transformation from a crawling to a swimming animal--with the
concomitant loss of locomotor appendages as such, and the alteration of the
shape of {387}the animal into the lithe fish-like form. The consideration
of the manner in which this latter change was brought about, takes us out
of the cranial into the spinal region.

If we take Limulus as the only living type of the Palæostraca, we are
struck with the fact that the animal consists to all intents and purposes
of prosomatic and mesosomatic regions only; the metasoma consisting of the
segments posterior to the mesosoma is very insignificant, so that the large
mass of the animal consists of what has become the head-region in the
vertebrate; the spinal region, which has become in the higher vertebrates
by far the largest region of the body, can hardly be said to exist in such
an animal as Limulus. As to the Eurypterids and others, similar remarks may
be made, though not to the same extent, for in them a distinct metasoma
does exist.

In this book I have considered up to the present the cranial region as a
system of segments, and shown how such segments are comparable, one by one,
with the corresponding segments in the prosoma and mesosoma of the presumed
arthropod ancestor.

In the spinal region such direct comparison is not possible, as is evident
on the face of it; for even among vertebrates themselves the spinal
segments are not comparable one by one, so great is the variation, so
unsettled is the number of segments in this region. This meristic
variation, as Bateson calls it, is the great distinctive character of the
spinal region, which distinguishes it from the cranial region with its
fixed number of nerves, and its substantive rather than meristic variation.
At the borderland, between the two regions, we see how the one type merges
into the other; how difficult it is to fix the segmental position of the
spino-occipital nerves; how much more variable in number are the segments
supplied by the vagus nerves than those anterior to them.

This meristic variation is a sign of instability, of want of fixedness in
the type, and is evidence, as already pointed out, that the spinal region
is newer than the cranial. This instability in the number of spinal
segments does not necessarily imply a variability in the number of segments
of the metasoma of the invertebrate ancestor; it may simply be an
expression of adaptability in the vertebrate phylum itself, according to
the requirements necessitated by the conversion of a crawling into a
swimming animal, and the subsequent conversion of the swimming into a
terrestrial or flying animal.

{388}However many may have been the original number of segments belonging
to the spinal region, one thing is certain--the segmental character of this
region is remarkably clearly shown, not only by the presence of the
segmental spinal nerves, but also by the marked segmentation of the
mesoblastic structures. The question, therefore, that requires elucidation
above all others is the origin of the spinal mesoblastic segments, _i.e._
of the coelomic cavities of the trunk-region, and the structures derived
from their walls.

Proceeding on the same lines as in the case of the cranial segments, it is
necessary in the first instance to inquire of the vertebrate itself as to
the scope of the problem in this region. In addition to the variability in
the number of segments so characteristic of the spinal region, the complete
absence in each spinal segment of a lateral root affords another marked
difference between the two regions. Here, except, of course, at the
junction of the spinal and cranial regions, each segmental nerve arises
from two roots only, dorsal and ventral, and these roots are separately
sensory and motor, and not mixed in function as was the lateral root of
each cranial segment. Now, these lateral roots were originally the nerves
supplying the prosomatic and mesosomatic appendages with motor as well as
sensory fibres. The absence, therefore, of lateral roots in the spinal
region implies that in the vertebrate none of the musculature belonging to
the metasomatic appendages has remained. Consequently, as far as muscles
are concerned, the clue to the origin of the spinal segments must be sought
for in the segmentation of the body-muscles.

Here, in contradistinction to the cranial region, the segmentation is most
marked, for the somatic spinal musculature of all vertebrates can be traced
back to a simple sheet of longitudinal ventral and dorsal muscles, such as
are seen in all fishes. This sheet is split into segments or myotomes by
transverse connective tissue septa or myo-commata; each myotome
corresponding to one spinal segment.

In addition to the evidence of segmentation afforded by the
body-musculature in all the higher vertebrates, similar evidence is given
by the segmental arrangement of parts of the supporting tissue to form
vertebræ. Such segments have received the name of sclerotomes, and each
sclerotome corresponds to one spinal segment.

Yet another marked peculiarity of this region is the segmental arrangement
of the excretory organs. Just as our body-musculature {389}has arisen from
the uniformly segmented simple longitudinal musculature of the lowest fish,
so, as we pass down the vertebrate phylum, we find more and more of a
uniform segmental arrangement in the excretory organs.

The origin of all these three separate segmentations may, in accordance
with the phraseology of the day, be included in the one term--the origin of
the spinal mesoblastic segments--_i.e._ of the coelomic cavities of the
trunk-region and the structures derived from their walls.


THE ORIGIN OF THE SEGMENTAL EXCRETORY ORGANS.

Of these three clues to the past history of the spinal region, the
segmentation manifested by the presence of vertebræ is the least important,
for in Ammocoetes there is no sign of vertebræ, and their indications only
appear at transformation. Especially interesting is the segmentation due to
the excretory organs, for the evidence distinctly shows that such excretory
organs have steadily shifted more and more posteriorly during the evolution
of the vertebrate.

In Limulus the excretory organs are in the prosomatic region--the coxal
glands; these become in the vertebrate the pituitary body.

In Amphioxus the excretory organs are in the mesosomatic region,
segmentally arranged with the gills.

In vertebrates the excretory organs are in the metasomatic region posterior
to the gills, and are segmentally arranged in this region. Their
investigation has demonstrated the existence of three distinct stages in
these organs: 1. A series of segmental excretory organs in segments
immediately following the branchial segments. This is the oldest of the
three sets, and to these organs the name of the _pronephros_ is given. 2. A
second series which extends more posteriorly than the first, overlaps them
to an extent which is not yet settled, and takes their place; to them is
given the name of the _mesonephros_. 3. A third series continuous with the
mesonephric is situated in segments still more posterior, supplants the
mesonephros and forms the kidneys of all the higher vertebrates. This forms
the _metanephros_.

These three sets of excretory organs are not exactly alike in their origin,
in that the pronephric tubules are formed from a different portion of the
coelomic walls to that from which the meso- and {390}metanephric tubules
are formed, and the former alone gives origin to a duct, which forms the
basis for the generative and urinary ducts, and is called the _segmental
duct_. The mesonephric tubules, called also the Wolffian body, open into
this duct.

In order to make the embryology of these excretory organs quite clear, I
will make use of van Wijhe's phraseology and also of his illustrations. He
terms the whole coelomic cavity the _procoelom_, which is divisible into a
ventral unsegmented part, the body-cavity or _metacoelom_, and a dorsal
segmented part, the _somite_. This latter part again is divided into a
dorsal part--the _epimere_--and a part connecting the dorsal part with the
body-cavity, to which therefore he gives the name of _mesomere_.

The cavity of the epimere disappears, and its walls form the muscle and
cutis plates of the body. The part which forms the muscles is known as the
_myotome_, which separates off from the mesomere, leaving the latter as a
blind sac--the _mesocoelom_--communicating by a narrow passage with the
body cavity or _metacoelom_. At the same time, from the mesomere is formed
the _sclerotome_, which gives rise to the skeletal tissues of the vertebræ,
etc., so that van Wijhe's epimere and mesomere together correspond to the
original term, protovertebra, or somite of Balfour; and when the myotome
and sclerotome have separated off, there is still left the intermediate
cell-mass of Balfour and Sedgwick, _i.e._ the sac-like mesocoele of van
Wijhe, the walls of which give origin to the mesonephrotome or
_mesonephros_. Further, according to van Wijhe, the dorsal part of the
unsegmented metacoelom is itself segmented, but not, as in the case of the
mesocoele, with respect to both splanchnopleuric and somatopleuric walls.
The segmentation is manifest only on the somatopleuric side, and consists
of a distinct series of hollow somatopleuric outgrowths, called by him
_hypomeres_, which give rise to the _pronephros_ and the segmental duct.

Van Wijhe considers that the whole metacoelom was originally segmented,
because in the lower vertebrates the segmentation reaches further
ventral-wards, so that in Selachia the body-cavity is almost truly
segmental. Also in the gill-region of Amphioxus the cavities which are
homologous with the body-cavity arise segmentally.

{391}[Illustration: FIG. 156.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF
THE VERTEBRATE COELOM. (After VAN WIJHE.)

_N._, central nervous system; _Nc._, notochord; _Ao._, aorta; _Mg._,
midgut. A, _My._, myocoele; _Mes._, mesocoele; _Met._, metacoele; _Hyp._,
hypomere (pronephric). B and C, _My._, myotome; _Mes._, mesonephros;
_S.d._, segmental duct (pronephric); _Met._, body cavity.]

{392}As is well known, Balfour and Semper were led, from their
embryological researches, to compare the nephric organs of vertebrates with
those of annelids, and, indeed, the nature of the vertebrate segmental
excretory organs has always been the fact which has kept alive the belief
in the origin of vertebrates from a segmented annelid. These segmental
organs thus compared were the mesonephric tubules, and doubts arose,
especially in the mind of Gegenbaur, as to the validity of such a
comparison, because the mesonephric tubules did not open to the exterior,
but into a duct--the segmental duct--which was an unsegmented structure
opening into the cloaca; also because the segmental duct, which was the
excretory duct of the pronephros, was formed first, and the mesonephric
tubules only opened into it after it was fully formed. Further, the
pronephros was said to arise from an outbulging of the somatopleuric
mesoblast, which extended over a limited number of metameres, and was not
segmental, but continuous. Gegenbaur and others therefore argued that the
original prevertebrate excretory organ was the pronephros and its duct, not
the mesonephros, from which they concluded that the vertebrate must have
been derived from an unsegmented type of animal, and not from the segmented
annelid type.

Such a view, however, has no further reason for acceptance, as it was based
on wrong premises, for Rückert has shown that the pronephros does arise as
a series of segmental nephric tubules, and is not unsegmented. He also has
pointed out that in Torpedo the anterior part of the pronephric duct shows
indications of being segmented, a statement fully borne out by the
researches of Maas on Myxine, who gives the clearest evidence that in this
animal the anterior part of the pronephric duct is formed by the fusion of
a series of separate ducts, each of which in all probability once opened
out separately to the exterior.

Rückert therefore concludes that Balfour and Semper were right in deriving
the segmental organs of vertebrates from those of annelids, but that the
annelid organs are represented in the vertebrate, not by the mesonephric
tubules, but by the pronephric tubules and their ducts, which originally
opened separately to the exterior. By the fusion of such tubules the
anterior part of the segmental duct was formed, while its posterior part
either arose by a later coenogenetic lengthening, or is the only remnant of
a series of pronephric tubules which originally extended the whole length
of the body, as suggested also by Maas and Boveri. Rückert therefore
supposed that the mesonephric tubules were a secondary set of nephric
organs, which were not necessarily directly derived from the annelid
nephric organs.

{393}At present, then, Rückert's view is the one most generally
accepted--the original annelid nephric organs are represented by the
pronephric tubules and the pronephric duct, not by the mesonephric tubules,
which are a later formation. This latter statement would hold good if the
mesonephric tubules were found entirely in segments posterior to those
containing the pronephric tubules; such, however, is said not to be the
case, for the two sets of organs are said to overlap in some cases; even
when they exist in the same segments, the former are said always to be
formed from a more dorsal part of the coelom than the pronephros, always to
be a later formation, and never to give any indication of communicating
with the exterior except by way of the pronephric duct.

The recent observations of Brauer on the excretory organs of the
Gymnophiona throw great doubt on the existence of mesonephric and
pronephric tubules in the same segment. He criticizes the observations on
which such statements are based, and concludes that, as in Hypogeophis, the
nephrotome which is cut off after the separation of the sclero-myotome
gives origin to the pronephros in the more anterior regions, just as it
gives origin to the mesonephros in the more posterior regions. In fact, the
observations of van Wijhe and others do not in reality show that two
excretory organs may be formed in one segment, the one mesonephric from the
remains of the mesomere and the other pronephric from the hypomere, but
rather that in such cases there is only one organ--the pronephros--part of
which is formed from the mesomere and part from the hypomere. Brauer goes
further than this, and doubts the validity of any distinction between
pronephros and mesonephros, on the ground of the former arising from a more
ventral part of the procoelom than the latter; for, as he says, it is only
possible to speak of one part of the somite as being more ventral than
another part when both parts are in the same segment; so that if pronephric
and mesonephric organs are never in the same segment, we cannot say with
certainty that the former arises more ventrally than the latter.

These observations of Brauer strongly confirm Sedgwick's original statement
that the pronephric and mesonephric organs are homodynamous organs, in that
they are both derived from the original serially situated nephric organs,
the differences between them being of a subordinate nature and not
sufficient to force us to believe that the mesonephros is an organ of quite
different origin to the {394}pronephros. So, also, Price, from his
investigations of the excretory organs of Bdellostoma, considers that in
this animal both pronephros and mesonephros are derived from a common
embryonic kidney, to which he gives the name _holonephros_.

Brauer also is among those who conclude that the vertebrate excretory
organs were derived from those of annelids; he thinks that the original
ancestor possessed a series of similar organs over the whole pronephric and
mesonephric regions, and that the anterior pronephric organs, which alone
form the segmental duct, became modified for a larval existence--that their
peculiarities were adaptive rather than ancestral. This last view seems to
me very far-fetched, without any sufficient basis for its acceptance.
According to the much more probable and reasonable view, the pronephros
represents the oldest and original excretory organs, while the mesonephros
is a later formation. Brauer's evidence seems to me to signify that the
pronephros, mesonephros, and metanephros are all serially homologous, and
that the pronephros bears much the same relation to the mesonephros that
the mesonephros does to the metanephros. The great distinction of the
pronephros is that it, and it alone, forms the segmental duct.

We may sum up the conclusions at which we have now arrived as follows:--

1.  The pronephric tubules and the pronephric duct are the oldest part of
the excretory system, and are distinctly in evidence for a few segments
only in the most anterior part of the trunk-region immediately following
the branchial region. They differ also from the mesonephric tubules by not
being so clearly segmental with the myotomes.

2.  The mesonephric tubules belong to segments posterior to those of the
pronephros, are strictly segmental with the myotomes, and open into the
pronephric duct.

3.  All observers are agreed that the two sets of excretory organs resemble
each other in very many respects, as though they arose from the same series
of primitive organs, and, according to Sedgwick and Brauer, no distinction
of any importance does exist between the two sets of organs. Other
observers, however, consider that the pronephric organs, in part at all
events, arise from a part of the nephrocoele more ventral than that which
gives origin to the mesonephric organs, and that this difference in
position of origin, combined {395}with the formation of the segmental duct,
does constitute a true morphological distinction between the two sets of
organs.

4. All the recent observers are in agreement that the vertebrate excretory
organs strongly indicate a derivation from the segmental organs of
annelids.

The very strongest support has been given to this last conclusion by the
recent discoveries of Boveri and Goodrich upon the excretory organs of
Amphioxus. According to Boveri, the nephric tubules of Amphioxus open into
the dorsal coelom by one or more funnels. Around each funnel are situated
groups of peculiar cells, called by him 'Fadenzellen,' each of which sends
a long process across the opening of the funnel. Goodrich has examined
these 'Fadenzellen,' and found that they are typical pipe-cells, or
solenocytes, such as he has described in the nephridial organs of various
members of the annelid group Polychæta. Also, just as in the Polychæta, the
ciliated nephric tubule has no internal funnel-shaped opening into the
coelom, but terminates in these groups of solenocytes. "Each solenocyte
consists of a cell-body and nucleus situated at the distal free extremity
of a delicate tube; the proximal end of the tube pierces the wall of the
nephridial canal and opens into its lumen. A single long flagellum arising
from the cells works in the tube and projects into the canal."

The exceedingly close resemblance between the organs of Amphioxus and those
of Phyllodoce, as given in his paper, is most striking, and, as he says,
leads to the conclusion that the excretory organs of Amphioxus are
essentially identical with the nephridia of certain polychæte worms.

It is to me most interesting to find that the very group of annelids, the
Polychæta, which possess solenocytes so remarkably resembling those of the
excretory organs of Amphioxus, are the highest and most developed of all
the Annelida. I have argued throughout that the law of evolution consists
in the origination of successive forms from the dominant group then alive,
dominance signifying the highest type of brain-power achieved up to that
time. The highest type among Annelida is found in the Chætopoda; from them,
therefore, the original arthropod type must have sprung. This original
group of Arthropoda gave rise to the two groups of Crustacea and Arachnida,
in my opinion also to the Vertebrata, and, as already mentioned, it is
convenient to give it a generalized {396}name, the Protostraca, from which
subsequently the Palæostraca arose.

The similarity between the excretory organs of Amphioxus and those of
Phyllodoce suggests that the protostracan ancestor of the vertebrates arose
from the highest group of the Chætopoda--the Polychæta. The evidence which
I have already given points, however, strongly to the conclusion that the
vertebrate did not arise from members of the Protostraca near to the
polychæte stock, but rather from members in which the arthropod characters
had already become well developed--members, therefore, which were nearer
the Trilobita than the Polychæta. Such early arthropods would very probably
have retained in part excretory organs of the same character as those found
in the original polychæte stock, and thus account for the presence of
solenocytes in the excretory organs of Amphioxus.

In connection with such a possibility, I should like to draw attention to
the observations of Claus and Spangenberg on the excretory organs of
Branchipus--that primitive phyllopod, which is recognized as the nearest
approach to the trilobites at present living. According to Claus, an
excretory apparatus exists in the neighbourhood of each nerve-ganglion, and
Spangenberg finds a perfectly similar organ in the basal segment of each
appendage--a system, therefore, of excretory organs as segmentally arranged
as those of Peripatus. Claus considers that although these organs formed an
excretory system, it is not possible to compare them with the annelid
segmental organs, because he thought the cells in question arose from
ectoderm. Now, the striking point in the description of the excretory cells
in these organs, as described both by Claus and Spangenberg, is that they
closely resemble the pipe-cells or solenocytes of Goodrich; each cell
possesses a long tube-like projection, which opens on the surface. They
appear distinctly to belong to the category of flame-cells, and resemble
solenocytes more than anything else. According to Goodrich, the solenocyte
is probably an ectodermal cell, so that even if it prove to be the case, as
Claus thought, that these pipe-cells of Branchipus are ectodermal, they
would still claim to be derived from the segmental organs of annelids,
especially of the Polychæta, being, to use Goodrich's nomenclature, true
nephridial organs, as opposed to coelomostomes.

These observations of Claus and Spangenberg suggest not only that the
primitive arthropod of the trilobite type possessed segmental {397}organs
in every segment directly derived from those of a polychæte ancestor, but
also that such organs were partly somatic and partly appendicular in
position. Such a suggestion is in strict accord with the observations of
Sedgwick on the excretory organs of the most primitive arthropod known,
viz. Peripatus, where also the excretory organs, which are true segmental
organs, are partly somatic and partly appendicular. Further, the excretory
organs of the Scorpion and Limulus group are again partly somatic and
partly appendicular, receiving the name of coxal glands, because there is a
ventral projection of the gland into the coxa of the corresponding
appendage.

Judging from all the evidence available, it is probable that when the
arthropod stock arose from the annelids, simultaneously with the formation
of appendages, the segmental somatic nephric organs of the latter extended
ventrally into the appendage, and thus formed a segmental set of excretory
organs, which were partly somatic, partly appendicular in position, and
might therefore be called coxal glands.

As already stated, all investigators of the origin of the vertebrate
excretory organs are unanimous in considering them to be derived from
segmental organs of the annelid type. I naturally agree with them, but, in
accordance with my theory, would substitute the words "primitive arthropod"
for the word "annelid," for all the evidence I have accumulated in the
preceding chapters points directly to that conclusion. Further, the most
primitive of the three sets of vertebrate segmental organs--the pronephros,
mesonephros, and metanephros--is undoubtedly the pronephros; consequently
the pronephric tubules are those which I consider to be more directly
derived from the coxal glands of the primitive arthropod ancestor. Such a
derivation appears to me to afford an explanation of the difficulties
connected with the origin of the pronephros and mesonephros respectively,
which is more satisfactory than that given by the direct derivation from
the annelid.

The only living animal which we know of as at all approaching the most
primitive arthropod type is, as pointed out by Korschelt and Heider,
Peripatus; and Peripatus, as is well known, possesses a true coelom and
true coelomic excretory organs in all the segments of the body. Sedgwick
shows that at first a true coelom, as typical as that of the annelids, is
formed in each segment of the body, and that then this coelom (which
represents in the vertebrate van Wijhe's pro-coelom) {398}splits into a
dorsal and a ventral part. In the anterior segments of the body the dorsal
part disappears (presumably its walls give origin to the mesoblast from
which the dorsal body-muscles arise), while the ventral part remains and
forms a nephrocoele, giving origin to the excretory organs of the adult.
According to von Kennel, the cavity becomes divided into three spaces,
which for a time are in communication--a lateral (I.), a median (II.), and
a dorso-median (III.). The dorso-median portion becomes partitioned off,
and this, as well as the greater part of the lateral portion, which lies
principally in the foot, is used up in providing elements for the formation
of the body- and appendage-muscles respectively and the connective tissue.

In Fig. 157 I reproduce von Kennel's diagram of a section across a
Peripatus embryo, in which I. represents the lateral appendicular part of
the coelom, II. the ventral somatic part, and III. the dorsal part which
separates off from the ventral and lateral parts, and, as its walls give
origin largely to the body-muscles, may be called the myocoele. The muscles
of the appendages are formed from the ventral part of the original
procoelom, just as I have argued is the case with the muscles of the
splanchnic segmentation in vertebrates.

Sedgwick states that the ventral part of the coelom extends into the base
of each appendage, and there forms the end-sac of each nephric tubule, into
which the nephric funnel opens, thus forming a coxal gland; this end-sac or
vesicle in the appendage is called by him the internal vesicle (_i.v._),
because later another vesicle is formed from the ventral coelom in the body
itself, close against the nerve-cord on each side, which he calls the
external vesicle (_e.v._). (_Cf._ Fig. 158, taken from Sedgwick.) This
second vesicle is, according to him, formed later in the development from
the nephric tubule of the internal vesicle, so that it discharges its
contents to the exterior by the same opening as the original tubule. Of
course, as he points out, the whole system of internal and external
vesicles and nephric tubules are all simply derivatives of the original
ventral part of the coelom or nephrocoele.

{399}[Illustration: FIG. 157.--TRANSVERSE SECTION OF PERIPATUS EMBRYO.
(After VON KENNEL.)

_Al._, alimentary canal; _N._, nerve-cord; _App._, appendage; _I_, _II_,
_III_, the three divisions (lateral, median, and dorso-median) of the
coelom.]

[Illustration: FIG. 158.--SECTION OF PERIPATUS. (After SEDGWICK.)

_Al._, alimentary canal; _N._, nerve-cord; _App._, appendage; _i.v._,
internal, and _e.v._, external vesicles of the segmented excretory tubule
(coxal gland).]

Here, then, in Peripatus, and presumably, therefore, in members of the
Protostraca, we see that the original segmental organs of the annelid have
become a series of nephric organs, which extended into the base of the
appendages, and may therefore be called coxal glands; also it is clear,
from Sedgwick's description, that if the appendages disappeared, the
nephric organs would still remain, not as coxal glands, but as purely
somatic excretory glands. They would still be homologous with the annelid
segmental organs, or with the coxal glands, but would arise _in toto_ from
a part of the ventral coelom or nephrocoele, more dorsal than the former
appendicular part, because the appendages and their enclosed coelom are
always situated ventrally to the body. Again, according to Sedgwick, the
nephric tubules are connected with two coelomic vesicles, the one in the
appendage the internal vesicle, and the other, the so-called bladder, or
the external vesicle, in the body itself, close against the nerve-cord.
Sedgwick appears to consider that either of these vesicles may form the
end-sac of a nephric tubule, for he discusses the question whether the
single vesicle, which in each case gives origin to the nephridia of the
first three legs, corresponds to the internal or external vesicle. He
{400}decides, it is true, in favour of the internal vesicle, and therefore
considers the excretory organ to be appendicular, _i.e._ a coxal gland, in
these segments as well as in those more posterior. Still, the very
discussion shows that in his opinion, at all events, the external vesicle
might represent the end-sac of the tubule, in the absence of the internal
or appendicular vesicle.

Such an arrangement as Sedgwick describes in Peripatus is the very
condition required to give rise to the pronephric and mesonephric tubules,
as deduced by me from the consideration of the vertebrate, and harmonizes
and clears up the controversy about the mesonephros and pronephros in the
most satisfactory manner. Both pronephros and mesonephros are seen to be
derivatives of the original annelid segmental organs, not directly from an
annelid, but by way of an arthropodan ancestor; the difference between the
two is simply that the pronephric organs were coxal glands, and indicate,
therefore, the presence of the original metasomatic appendages, while the
mesonephric organs were homologous organs, formed in segments of later
origin which had lost their appendages. For this reason the pronephros is
said to be formed, in part at least, from a portion of the coelom situated
more ventrally than the purely somatic part which gives rise to the
mesonephros. For this reason Sedgwick, Brauer, etc., can say that the
mesonephros is strictly homodynamous with the pronephros; while equally
Rückert, Semon, and van Wijhe can say it is not homodynamous, in so far
that the two organs are not derived strictly from absolutely homologous
parts of the coelom. For this reason Semon can speak of the mesonephros as
a dorsal derivative of the pronephros, just as Sedgwick says that the
external or somatic vesicle of Peripatus is a derivative of the
appendicular nephric organ. For this reason the pronephros, or rather a
part of it, is always derived from the somatopleuric layer, for, as is
clear from Miss Sheldon's drawing, the part of the coelom in Peripatus
which dips into the appendage is derived from the somatopleuric layer
alone.

Such a coelom as that of Peripatus, Fig. 157, would represent the origin of
the vertebrate coelom, and would therefore represent the procoelom of van
Wijhe. In strict accordance with this, we see that it separates into a
dorsal part, the walls of which give origin to the somatic muscles, or at
all events to the great longitudinal dorsal muscles of the animal, and a
ventral part, which forms a nephrocoele, {401}dips into the appendage, and
gives origin to the muscles of the appendage. In the vertebrate, after the
somatic dorsal part or myocoele has separated off, a ventral part is left,
which forms a nephrocoele in the trunk-region, and gives origin to the
splanchnic striated muscles in the cranial region, _i.e._ to the muscles
which, according to my theory, were once appendicular muscles. This ventral
nephrocoelic part is divisible in the trunk into a segmented part, which
forms the excretory organs proper, and an unsegmented part, the metacoele
or true body-cavity of the vertebrate.

This comparison of the procoelom of the vertebrate and arthropod signifies
that the vertebrate metacoele was directly derived by ventral downgrowth
from the arthropod nephrocoele, so that if, as I suppose, the vertebrate
nervous system represents the conjoined nervous system and alimentary canal
of the arthropod, then the vertebrate metacoele, or body-cavity, must have
been originally confined to the region on each side of the central nervous
system, and from this position have spread ventrally, to enclose ultimately
the new-formed vertebrate gut. This means that the body-cavity (metacoele)
of the vertebrate is not the same as the body-cavity of the annelid, but
corresponds to a ventral extension of the nephrocoele, or ventral part of
such body-cavity.

Such a phylogenetic history is most probable, because it explains most
naturally and simply the facts of the development of the vertebrate
body-cavity; for the mesoblast always originates in the neighbourhood of
the notochord and central nervous system, and the lumen of the body-cavity
always appears first in that region, and then extends laterally and
ventrally on each side until it reaches the most ventral surface of the
embryo, thus forming a ventral mesentery, which ultimately disappears, and
the body-cavity surrounds the gut, except for the dorsal mesentery. Thus
Shipley, in his description of the formation of the mesoblastic plates
which line the body-cavity in Ammocoetes, describes them as commencing in
two bands of mesoblast situated on each side, close against the commencing
nervous system:--

"These two bands are separated dorsally by the juxtaposition of the dorsal
wall of the mesenteron and the epiblast, and ventrally by the hypoblastic
yolk-cells which are in contact with the epiblast over two-thirds of the
embryo. Subsequently, but at a much later date, the mesoblast is completed
ventrally by the downgrowth on {402}each side of these mesoblastic plates.
The subsequent downward growth is brought about by the cells proliferating
along the free ventral edge of the mesoblast, these cells then growing
ventralwards, pushing their way between the yoke-cells and epiblast."

The derivation of the vertebrate pronephric segmental organs from the
metasomatic coxal glands of a primitive arthropod would mean, if the
segmental organs of Peripatus be taken as the type, that such glands opened
to the exterior on every segment, either at the base of the appendage or on
the appendage itself. It is taken for granted by most observers that the
pronephric segmental organs once opened to the exterior on each segment,
and then, from some cause or other, ceased to do so, and the separate
ducts, by a process of fusion, came to form a single segmental duct, which
opened into the cloaca. Many observers have been led to the conclusion that
the pronephric duct is epiblastic in origin, although from its position in
the adult, it appears far removed from all epiblastic formations. However,
at no time in the developmental history is there any clear evidence of
actual fusion of any part of the pronephric organ with the epidermis, and
the latest observer, Brauer, is strongly of opinion that there is never
sufficiently close contact with the epidermis to warrant the statement that
the epiblastic cells take part in the formation of the duct. All that can
be said is, that the formation of the duct takes place at a time when the
pronephric diverticulum is in close propinquity to the epidermis, before
the ventral downgrowth of the myotome has taken place.

The formation of the anterior portion of the pronephric duct is, according
to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly brought about by
the fusion of a number of pronephric tubules, which, according to Maas, are
clearly seen in the youngest specimens as separate segmental tubes; each of
these tubules is supplied by a capillary network from a segmental branch of
the aorta, as in the tubules of Amphioxus according to Boveri, and does not
possess a glomerulus.

The posterior part of the duct into which the mesonephric tubules enter
possesses also a capillary network, which Maas considers to represent the
original capillary network of a series of pronephric tubules, the only
remnant of which is the duct into which the mesonephric tubules open. He
therefore argues that the pronephric duct indicates a series of pronephric
tubules, which originally extended {403}along the whole length of the body,
and were supplanted by the mesonephric tubules, which also belonged to the
same segments.

I also think that the paired appendages which have left the pronephric
tubules as signs of their past existence, existed originally, in the
invertebrate stage, on every segment of the body. But I do not consider
that such a statement is at all equivalent to saying that such pairs of
tubules must have existed upon every one of the segments existing at the
present day; for it seems to me that Rückert is much more likely to be
right when he says that in Selachians the duct clearly does grow back, and
is not formed throughout _in situ_; so that he gives a double explanation
of the formation of the duct--a palingenetic anterior part formed by the
fusion of the extremities of the original excretory tubules, to which a
posterior coenogenetic lengthening has been added.

It does not seem to me at all necessary that the immediate invertebrate
ancestor of the vertebrate should have possessed excretory organs which
opened out separately to the exterior on each segment; a fusion may already
have taken place in the invertebrate stage, and so a single duct have been
acquired for a number of organs. Such a suggestion has been made by
Rückert, because of the fact discovered by Cunningham and E. Meyer, that
the segmental organs of _Lanice conchilega_ are on each side connected
together by a single strong longitudinal canal. I would, however, go
further than this and say, that even although the nephric organs of the
polychæte ancestor opened out on every segment, and although the primitive
arthropodan ancestor derived from such polychæte possessed coxal glands
which opened out either on to or at the base of each appendage, similarly
to those of Peripatus, yet the immediate arthropodan ancestor, with its
palæostracan affinities, may already have possessed metasomatic coxal
glands, all of which opened into a single duct, with a single opening to
the exterior.

Judging from Limulus, such was very probably the case, for Patten and Hazen
have shown (1) that the coxal glands of Limulus are segmental organs
belonging to the prosomatic segments; (2) that the organs belonging to the
cheliceral and ectognathal segments are not developed; (3) that the four
glands belonging to the endognaths become connected together by a _stolon_,
which communicates with a single nephric duct, opening to the exterior on
the basal segment of the 5th prosomatic appendage (the last endognath). At
{404}no time is there any evidence of any separate openings or any fusion
with the ectoderm, such as might indicate separate openings of these
prosomatic coxal segmental organs. Thus we see that in Limulus, which is
presumably much nearer the annelid condition than the vertebrate, all
evidence of separate nephric ducts opening to the exterior on each
prosomatic segment has entirely disappeared, just as is the case in the
metasomatic coxal glands (_i.e._ the pronephros) of the vertebrate. What is
seen in the prosomatic region of Limulus, and doubtless also of the
Eurypterids, may very probably have occurred in the metasomatic region of
the immediate invertebrate ancestors of the vertebrate, and so account for
the single pronephric duct belonging to a number of pronephric organs.

The interpretation of these various embryological investigations may be
summed up as follows:--

1.  The ancestor of the vertebrates possessed a pair of appendages on each
segment; into the base of each of these appendages the segmental excretory
organ sent a diverticulum, thus forming a coxal gland.

2.  Such coxal glands, even in the invertebrate stage, may have discharged
into a common duct which opened to the exterior most posteriorly.

3.  Then, from some cause, the appendages were rendered useless, and
dwindled away, leaving only the pronephric organs to indicate their former
presence. At the end of this stage the animal possessed vertebrate
characteristics.

4.  For the purpose of increasing mobility, of forming an efficient
swimming instead of a crawling animal, the body-segments increased in
number, always, as is invariably the case, by the formation of new ones
between those already formed and the cloacal region, and so of necessity
caused an elongation of the pronephric duct. Into this there now opened the
ducts of the segmental organs formed by recapitulation, those, therefore,
belonging to the body-segments--mesonephric--having nothing to do with
appendages, for the latter had already ceased to exist functionally, and
would not, therefore, be repeated with each meristic repetition.

This, so to speak, passive lengthening of the pronephric duct in
consequence of the lengthening of the early vertebrate body by the addition
of metameres, each of which contained only mesonephric and no pronephric
tubules, is, to my mind, an example of a principle {405}which has played an
important part in the formation of the vertebrate, viz. that the meristic
variation by which the spinal region of even the lowest of existing
vertebrates has been formed, has largely taken place in the vertebrate
phylum itself, and that such changes must be eliminated before we can
picture to ourselves the pre-vertebrate condition. As an example, I may
mention the remarkable repetition of similar segments pictured by Bashford
Dean in Bdellostoma. Such repetition leads to passive lengthening of such
parts as are already formed but are not meristically repeated: such are the
notochord, the vertebrate intestine, the canal of the spinal cord, and
possibly the lateral line nerve. The fuller discussion of this point means
the discussion of the formation of the vertebrate alimentary canal; I will
therefore leave it until I come to that part of my subject, and only say
here that the evidence seems to me to point to the conclusion that at the
time when the vertebrate was formed, the respiratory and cloacal regions
were very close together, the whole of the metasoma being represented by
the region of the pronephros alone.

Here, as always, the evidence of Ammocoetes tends to give definiteness to
our conceptions, for Wheeler points out that up to a length of 7 mm. the
pronephros only is formed; there is no sign of the more posteriorly formed
mesonephros. Now we know, as pointed out in Chapter VI., p. 228, this is
the time of Kupffer's larval stage of Ammocoetes. This is the period during
which the invertebrate stage is indicated in the ontogeny, so that, in
accordance with all that has gone before, this means that the metasoma of
the invertebrate ancestor was confined to the region of the pronephros.

Again, take Shipley's account of the development of Petromyzon. He says--

"The alimentary canal behind the branchial region may be divided into three
sections. Langerhans has termed these the stomach, midgut, and hindgut, but
as the most anterior of these is the narrowest part of the whole intestine,
it would, perhaps, be better to call it oesophagus. This part of the
alimentary canal lies entirely in front of the yolk, and is, with the
anterior region, which subsequently bears the gills, raised from the rest
of the egg when the head is folded off. It is supported by a dorsal
mesentery, on each side of which lies the head-kidney (pronephros)."

Further on he says--

{406}"The hindgut is smaller than the midgut; its anterior limit is marked
by the termination of the spiral valve, which does not extend into this
region. The two segmental ducts open into it just where it turns ventrally
to open to the exterior by a median ventral anus. Its lumen is from an
early stage lined with cells which have lost their yolk, and it is in wide
communication with the exterior from the first. This condition seems to be,
as Scott suggests, connected with the openings of the ducts of the
pronephros, for this gland is completed and seems capable of functioning
long before any food could find its way through the midgut, or, indeed,
before the stomodæum has opened."

Is there no significance in this statement of Shipley? Even if it be
possible to find some special reason why the branchial and cloacal parts of
the gut are freed from yolk and lined with serviceable epithelium a long
time before the midgut, why should a bit of the midgut, which Shipley calls
the oesophagus, which is connected with the region of the pronephros and
not of the branchiæ, differ so markedly from the rest of the midgut? Surely
the reason is that the branchial region of the gut, the pronephric region
of the gut, and the cloacal region of the gut, belong to a different and
earlier phase in the phylogenetic history of the Ammocoetes than does the
midgut between the pronephric and cloacal regions. This observation of
Shipley fits in with and emphasizes the view that the original animal from
which the vertebrate arose consisted of a cephalic and branchial region,
followed by a pronephric and cloacal region; the whole intermediate part of
the gut, which forms the midgut, with its large lumen and spiral valve, and
belongs to the mesonephric region, being a later formation brought about by
the necessity of increasing the length of the body.


THE ORIGIN OF THE SOMATIC TRUNK-MUSCULATURE AND THE FORMATION OF AN ATRIAL
CAVITY.

Next comes the question, why was the pronephros not repeated in the
meristic repetition that took place during the early vertebrate stage?
What, in fact, caused the disappearance of the metasomatic appendages, and
the formation of the smooth body-surface of the fish?

The embryological evidence given by van Wijhe and others of the manner in
which the original superficially situated pronephros is {407}removed from
the surface and caused to assume the deeper position, as seen in the later
embryo, is perfectly clear and uniform in all the vertebrate groups. The
diagrams at the end of van Wijhe's paper, which I reproduce here,
illustrate the process which takes place. At first the myotome (Fig. 159,
A) is confined to the dorsal region on each side of the spinal cord and
notochord. Then (Fig. 159, B) it separates from the rest of the somite and
commences to extend ventrally, thus covering over the pronephros and its
duct, until finally (Fig. 159, C) it reaches the mid-ventral line on each
side, and the foundations of the great somatic body-muscles are finally
laid.

In order, therefore, to understand how the obliteration of the appendages
took place, we must first find out what is the past history of the
myotomes. Why are they confined at first to the dorsal region of the body,
and extend afterwards to the ventral region, forcing by their growth an
organ that was originally external in situation to become internal?

In the original discussion at Cambridge, I was accused of violating the
important principle that in phylogeny we must look at the most elementary
of the animals whose ancestors we seek, and was told that the lowest
vertebrate was Amphioxus, not Ammocoetes; that therefore any argument as to
the origin of vertebrates must proceed from the consideration of the former
and not the latter animal. My reply was then, and is still, that I was
considering the cranial region in the first place, and that therefore it
was necessary to take the lowest vertebrate which possessed cranial nerves
and sense-organs of a distinctly vertebrate character, a criterion
evidently not possessed by Amphioxus. Such argument does not apply to the
spinal region, so that, now that I have left the cranial region and am
considering the spinal, I entirely agree with my critics that Amphioxus is
likely to afford valuable help, and ought to be taken into consideration as
well as Ammocoetes. The distinction between the value of the spinal
(including respiratory) and cranial regions of Amphioxus for drawing
phylogenetic conclusions is recognized by Boveri, who says that, in his
opinion, "Amphioxus shows simplicity and undifferentiation rather than
degeneration. If truly Amphioxus is somewhat degenerated, then it is so in
its prehensile and masticatory apparatus, its sense organs, and perhaps its
locomotor organs, owing to its method of living."

{408}[Illustration: FIG. 159.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF
THE VERTEBRATE COELOM. (After VAN WIJHE.)

_N._, central nervous system; _Nc._, notochord; _Ao._, aorta; _Mg._,
midgut. A, _My._, myocoele; _Mes._, mesocoele; _Met._, metacoele; _Hyp._,
hypomere (pronephric). B and C, _My._, myotome; _Mes._, mesonephros;
_S.d._, segmental duct (pronephric); _Met._, body-cavity.]

{409}Hatschek describes in Amphioxus how the coelom splits into a dorsal
segmented portion, the protovertebra, and a ventral unsegmented portion,
the lateral plates. He describes in the dorsal part the formation of
myotome and sclerotome, as in the Craniota. Also, he describes how the
myotome is at first confined to the dorsal region in the neighbourhood of
the spinal cord and notochord, and subsequently extends ventrally, until,
just as in Ammocoetes, the body is enveloped in a sheet of somatic
segmented muscles, the well-known myomeres.

The conclusion to be drawn from this is inevitable. Any explanation of the
origin of the somatic muscles in Ammocoetes must also be an explanation of
the somatic muscles in Amphioxus, and conversely; so that if in this
respect Amphioxus is the more primitive and simpler, then the condition in
Ammocoetes must be looked upon as derived from a more primitive condition,
similar to that found in Amphioxus. Now, it is well known that a most
important distinction exists between Amphioxus and Ammocoetes in the
topographical relation of the ventral portion of this muscle-sheet, for in
the former it is separated from the gut and the body-cavity by the atrial
space, while in the latter there is no such space. Fürbringer therefore
concludes, as I have already mentioned, that this space has become
obliterated in the Craniota, but that it must be taken into consideration
in any attempt at formulating the nature of the ancestors of the
vertebrate.

Kowalewsky described this atrial space as formed by the ventral downgrowth
of pleural folds on each side of the body, which met in the mid-ventral
line and enclosed the branchial portion of the gut. According to this
explanation, the whole ventral portion of the somatic musculature of the
adult Amphioxus belongs to the extension of the pleural folds, the original
body-musculature being confined to the dorsal region. This is expressed
roughly on the external surface of Amphioxus by the direction of the
connective tissue septa between the myotomes (_cf._ Fig. 162, B). These
septa, as is well known, bend at an angle, the apex of which points towards
the head. The part dorsal to the bend represents the part of the muscle
belonging to the original body; the part ventral to the bend is the pleural
part, and represents the extension into the pleural folds.

Lankester and Willey have attempted to give another explanation of the
formation of the atrial cavity; they look upon it as originating from a
ventral groove, which becomes a canal by the meeting of two {410}outgrowths
from the metapleure on each side. This canal then extends dorsalwards on
each side, and so forms the atrial cavity; the metapleure still remains in
the adult; the somatic muscles in the epipleure of the adult are the
original body-muscles, and not extensions into an epipleuric fold, for
there is no such fold.

This explanation is a possible conception for the post-branchial portion of
the atrium, but is impossible for the branchial region; for, as Macbride
points out, as must necessarily be the case, the point of origin of the
atrial wall is, in all stages of development, situated at the end of the
gill-slit. It shifts in position with the position of the gill-slit, but
there can be no backwards extension of the cavity. Macbride therefore
agrees with Kowalewsky that the atrial cavity is formed by the simultaneous
ventral extension of pleural folds, and of the branchial part of the
original pharynx. Thus, in his summing up, he states: "In the larva
practically the whole sides and dorsal portion of the pharynx represent
merely the hyper-pharyngeal groove and the adjacent epithelium of the
pharynx of the adult, the whole of the branchial epithelium of the adult
being represented by a very narrow strip of the ventral wall of the pharynx
of the larva. The subsequent disproportionate growth of this part of the
pharynx of the larva, and of the adjacent portion of the atrial cavity, has
given the impression that the atrial cavity grew upwards and displaced
other structures, which is not the case."

Further, van Wijhe states that the atrium extends beyond the atriopore
right up to the anus, just as must have been the case if the pleural folds
originally existed along the whole length of the body. His words are:
"Allerdings hat sich das Atrium beim _Amphioxus lanceolatus_ eigenthümlich
ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus,
d.h. bis an die Wurzel des Schwanzes ausdehnt."

We get, therefore, this conception of the origin of the somatic musculature
of the vertebrate. The invertebrate ancestor possessed on each side, along
the whole length of its body, a lateral fold or pleuron which was segmented
with the body, and capable of movement with the body, because the dorsal
longitudinal somatic muscles extended segmentally into each segment of the
pleuron. By the ventral extension of these pleural folds, not only was the
smooth body-surface of the vertebrate attained, but also the original
appendages obliterated as such, leaving only as signs of their existence
the {411}branchiæ, the pronephric tubules, and the sense-organs of the
lateral line system.

Such an explanation signifies that the somatic trunk-musculature of the
vertebrate was derived from the dorsal longitudinal musculature of the body
of the arthropod, and not from the ventral longitudinal musculature, and
that therefore in the primitive arthropod stage the equivalent of the
myotome of the vertebrate did not give origin to the ventral longitudinal
muscles of the invertebrate ancestor. Now, as I have said, von Kennel
states that in the procoelom of Peripatus a dorsal part (III. in Fig. 157)
is cut off which gives origin to the dorsal body-musculature, while the
ventral part which remains (I. and II. in Fig. 157) gives origin in its
appendicular portion (I.) to the muscles of the appendage, and presumably
in its ventral somatic portion (II.) to the ventral longitudinal muscles of
the body. This dorsal cut-off part might be called the myotome, in the same
sense as the corresponding part of the procoelom in the vertebrate is
called the myotome. In both cases the muscles derived from it form only a
part of the voluntary musculature of the animal, and in both cases the
muscles in question are the dorsal longitudinal muscles of the body, to
which must be added the dorso-ventral body-muscles. Now, the whole of my
theory of the origin of vertebrates arose from the investigation of the
structure of the cranial nerves, which led to the conception that their
grouping is not, like the spinal, a dual grouping of motor and sensory
elements, but a dual grouping to supply two sets of segments, characterized
especially by the different embryological origin of their musculature. The
one set I called the somatic segmentation, because the muscles belonging to
it were the great longitudinal body-muscles; the other I called the
splanchnic segmentation, because its muscles were those connected with the
branchial and visceral arches. According to my theory, this latter
segmentation was due to the segmentation of the appendages in the
invertebrate ancestor; and in previous chapters, dealing as they do with
the cranial region, attention was especially directed to the way in which
the position of the striated splanchnic musculature could be explained by a
transformation of the prosomatic and mesosomatic appendages. Now, I am
dealing with the metasomatic region, in which it is true the appendages
take a very subordinate place, but still something corresponding to the
splanchnic segments of the cranial region might fairly be expected to
exist, and I therefore {412}desire to emphasize what appears to me to be
the fact, that the musculature, which in the region of the trunk would
correspond to that derived from the ventral segmentation of the mesoblast
in the region of the head, may have arisen not only from the musculature of
the appendages, but also from the ventral longitudinal musculature of the
body of the invertebrate ancestor, for it seems probable that this latter
musculature had nothing to do with the origin of the great longitudinal
muscles of the vertebrate body, either dorsal or ventral.

The way in which I imagine the obliteration of the atrial cavity to have
taken place is indicated in Fig. 160, B, which is a modification of a
section across a trilobite-like animal as represented in Fig. 160, A. As is
seen, the pleural folds on each side have nearly met the bulged-out ventral
body-surface. A continuation of the same process would give Fig. 160, C,
which is, to all intents and purposes, the same as Fig. 159, C, taken from
van Wijhe, and shows how the segmental duct is left in the remains of the
atrial cavity. The lining walls of the atrial cavity are represented very
black, in order to indicate the presence of pigment, as indeed is seen in
the corresponding position in Ammocoetes. In these diagrams I have
represented the median ventral surface as a large bulged-out bag, without
indicating any structures in it except the ventral extension of the
procoelom to form the metacoelom. At present I will leave the space between
the central nervous system and the ventral mesentery blank, as in the
diagrams; in my next chapter I will discuss the possible method of
formation within this blank space of the notochord and midgut. Boveri
considers that the obliteration of the atrial cavity in the higher
vertebrates is not complete, but that its presence is still visible in the
shape of the pronephric duct. The evidence of Maas and others that the duct
is formed by the fusion of the pronephric tubules is, it seems to me,
conclusive against Boveri's view; but yet, as may be seen from my
diagrammatic figures, the very place where one would expect to find the
last remnant of the atrial cavity is exactly where the pronephric duct is
situated. For my own part I should expect to find evidence of a former
existence of an atrial cavity rather in the pigment round the pronephros
and its duct than in the duct itself.

{413}[Illustration: FIG. 160.--A, DIAGRAM OF SECTION THROUGH A
TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE SUGGESTED OBLITERATION OF
APPENDAGES AND THE FORMATION OF AN ATRIAL CAVITY BY THE VENTRAL EXTENSION
OF THE PLEURAL FOLDS; C, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE
VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE
AND THE OBLITERATION OF THE ATRIAL CAVITY.

_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._,
pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele;
_S.d._, segmental duct; _At._, atrial chamber; _V.Mes._, ventral mesentery;
_Mes._, mesonephros. The dotted line represents the splanchnopleuric
mesoblast in all figures.]

{414}The conception that Amphioxus shows us how to account for the great
envelope of somatic muscles which wraps round the vertebrate body, in that
the ancestor of the vertebrate possessed on each side the body a segmented
pleuron, is exactly in accordance with the theory of the origin of
vertebrates deduced from the study of Ammocoetes, as already set forth in
previous chapters. For we see that one of the striking characteristics of
such forms as Bunodes, Hemiaspis, etc., is the presence of segmented
pleural flaps on each side of the main part of the body; and if we pass
further back to the great group of trilobites, we find in the most manifold
form, and in various degrees of extent, the most markedly segmented pleural
folds. In fact, the hypothetical figure (Fig. 160, A) which I have deduced
from the embryological evidence, might very well represent a cross-section
of a trilobite, provided only that each appendage of the trilobite
possessed an excretory coxal gland.

The earliest fishes, then, ought to have possessed segmented pleural folds,
which were moved by somatic muscles, and enveloped the body after the
fashion of Ammocoetes and Amphioxus, and I cannot help thinking that
Cephalaspis shows, in this respect also, its relation to Ammocoetes. It is
well known that some of the fossil representatives of the Cephalaspids show
exceedingly clearly that these animals possessed a very well-segmented
body, and it is equally recognized that this skeleton is a calcareous, not
a bony skeleton, and does not represent vertebræ, etc. It is generally
called an aponeurotic skeleton, meaning thereby that what is preserved
represents not dermal plates alone, or a vertebrate skeleton, but the
calcified septa or aponeuroses between a number of muscle-segments or
myomeres, precisely of the same kind as the septa between the myomeres in
Ammocoetes. The termination of such septa on the surface would give rise to
the appearance of dermal plates or scutes, or the septa may even have been
attached to something of the nature of dermal plates. The same kind of
picture would be represented if these connective tissue dissepiments of
Ammocoetes were calcified, and the animal then fossilized. In agreement
with this interpretation of the spinal skeleton of Cephalaspis, it may be
noted that again and again, in parts of these dissepiments, I have found in
old specimens of Ammocoetes nodules of cartilage formed, and at
transformation it is in this very tissue that the spinal cartilages are
formed.

{415}[Illustration: FIG. 161.--A, FACSIMILE OF WOODWARD'S DRAWING OF A
SPECIMEN OF _Cephalaspis Murchisoni_, AS SEEN FROM THE SIDE. THE CEPHALIC
SHIELD IS ON THE RIGHT AND CAUDAL TO IT THE PLEURAL FRINGES ARE WELL SHOWN;
B, ANOTHER SPECIMEN OF _Cephalaspis Murchisoni_ TAKEN FROM THE SAME BLOCK
OF STONE, SHOWING THE DERMOSEPTAL SKELETON AND IN ONE PLACE THE PLEURAL
FRINGES, _bc._]

Now, the specimens of Cephalaspis all show, as seen in Fig. 161, that the
skeletal septa cover the body regularly, and then along one line are bent
away from the body to form, as it were, a fringe, or rather a free pleuron,
which has been easily pushed at an angle to the body-skeleton in the
process of fossilization. Patten thinks that this fringed appearance is
evidence of a number of segmental appendages which were jointed to the
corresponding body-segments, and in the best specimen at the South
Kensington Natural History Museum he thinks such joints are clearly
visible. He concludes, therefore, that the cephalaspids were arthropods,
and not vertebrates. I have also carefully examined this specimen, and do
not consider that what is seen resembles the joint of an arthropod
appendage; the appearance is rather such as would be produced if the line
of attachment of Patten's appendages to the body were the place where the
pleural body folds became free from the body, and so with any pressure a
{416}bending or fracture of the calcified plates would take place along
this line. There is, undoubtedly, an appearance of finish at the
termination of these skeletal fringes, as though they terminated in a
definitely shaped spear-like point, just as is seen in the trilobite
pleuræ. This, again, to my mind, is rather evidence of pleural fringes than
of true appendages.

[Illustration: FIG. 162.--A, ARRANGEMENT OF SEPTA IN AMMOCOETES (_NC._,
position of notochord); B, ARRANGEMENT OF SEPTA IN AMPHIOXUS.]

As already argued, I look upon Ammocoetes as the only living fish at all
resembling the cephalaspids; it is therefore instructive to compare the
arrangement of this spinal dermo-septal skeleton of Cephalaspis with that
of the septa between the myomeres in the trunk-region of Ammocoetes and
Amphioxus. Such a skeleton in Ammocoetes would be represented by a series
of plates overlapping each other, arranged as in Fig. 162, A, and in
Amphioxus as in Fig. 162, B. I have lettered the corresponding parts of the
two structures by similar letters, _a_, _b_, _c_. Ammocoetes differs in
configuration from Amphioxus in that it possesses an extra dorsal (_a_,
_d_) and an extra ventral bend. Ammocoetes is a much rounder animal than
Amphioxus, and both the dorsal and ventral bends are on the extreme ventral
and dorsal surfaces--surfaces which can hardly be said to exist in
Amphioxus. The part, then, of such an aponeurotic skeleton {417}in
Ammocoetes which I imagine corresponds to _b_, _c_ in Amphioxus, and
therefore would represent the pleural fold, is the part ventral to the bend
at _b_. In both the animals this bend corresponds to the position of the
notochord NC.

The skeleton of Cephalaspis compares more directly with that of Ammocoetes
than that of Amphioxus, for there is the same extra dorsal bend (Fig. 161,
_a_, _d_) as in Ammocoetes; the lateral part of the skeleton again gives an
angle _a_, _b_, _c_; the part from _b_ to _c_ would therefore represent the
pleural fold. I picture to myself the sequence of events somewhat as
follows:--

First, a protostracan ancestor, which, like Peripatus, possessed appendages
on every segment into which coelomic diverticula passed, forming a system
of coxal glands; such glands, being derived from the segmental organs of
the Chætopoda, discharged originally to the exterior by separate openings
on each segment. It is, however, possible, and I think probable, that a
fusion of these separate ducts had already taken place in the protostracan
stage, so that there was only one external opening for the whole of these
metasomatic coxal glands, just as there is only one external opening for
the corresponding prosomatic coxal glands of Limulus. Then, by the ventral
growth of pleural body-folds, such appendages became enclosed and useless,
and the coxal glands of the post-branchial segments, with their segmental
or pronephric duct, were all that remained as evidence of such appendages.
This dwindling of the metasomatic appendages was accompanied by the
getting-rid of free appendages generally, in the manner already set forth,
with the result that a smooth fish-like body-surface was formed; then the
necessity of increasing mobility brought about elongation by the addition
of segments between those last formed and the cloacal region. Each of such
new-formed segments was appendageless, so that its segmental organ was not
a coxal gland, but entirely somatic in position, and formed, therefore, a
mesonephric tubule, not a pronephric one. Such glands could no longer
excrete to the exterior, owing to the enclosing shell of the pleural folds;
but the pronephric duct was there, already formed, and so these nephric
tubules opened into that, instead of, as in the case of the branchial
slits, forcing their way through the pleural walls when the atrium became
closed.


{418}THE MEANING OF THE DUCTLESS GLANDS.

If it is a right conception that the excretory organs of the protostracan
group, which gave origin to the vertebrates as well as to the crustaceans
and arachnids, were of the nature of coxal glands, then it follows that
such coxal glands must have existed originally on every segment, because
they themselves were derived from the segmental organs of the annelids; it
is therefore worth while making an attempt to trace the fate of such
segmental organs in the vertebrate as well as in the crustacean and
arachnid.

Such an attempt is possible, it seems to me, because there exists
throughout the animal kingdom striking evidence that excretory organs which
no longer excrete to the exterior do not disappear, but still perform
excretory functions of a different character. Their cells still take up
effete or injurious substances, and instead of excreting to the exterior,
excrete into the blood, forming either ductless glands of special
character, or glands of the nature of lymphatic glands.

The problem presented to us is as follows:--

The excretory organs of both arthropods and vertebrates arose from those of
annelids, and were therefore originally present in every segment of the
body. In most arthropods and vertebrates they are present only in certain
regions; in the former case, as the coxal glands of the prosomatic or
head-region; in the latter, as the nephric glands of the metasomatic or
trunk-region, and, in the case of Amphioxus, of the mesosomatic or
branchial region.

In the original arthropod, judging from Peripatus, they were present, as in
the annelid, in all the segments of the body, and formed coxal glands.
Therefore, in the ancestors of the living Crustacea and Arachnida, coxal
glands must have existed in all the segments of the body, and we ought to
be able to find the vestiges of them in the mesosomatic or branchial and
metasomatic or abdominal regions of the body.

Similarly, in the vertebrates, derived, as has been shown, not from the
annelids, but from an arthropod stock, evidence of the previous existence
of coxal glands ought to be manifested in the prosomatic or trigeminal
region, in the mesosomatic or branchial region, as well as in the
metasomatic or post-branchial region.

How does an excretory organ change its character when it ceases {419}to
excrete to the exterior? What should we look for in our search after the
lost coxal glands?

The answer to these questions is most plainly given in the case of the
pronephros, especially in Myxine, where Maas has been able to follow out
the whole process of the conversion of nephric tubules into a tissue
resembling that of a lymph-gland.

He states, in the first place, that the pronephros possesses a capillary
network, which extends over the pronephric duct, while the tubules of the
mesonephros possess not only this capillary network, equivalent to the
capillaries over the convoluted tubules in the higher vertebrates, but also
a true glomerulus, in that the nephric segmental arteriole forms a coil
(Knauel), and pushes in the wall of the mesonephric tubule. He describes
the pronephros of large adult individuals as consisting of--

1.  Tubules with funnels which open into the pericardial coelom.

2.  A large capillary network (the glomus) at the distal end.

3.  A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel
controversy), which Spengel considers to be composed of the altered
epithelium of pronephric tubules, while Semon looks on it as an
amalgamation of glomeruli.

Maas is entirely on the side of Spengel, and shows that this peculiar
tissue is actually formed by modified pronephric tubules, which become more
and more lymphatic in character.

He says: "The pronephros consists of a number of nephric tubules, placed
separately one behind the other, which were originally segmental in
character, each one of which is supplied by a capillary network from a
segmental branch of the aorta. The tubules begin with many mouths
(dorso-lateral and medial-ventral) in the pericardial cavity; on their
other blind end they have lost their original external opening, and there,
in the cranial portion of the head-kidney, before they have joined together
to form a collecting duct, they, together with the vascular network, are
transformed into a peculiar adrenal-like tissue. The most posterior of the
segmental capillary nets retain their original character, and are
concentrated into the separate capillary mass known as the glomus."

Later on he says: "Further, the separate head-kidney is more and more
removed in structure from an excretory organ in the ordinary sense. One
cannot, however, speak of it as an organ becoming rudimentary; this is
proved not only by the progressive transformation {420}of its internal
tissue into a tissue of a very definite character, but also by the cilia in
its canals, and the steady increase in the number of its funnels. It
appears, therefore, to be the conversion of an excretory organ into an
organ for the transference of fluid out of the coelom into a special
tissue, _i.e._ into its blood-sinus; in other words, into an organ which
must be classed as belonging to the lymph-system."

In exact correspondence with this transformation of a nephric tubule into a
ductless gland of the nature of a lymphatic gland, is the formation of the
head-kidney in the Teleostea. Thus, Weldon points out that, though the
observations of Balfour left it highly probable that the "lymphatic" tissue
described by him was really a result of the transformation of part of the
embryonic kidney, he did not investigate the details of its development.
This was afterwards done by Emery, with the following results: "In those
Teleostea which he has studied, Professor Emery finds that at an early
stage the kidney consists entirely of a single pronephric funnel, opening
into the pericardium, and connected with the segmental duct, which already
opens to the exterior. Behind this funnel, the segmental duct is surrounded
by a blastema, derived from the intermediate cell-mass, which afterwards
arranges itself more or less completely into a series of solid cords,
attaching themselves to the duct. These develop a lumen, and become normal
segmental tubules, but it is, if I may be allowed the expression, a matter
of chance how much of the blastema becomes so transformed into kidney
tubules, and how much is left as the 'lymphatic' tissue of Balfour, this
'lymphatic' tissue remaining either in the pronephros only, or in both pro-
and meso-nephros."

If we turn now to the invertebrates, we see also how close a connection
exists between lymphatic and phagocytic organs and excretory organs. The
chief merit for this discovery is due to Kowalewsky, who, taking a hint
from Heidenhain's work on the kidney, in which he showed how easy it was to
find out the nature of different parts of the mammalian excretory organ by
the injection of different substances, such as a solution of ammoniated
carmine, or of indigo-carmine, has injected into a large number of
different invertebrates various colouring matters, or litmus, or bacilli,
and thus shown the existence, not only of known excretory organs, but also
of others, lymphatic or lymphoid in nature, not hitherto suspected.

In all cases he finds that a phagocytic action with respect to solid
{421}bodies is a property of the leucocytes, and that these leucocytes
which are found in the coelomic spaces of the Annelida, etc., are
apparently derived from the epithelium of such spaces. Also by the
proliferation of such epithelium in places, _e.g._ the septal glands of the
terrestrial Oligochæta, segmental glandular masses of such tissue are
formed which take up the colouring matter, or the bacilli. In the
limicolous Oligochæta such septal glands are not found, but at the
commencement of the nephridial organ, immediately following upon the
funnel, a remarkable modification of the nephridial wall takes place to
form a large cellular cavernous mass, the so-called filter, which in Euaxes
is full of leucocytes; the cells are only definable by their nuclei, and
look like and act in the same way as the free leucocytes outside this
nephridial appendage. As G. Schneider points out, the whole arrangement is
very like that described by Kowalewsky in the leeches Clepsine and
Nephelis, where, also immediately succeeding the funnel of the nephridial
organ, a large accessory organ is found, which is part of the nephridium,
and is called the nephridial capsule. This is the organ _par excellence_
which takes up the solid carmine-grains and bacilli, and apparently, from
Kowalewsky's description, contains leucocytes in large numbers. We see,
then, that in such invertebrates, just as in the vertebrate, modifications
of the true excretory organ may give rise to phagocytic glands of the
nature of lymphatic glands. Further, these researches of Kowalewsky suggest
in the very strongest manner that whenever by such means new, hitherto
unsuspected glands are discovered, such glands must belong to the excretory
system, _i.e._ must be derived from coelomic epithelium, even when all
evidence of any coelom has disappeared. Kowalewsky himself was evidently so
impressed with the same feeling that he heads one of his papers "The
Excretory Organs of the Pantopoda," although the organs in question had
been discovered by him by this method, and appeared as ductless glands with
no external opening.

To my mind these observations of Kowalewsky are of exceeding interest, for
it is immediately clear that if the segmental organs of the annelids, which
must have existed on all the segments of the forefathers of the Crustacea
and Arachnida (the Protostraca), have left any sign of their existence in
living crustaceans and arachnids, then such indication would most likely
take the form of lymphatic glands in the places where the excretory organs
ought to have been.

Now, as already pointed out in Peripatus, such segmental organs {422}were
formed by the ventral part of the coelom, and dipped originally into each
appendage. We know also that each segment of an arachnid embryo possesses a
coelomic cavity in its ventral part which extends into the appendage on
each side; this cavity afterwards disappears, and is said to leave no trace
in the adult of any excretory coxal gland derived from its walls. If,
however, it is found that in the very position where such organ ought to
have been formed a segmentally arranged ductless gland is situated, the
existence of which is shown by its taking up carmine, etc., then it seems
to me that in all probability such gland is the modification of the
original coxal gland.

This is what Kowalewsky has done. Thus he states that Metschnikoff had fed
Mysis with carmine-grains, and found tubules at the base of the thoracic
feet coloured red with carmine. He himself used an allied species,
_Parapodopsis cornutum_, and found here also that the carmine was taken up
by tubules situated in the basal segments of the feet. In Nebalia, feeding
experiments with alizarin blue and carmine stained the antennal glands, and
showed the existence of glands at the base of the eight thoracic feet.
These glands resemble the foot-glands of Mysis, Parapodopsis, and Palæmon,
and lie in the space through which the blood passes from the thoracic feet,
_i.e._ from the gills, to the heart. In Squilla also, in addition to the
shell-glands, special glands were discovered on the branchial feet on the
path of the blood to the heart. These glands form continuous masses of
cells which constitute large compact glands at the base of the branchial
feet. Single cells of the same sort are found along the whole course of the
branchial venous canal, right up to the pericardium.

These observations show that the Crustacea possess not only true excretory
organs in the shape of coxal glands, _i.e._ antennary glands, shell-glands,
etc., in the cephalic region, but also a series of segmental glands
situated at the base of the appendages, especially of the respiratory
appendages: a system, that is to say, of coxal glands which have lost their
excretory function, through having lost their external opening, but have
not in consequence disappeared, but still remain _in situ_, and still
retain an important excretory function, having become lymphatic glands
containing leucocytes. Such glands are especially found in the branchial
appendages, and are called branchial glands by Cuénot, who describes them
for all Decapoda.

Further, it is significant that the same method reveals the {423}existence
in Pantopoda of a double set of glands of similar character, one set in the
basal segments of the appendage, and the other in the adjacent part of the
body.

In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ
situated along the whole length of the nerve-cord in the abdominal region
takes up carmine grains and bacilli; an organ which in Androctonus does not
form one continuous gland, but a number of separate, apparently irregularly
grouped, glandular bodies.

In addition to this median lymphatic gland, Kowalewsky has discovered in
the scorpion a pair of lateral glands, to which he gives the name of
lymphoid glands, which communicate with the thoracic body-cavity (_i.e._
the pseudocoele), are phagocytic, and, according to him, give origin to
leucocytes by the proliferation of their lining cells, thus, as he remarks,
reminding us of the nephridial capsules of Clepsine. These glands are so
closely related in position to the coxal glands on each side that he has
often thought that the lumen of the gland communicated with that of the
coxal gland; he, however, has persuaded himself that there is no true
communication between the two glands. Neither of these organs appears to be
segmental, and until we know how they are developed it is not possible to
say whether they represent fused segmental organs or not.

The evidence, then, is very strong that in the Crustacea and Arachnida the
original segmental excretory organs do not disappear, but remain as
ductless glands, of the nature of lymphatic glands, which supply leucocytes
to the system.

Further, the evidence shows that the nephric organs, or parts of the coelom
in close connection with these organs, may be transformed into ductless
glands, which do not necessarily contain free leucocytes as do
lymph-glands, but yet are of such great importance as excretory organs that
their removal profoundly modifies the condition of the animal. Such a gland
is the so-called adrenal or suprarenal body, disease of which is a feature
of Addison's disease; a gland which forms and presumably passes into the
blood a substance of remarkable power in causing contraction of
blood-vessels, a substance which has lately been prepared in crystalline
form by Jokichi Takamine, and called by him "adrenalin"; a gland,
therefore, of very distinctly peculiar properties, which cannot be regarded
as rudimentary, but is of vital importance for the due maintenance of the
healthy state.

In the Elasmobranchs two separate glandular organs have been {424}called
suprarenal; a segmental series of paired organs, each of which possesses a
branch from the aorta and a sympathetic ganglion, and an unpaired series in
close connection with the kidneys, to which Balfour gave the name of
interrenal glands. Of these two sets of glands, Swale Vincent has shown
that the extract of the interrenals has no marked physiological effect, in
this respect resembling the extract of the cortical part of the mammalian
gland, while the extract of the paired segmental organs of the Elasmobranch
produces the same remarkable rise of blood-pressure as the extract of the
medullary portion of the mammalian gland.

The development also of these two sets of glands is asserted to be
different. Balfour considered that the suprarenals were derived from
sympathetic ganglion-cells, but left the origin of the interrenals
doubtful. Weldon showed that the cortical part of the suprarenals in the
lizard was derived from the wall of the glomerulus of a number of
mesonephric tubules. In Pristiurus, he stated that the mesoblastic rudiment
described by Balfour as giving origin to the interrenals is derived from a
diverticulum of each segmental tubule, close to the narrowing of its
funnel-shaped opening into the body-cavity. With respect to the paired
suprarenals he was unable to speak positively, but doubted whether they
were derived entirely from sympathetic ganglia.

Weldon sums up the results of his observations by saying: "That all
vertebrates except Amphioxus have a portion of the kidney modified for some
unknown purpose not connected with excretion; that in Cyclostomes the
pronephros alone is so modified, in Teleostei the pro- and part of the
meso-nephros; while in the Elasmobranchs and the higher vertebrates the
mesonephros alone gives rise to this organ, which has also in these forms
acquired a secondary connection with certain of the sympathetic ganglia."

Since Weldon's paper, a large amount of literature on the origin of the
adrenals has appeared, a summary of which, up to 1891, is given by Hans
Rabl in his paper, and a further summary by Aichel in his paper published
in 1900. The result of the investigations up to this latter paper may be
summed up by saying that the adrenals, using this term to include all these
organs of whatever kind, are in all cases, partly at all events, derived
from some part of the walls of either the mesonephric or pronephric
excretory organs, but that in addition a separate origin from the
sympathetic nervous system must {425}be ascribed to the medullary part of
the organ and to the separate paired organs in the Elasmobranchs, which are
equivalent to the medullary part in other cases.

The evidence, then, of the transformation of the known vertebrate excretory
organs--the pronephros and the mesonephros--leads to the conclusion that in
our search for the missing coxal glands of the meso- and pro-somatic
regions, we must look for either lymphatic glands, or ductless glands of
distinct importance to the body. I have already considered the question in
the prosomatic region, and have given my reasons why the pituitary gland
must be looked upon as the descendant of the arthropod coxal gland. In this
case also the resulting ductless gland is still of functional importance,
for disease of it is associated with acromegaly. If, as is possible, it is
homologous with the Ascidian hypophysial gland, then it is confirmatory
evidence that this latter is said by Julin to be an altered nephridial
organ.

Finally, I come to the mesosomatic or branchial region; and here,
strikingly enough, we find a perfectly segmental glandular organ of
mysterious origin--the thymus gland--segmental with the branchiæ, not
necessarily with the myotomes, belonging, therefore, to the appendicular
system; and since the branchiæ represent, according to my theory, the basal
part of the appendage, such segmental glands would be in the position of
coxal glands. Here, then, in the thymus may be the missing mesosomatic
coxal glands.

What, then, is the thymus?

The answer to this question has been given recently by Beard, who strongly
confirms Kölliker's original view that the thymus is a gland for the
manufacture of leucocytes, and that such leucocytes are directly derived
from the epithelial cells of the thymus. Kölliker also further pointed out
that the blood of the embryo is for a certain period destitute of
leucocytes. Beard confirms this last statement, and says that up to a
certain stage (varying from 10 to 16 mm. in length of the embryo) the
embryos of _Raja batis_ have no leucocytes in the blood or elsewhere. Up to
this period the thymus-placode is well formed, and the first leucocytes can
be seen to be formed in it from its epithelial cells; then such formation
takes place with great rapidity, and soon an enormous discharge of
leucocytes occurs from the thymus into the tissue-spaces and blood. He
therefore concludes that all lymphoid tissues in the body arise originally
from the thymus gland, _i.e._ from leucocytes discharged from the thymus.

{426}The segmental branchial glands, known by the name of thymus, are,
according to this view, the original lymphatic glands of the vertebrate;
and it is to be noted that, in fishes and in Amphibia, lymphatic glands,
such as we know them in the higher mammals, do not exist; they are
characteristic of the higher stages of vertebrate evolution. In the lower
vertebrates, the only glandular masses apart from the cell-lining of the
body-cavity itself, which give rise to leucocyte-forming tissue, are these
segmental branchial glands, or possibly also the modified post-branchial
segmental glands, known as the head-kidney in Teleostea, etc.

The importance ascribed by Beard to the thymus in the formation of
leucocytes in the lowest vertebrates would be considerably reduced in value
if the branchial region of Ammocoetes possessed neither thymus glands nor
anything equivalent to them. Such, however, is not the case. Schaffer has
shown that in the young Ammocoetes masses of lymphatic glandular tissue are
found segmentally arranged in the neighbourhood of each gill-slit--tissue
which soon becomes converted into a swarming mass of leucocytes, and shows
by its staining, etc., how different it is from a blood-space. The presence
of this thymus leucocyte-forming tissue, as described by Schaffer, is
confirmed by Beard, and I myself have seen the same thing in my youngest
specimen of Ammocoetes.

Further, the very methods by which Kowalewsky has brought to light the
segmental lymph-glands of the branchial region of the Crustacea, etc., are
the same as those by which Weiss discovered the branchial nephric glands in
Amphioxus--excretory organs which Boveri considers to represent the
pronephros of the Craniota. In this supposition Boveri is right, in so far
that both pronephros and the tubules in Amphioxus belong to the same system
of excretory organs; but I entirely agree with van Wijhe that the region in
Amphioxus is wrong. The tubules in Amphioxus ought to be represented in the
branchial region of the Craniota, not in the post-branchial region; van
Wijhe therefore suggests that further researches may homologize them with
the thymus gland in the Craniota, not with the pronephros. This suggestion
of van Wijhe appears to me a remarkably good one, especially in view of the
position of the thymus glands in Ammocoetes and the nephric branchial
glands in Amphioxus. If, as I have pointed out, the atrial cavity of
Amphioxus has been closed in Ammocoetes by the apposition of {427}the
pleural fold with the branchial body-surface, then the remains of the
position of the atrial chamber must exist in Ammocoetes as that
extraordinary space between the somatic muscles and the branchial
basket-work filled with blood-spaces and modified muco-cartilage. It is in
this very space, close against the gill-slits, that the thymus glands of
Ammocoetes are found, in the very place where the nephric tubules of
Amphioxus would be found if its atrial cavity were closed completely.
Instead, therefore, of considering with Boveri that the branchial nephric
tubules of Amphioxus still exist in the Craniota as the pronephros, and
that the atrial chamber has narrowed down to the pronephric duct, I would
agree with van Wijhe that the pronephros is post-branchial, and suggest
that by the complete closure of the atrial space in the branchial region
the branchial nephric tubules have lost all external opening, and
consequently, as in all other cases, have changed into lymphatic tissue and
become the segmental thymus glands.

As van Wijhe himself remarks, the time is hardly ripe for making any
positive statement about the relationship between the thymus gland and
branchial excretory organs. There is at present not sufficient consensus of
opinion to enable us to speak with any certainty on the subject, yet there
is so much suggestiveness in the various statements of different authors as
to make it worth while to consider the question briefly.

On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and
parathymus, are all stated to be derivatives of the epithelium lining the
gill-slits, and Maurer would draw a distinction between the organs derived
from the dorsal side of the gill-cleft and those derived from the ventral
side--the former being thymus, the latter forming the epithelial
cell-nests, _i.e._ parathyroids. The thymus in Ammocoetes, according to
Schaffer, lies both ventral and dorsal to the gill-cleft; Maurer thinks
that only the dorsal part corresponds to the thymus, the ventral part
corresponding to the parathyroids, etc. Structurally, the thymus,
parathyroids, and the epithelial cell-nests are remarkably similar, so that
the evidence appears to point to the conclusion that, in the neighbourhood
of the gill-slits, segmentally arranged organs of a lymphatic character are
situated, which give origin to the thymus, parathyroids, tonsils, etc. Now,
among these organs, _i.e._ among those ventrally situated, Maurer places
the carotid gland, so that, if he is right, the origin of the carotid gland
{428}might be expected to help in the elucidation of the origin of the
thymus.

The origin of the carotid gland has been investigated recently by Kohn, who
finds that it is associated with the sympathetic nervous system in the same
way as the suprarenals. He desires, in fact, to make a separate category
for such nerve-glands, or paraganglia, as he calls them, and considers them
all to be derivatives of the sympathetic nervous system, and to have
nothing to do with excretory organs. The carotid gland is, according to
him, the foremost of the suprarenal masses in the Elasmobranchs, viz. the
so-called axillary heart.

In my opinion, nests of sympathetic ganglion-cells necessarily mean the
supply of efferent fibres to some organ, for all such ganglia are efferent,
and also, if they are found in the organ, would have been brought into it
by way of the blood-vessels supplying the organ, so that Aichel's statement
of the origin of the suprarenals in the Elasmobranchs seems to me much more
probable than a derivation from nerve-cells. If, then, it prove that Aichel
is right as to the origin of the suprarenals, and Kohn is right in
classifying the carotid gland with the suprarenals, then Maurer's
statements would bring the parathyroids, thymus, etc., into line with the
adrenals, and suggest that they represent the segmented glandular excretory
organs of the branchial region, into which, just as in the interrenals of
Elasmobranchs, or the cortical part of the adrenals of the higher
vertebrates, there has been no invasion of sympathetic ganglion-cells.

Wheeler makes a most suggestive remark in his paper on Petromyzon: he
thinks he has obtained evidence of serial homologues of the pronephric
tubules in the branchial region of Ammocoetes, but has not been able up to
the present to follow them out. If what he thinks to be serial homologues
of the pronephric tubules in the branchial region should prove to be the
origin of the thymus glands of Schaffer, then van Wijhe's suggestion that
the thymus represents the excretory organs of the branchial region would
gain enormously in probability. Until some such further investigation has
been undertaken, I can only say that it seems to me most likely that the
thymus, etc., represent the lymphatic branchial glands of the Crustacea,
and therefore represent the missing coxal glands of the branchial region.

This, however, is not all, for the appendages of the mesosomatic region, as
I have shown, do not all bear branchiæ; the foremost or {429}opercular
appendage carries the thyroid gland. Again, the basal part of the appendage
is all that is left; the thyroid gland is in position a coxal gland. It
ought, therefore, to represent the coxal gland of this appendage, just as
the thymus, tonsils, etc., represent the coxal glands of the rest of the
mesosomatic appendages. In the thyroid gland we again see a ductless gland
of immense importance to the economy, not a useless organ, but one, like
the other modified coxal glands, whose removal involves far-reaching vital
consequences. Such a gland, on my theory, was in the arthropod a part of
the external genital ducts which opened on the basal joint of the
operculum. What, then, is the opinion of morphologists as to the meaning of
these external genital ducts?

In a note to Gulland's paper on the coxal glands of Limulus, Lankester
states that the conversion of an externally-opening tubular gland (coxal
gland) into a ductless gland is the same kind of thing as the history of
the development of the suprarenal from a modified portion of mesonephros,
as given by Weldon. Further, that in other arthropods with glands of a
tubular character opening to the exterior at the base of the appendages, we
also have coxal nephridia, such as the shell-glands of the Entomostraca,
green glands of Crustacea (antennary coxal gland); and further on he
writes: "When once the notion is admitted that ducts opening at the base of
limbs in the Arthropoda are possibly and even probably modified nephridia,
we immediately conceive the hypothesis that the genital ducts of the
Arthropoda are modified nephridia."

So, also, Korschelt and Heider, in their general summing up on the
Arthropoda, say: "In Peripatus, where the nephridia appear, as in the
Annelida, in all the trunk-segments, a considerable portion of the
primitive segments is directly utilized for the formation of the nephridia.
In the other groups, the whole question of the rise of the organs known as
nephridia is still undecided, but it may be mentioned as very probable that
the salivary and anal glands of Peripatus, the antennal and shell-glands of
the Crustacea, the coxal glands of Limulus and the Arachnida, as well as
the efferent genital ducts, are derived from nephridia, and in any case are
mesodermal in origin."

The necessary corollary to this exceedingly probable argument is that
glandular structures such as the uterine glands of the scorpion already
described, which are found in connection with these terminal {430}genital
ducts, may be classed as modified nephridial glands, and that therefore the
thyroid gland of Ammocoetes, which, on the theory of this book, arose in
connection with the opercular genital ducts of the palæostracan ancestor,
represents the coxal glands of this fused pair of appendages. Such a gland,
although its function in connection with the genital organs had long
disappeared, still, in virtue of its original excretory function,
persisted, and even in the higher vertebrates, after it had lost all
semblance of its former structure and become a ductless gland of an
apparently rudimentary nature, still, by its excretory function,
demonstrates its vital importance even to the highest vertebrate.

By this simple explanation we see how these hitherto mysterious ductless
glands, pituitary, thymus, tonsils, thyroid, are all accounted for, are all
members of a common stock--coxal glands--which originally, as in Peripatus,
excreted at the base of the prosomatic and mesosomatic appendages, and are
still retained because of the importance of their excretory function,
although ductless owing to the modification of their original appendages.

Finally, there is yet another organ in the vertebrate which follows the
same law of the conversion of an excretory organ into a lymphatic organ
when its connection with the exterior is obliterated, and that is the
vertebrate body-cavity itself. According to the scheme here put forth, the
body-cavity of the vertebrate arose by the fusion of a ventral prolongation
of the original nephrocoele on each side; prolongations which accompanied
the formation of the new ventral midgut, and by their fusion formed
originally a pair of cavities along the whole length of the abdomen, being
separated from each other by the ventral mesentery of the gut.
Subsequently, by the ventral fusion of these two cavities, the body-cavity
of the adult vertebrate was formed.

This is simply a statement of the known method of formation of the
body-cavity in the embryo, and its phylogenetic explanation is that the
body-cavity of the vertebrate must be looked upon as a ventral prolongation
of the original ancestral body-cavity. Embryology clearly teaches that the
original body-cavity or somite was confined to the region of the notochord
and central nervous system, and there, just as in Peripatus, was divisible
into a dorsal part, giving origin to the myocoele, and a ventral part,
forming the nephrocoele. From this original nephrocoele are formed the
pronephric excretory organs, the mesonephric excretory organs, and the
body-cavity.

{431}That the vertebrate body-cavity was originally a nephrocoele is
generally accepted, and its excretory function is shown by the fact that it
communicates with the exterior in all the lower vertebrates, either through
abdominal pores or by way of nephridial funnels. Bles has shown how largely
these two methods of communicating with the exterior mutually exclude each
other. In the higher vertebrates both channels become closed, except in the
case of the Fallopian tubes, and thus, so to speak, the body-cavity becomes
a ductless gland, still, however, with an excretory function, but now, as
in all other cases, forming a part of the lymphatic rather than of the true
excretory system.


SUMMARY.

  The consideration of the formation of the vertebrate cranial region, as
  set forth in previous chapters, indicates that the ancestor of the
  vertebrates was not an arachnid purely or a crustacean purely, but
  possessed partly crustacean and partly arachnid characters. In order to
  express this conclusion, I have used the term Protostraca, invented by
  Korschelt and Heider, to indicate a primitive arthropod group, from which
  both arachnids and crustaceans may be supposed to have arisen, and have
  therefore stated that the vertebrate did not arise directly from the
  annelids, but from the Protostraca. Such an origin signifies that the
  origin of the excretory organs of the vertebrate must not be looked for
  in the segmental organs of the annelid, but rather in such modified
  annelid organs as would naturally exist in a primitive arthropod group.
  The nature of such organs may be inferred, owing to the fortunate
  circumstance that so primitive an arthropod as Peripatus still exists,
  and we may conclude that the protostracan ancestor possessed in every
  segment a pair of appendages and a pair of coelomic cavities, which
  extended into the base of these appendages. The ventral portion of each
  of these coelomic cavities separated off from the dorsal and formed a
  nephrocoele, giving origin to a segmental excretory organ, which, seeing
  that its end-vesicle was in the base of the appendage, and seeing also
  the nature of the known arachnid and crustacean excretory organs, may
  fitly be termed a coxal gland. This, then, is the working hypothesis to
  explain the difficulties connected with the origin of the pronephros and
  mesonephros--that the original segmental organs were coxal glands, and
  therefore indicated the presence of appendages. This hypothesis leads to
  the following conclusions:--

  1. The coxal glands belonging to the post-branchial appendages of the
  invertebrate ancestor are represented by the pronephric tubules, and
  existed over the whole metasomatic region.

  2. Such glands discharged into a common duct--the pronephric duct--which
  opened into the cloacal region, either in the protostracan stage, when
  the metasomatic appendages were still in existence, just as the coxal
  glands of the prosomatic region in Limulus discharge into a common duct,
  or else the pronephric duct was formed when the appendages were
  obliterated.

  {432}3. The metasomatic appendages disappeared owing to their enclosure
  by pleural folds, which, meeting in the mid-ventral line, not only caused
  the obliteration of the appendages, and gave a smooth fish-like
  body-surface to the animal, but also caused the formation of an atrial
  cavity.

  4. Into these pleural folds the dorsal longitudinal muscles of the body
  extended, and ultimately reached to the ventral surface, thus forming the
  somatic muscles of the vertebrate body.

  5. When the pleural folds had met in the mid-ventral line the animal had
  become a vertebrate, and was dependent for its locomotion on the
  movements of these somatic muscles, and not on the movements of
  appendages. Consequently, elongation of the trunk-region took place, for
  the purpose of increasing mobility, by the formation of new metameres.

  6. Each of such metameres possessed its own segmental excretory organ,
  formed in the same way as the previous pronephric organs, but, as there
  were no appendages in these new-formed segments, the excretory organs
  took on the characters of a mesonephros, not a pronephros, and opened
  into the pronephric duct, because the direct way to the exterior was
  blocked by the enveloping pleural folds.

  7. The group of annelids from which the protostracan ancestor of the
  vertebrates arose was the highest annelidan group, viz. the Polychæta, as
  shown by the nature of the excretory organs in Amphioxus.

  8. The coxal glands of the protostracan ancestor existed on all the
  segments, and were, therefore, divisible into three groups, prosomatic,
  mesosomatic, and metasomatic; these three groups of coxal glands still
  exist in the vertebrate as ductless glands.

  9. The prosomatic coxal glands form the pituitary body.

  10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids,
  tonsils, etc.

  11. The metasomatic coxal glands form the adrenals.

  12. The procoelom of the vertebrate is the procoelom of the protostracan
  ancestor, which splits into a dorsal part, the myocoele, and a ventral
  part, the nephrocoele. This latter part not only forms the pronephros and
  mesonephros, but also by a ventral extension gives origin to the walls of
  the vertebrate body-cavity or metacoele.

  13. This ventral extension of the original nephrocoele at first excreted
  to the exterior, through abdominal pores, or through peritoneal funnels.
  When such paths to the exterior became closed, it also became a ductless
  gland, belonging to the lymphatic system.




{433}CHAPTER XIII

_THE NOTOCHORD AND ALIMENTARY CANAL_

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


In the previous chapters all the important organs of the arthropod have
been found in the vertebrate in their appropriate place, of similar
structure, and innervated from corresponding parts of the central nervous
system. Such comparison is possible only as long as the ventral and dorsal
surfaces of the vertebrate correspond with the respective surfaces of the
arthropod, and no reversal is assumed. This method of comparative anatomy
is the surest and most certain guide to the relationship between two
animals, and when the facts obtained by the anatomical method are so
strikingly confirmatory of the palæontological evidence, the combined
evidence becomes so strong as to amount almost to a certainty that
vertebrates did arise from arthropods in the manner mapped out in previous
chapters, and not from a hypothetical group of animals, such as is
postulated in the theory of their origin from forms like Balanoglossus.

The latter theory derives the alimentary canal of the vertebrate from that
of the invertebrate, and finds in the latter the commencement of the
notochord. In the comparison which I have made the alimentary canal of the
invertebrate ancestor has become the tube of the central nervous system of
the vertebrate, and there is no sign of a notochord whatever. All the
organs of the arthropod have already been allocated; where the notochord is
situated in the {434}vertebrate there is nothing but a gap in the
invertebrate, but the position of that gap can be settled with great
accuracy from the previous comparison of organs in the two groups. So,
also, the alimentary canal of the vertebrate is from the very nature of the
case a new organ, yet, as has been shown in Chapter V., the comparison of
the respiratory organs in the two groups gives a strong suggestion of the
manner in which such a canal was formed.


THE ORIGIN OF THE NOTOCHORD.

The time has now come to endeavour to frame a plausible theory of the
method of formation of the notochord and the new alimentary canal, and thus
to complete the diagram on p. 413. The comparative method is no longer
available, for these structures are both unrepresented as such in the
arthropod; any suggested explanation, therefore, must be more tentative,
and cannot give the same feeling of certainty as is the case with all the
organs already considered. Our only chance of finding out the past history
of the notochord lies in the embryological method, in the hope that,
according to the 'law of recapitulation,' the ancestral history may be
repeated in the ontogeny with sufficient clearness to enable some
conclusion to be drawn.

At the outset, one point comes out clearly--the close relationship between
the notochord and the vertebrate gut; they are both derived from the same
layer, both parts of the same structure. On this point all embryologists
are agreed; it is expressed in such statements as, "the notochord, as well
as the alimentary canal, is formed from hypoblast"; "the notochord arises
as a thickening in the dorsal wall of the alimentary canal." The two
structures are so closely connected together that they must be considered
together. If we can conjecture the origin of the one, we may be sure that
we have the clue to the origin of the other. The two together form the one
new organ which distinguishes the vertebrate from the arthropod, the only
thing left which requires explanation for the completion of this strange
history.

What, then, is the notochord? What are its characteristics? In the highest
vertebrates it is conspicuous only in the embryo; with the development of
the axial skeleton it is more and more squeezed out of existence, until in
the adult it is no longer visible. By the 'law of recapitulation' this
developmental history implies that, as we descend the vertebrate phylum,
the notochord ought to be more and {435}more conspicuous, more and more
permanent during the life of the animal. Such is, indeed, found to be the
case, until at last, in the lowest vertebrates, such as the lamprey, and in
forms like Amphioxus, the notochord persists throughout the life of the
animal as a large important axial supporting rod.

This rod has a number of striking characteristics which distinguish it from
all other structures, and are the only means of guessing its probable
origin. Its position in the body is always the same in all vertebrates and
is very significant, for it lies just ventrally to the central nervous
system, along nearly the whole length of the animal, not quite the whole
length, for it invariably terminates close to the place where the
infundibulum comes to the surface of the brain; it is, in fact, always
confined to the infra-infundibular and spinal cord part of the central
nervous system. Interpreting this into the language of the arthropod, it
means that a rod was formed just ventrally to the nervous system, which
extended the whole length of the infraoesophageal and ventral chain of
ganglia, and terminated at the orifice of the mouth. Moreover, this rod was
unsegmented, for the notochord is devoid of segmentation.

At the anterior end the rod tapers to a point, as in Fig. 166. In its
middle part it is very large and conspicuous, cylindrical in shape; its
interior is filled with a peculiar vacuolated tissue, different to any
other known vertebrate tissue, which has therefore received the name of
notochordal tissue. Outside this is a thick sheath formed of many layers,
of which the external one gives the staining reactions of elastin, and is
called the external elastic layer. Between this sheath and the notochordal
tissue a thin layer of lining cells, of normal appearance, is conspicuous
in Ammocoetes. These cells secrete the layers of the sheath, and have
originally, by proliferation, given rise to the notochordal tissue. In the
notochord of Ammocoetes there is no sign of either nerves, blood-vessels,
or muscles.

The centre of the notochord presents the appearance of a slight slit, as
though it had originated from a tube, and that is the opinion now generally
held, for its mode of formation in the embryo is as that of a tube formed
from an open groove, as will be explained immediately.

We may, then, conceive of the notochord as originally a tube lying in the
mid-line just ventrally to the central nervous system, and extending from
the original mouth to the end of the body. Translate this into the language
of the arthropod and it denotes a tube on the {436}mid-ventral surface of
the body, which extended from mouth to anus. Such a tube might be formed
from the mid-ventral surface as follows:--

In Fig. 163, A, the lining of the ventral surface between two appendages is
represented flat, in B is shown how the formation of a solid rod may arise
from the bulging of that ventral surface, and in C how a groove on that
surface may lead to the formation of a tube between the two appendages. The
difference between a notochordal rod formed as in B from that in C would be
shown in the sheath, for in B the sheath would be formed from the cuticle
of the lining cells, and in C from the basement membrane. The structure of
the sheath is in accordance with the embryological evidence that the
notochord is formed as a tube from a groove, as in C, and not as a solid
rod as in B, for it possesses a well-marked elastin layer, and elastin has
never yet been found as a constituent of any cuticular secretion, but
invariably in connection with basement-membranes.

[Illustration: FIG. 163.--DIAGRAM OF TWO POSSIBLE METHODS OF THE FORMATION
OF A NOTOCHORD.]

The position, then, of the notochord and its method of formation suggests
that the mid-ventral surface of the arthropod ancestor of the vertebrate
formed a deep groove between the bases of all the prosomatic, mesosomatic,
and metasomatic appendages, which was subsequently converted into a tube
extending along the whole of the body between mouth and anus, and finally,
by the proliferation of its lining cells and their conversion into
notochordal tissue, became the notochordal rod of the vertebrate.

As already frequently stated, Apus and Branchipus are the two living
arthropods which most nearly resemble the extinct trilobites. The beautiful
specimens of Triarthrus (Fig. 165) found by Beecher give an idea of the
under surface of the trilobite such as has never been obtained before, and
demonstrate how closely the condition of things found in Apus (Fig. 164)
was similar to that occurring in the trilobites. In both cases the
mid-ventral surface of the animal formed a deep groove which extended the
whole length of the {437}animal; on each side of this groove in Apus are
closely set the gnatho-bases of the appendages, in such a manner that the
groove can be easily converted into a canal by the movements of these
bases--a canal which, owing to the great number of the appendages and their
closeness to each other, can be completely and efficiently closed.

[Illustration: FIG. 164.--UNDER-SURFACE OF APUS. (After BRONN.)]

[Illustration: FIG. 165.--UNDER-SURFACE OF A TRILOBITE (_Triarthrus_).
(From BEECHER.)]

All those who have seen Apus in the living state assert that this canal so
formed is actually used by the animal for feeding purposes. By the
movements of the gnatho-bases food is passed up from the hind end of the
animal along the whole length of this ventral canal to the mouth, where it
is taken in and swallowed. In this way Apus has been seen to swallow its
own eggs.

In the trilobites there is a similar deep channel formed by the mid-ventral
surface, similar gnatho-bases, and closely set appendages, and the membrane
of this ventral groove was extremely thin.

Here, then, in the very group of animals which were the progenitors of the
presumed palæostracan ancestor of the vertebrate--a group which is
characterized by its extensive prevalence and its {438}enormous variety of
form during the great trilobite era--the formation of a mid-ventral canal
out of this deep ventral groove is seen to be not only easy to imagine, but
most probable, provided that a necessity arose for such a conversion.

For what purpose might such a tube have been formed? I would suggest that
it might have acted as an accessory food-channel, which was of sufficient
value at the time to give some advantage in the struggle for existence to
those members of the group who were thus able to supplement their intake of
food, but at the same time was so inefficient that it was quickly
superseded by the new alimentary canal, and thus losing its temporary
function, became solid, and was utilized to form an axial supporting rod.

There is a very considerable amount of evidence in favour of the view that
the notochord was originally a digestive tube; in fact, as far as I know,
this conclusion is universally accepted. The evidence is based essentially
upon its development and upon its structure. It is formed in the vertebrate
from the same layer as the alimentary canal, _i.e._ the hypoblast, and in
Amphioxus it commences as a groove in the dorsal wall of the future
alimentary canal; this groove then closes to form the tube of the
notochord, and separates from the alimentary canal. Embryologically, then,
the notochord is looked upon as a tube formed directly from the alimentary
canal.

As regards its structure, its tissue is, as already stated, something _sui
generis_. Notochordal tissue has no resemblance to bone or cartilage, or
any of the usual supporting tissues. Such a tissue is not, however,
entirely confined to the notochord of the vertebrates, but tissue closely
resembling it has been found not only in Amphioxus and the Tunicata, but in
certain other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in
Cephalodiscus, and in Actinotrocha. In all these latter cases, such a
tissue is invariably found in disused portions of the alimentary canal; a
diverticulum of the alimentary canal becomes closed, vacuolation of its
lining cells takes place, and a tissue resembling notochordal tissue is
formed.

Owing to the notochord being invariably so striking and mysterious a
feature of the lowest vertebrates, the term vertebrate, which is
inappropriate in the members of the group which do not yet possess
vertebræ, has been largely superseded by the term chordate, with the result
of attributing an undue preponderance to this tissue in any system of
classification. Hence, wherever any animal has been found {439}with a
tissue resembling that of the notochord, enthusiasts have immediately
jumped to the conclusion that a relationship must exist between it and the
chordate animals; and, accordingly, they have classified such animals as
follows: Amphioxus belongs to the group _Cephalochorda_ because the
notochord projects beyond the central nervous system; the Tunicata are
called _Urochorda_ because it is confined to the tail; the Enteropneusta,
_Hemichorda_, because this tissue is confined to a small diverticulum of
the gut, and, finally, _Diplochorda_ has been suggested for Actinotrocha
and Phoronis because two separate portions of the gut are transformed in
this way.

This exaggerated importance given to any tissue resembling in structure
that of the notochord is believed in by many of those who profess to be our
teachers on this subject, the very men who can deliberately shut their eyes
to the plain reading of the story of the pineal eyes, and say, "In our
opinion this pineal organ was not an eye at all."

The only legitimate inference to be drawn from the similarity of structure
between the notochord and these degenerated gut-diverticula, is that the
structure of the notochord may have arisen in the same way, and that
therefore the notochord may once have functioned as a gut. With cessation
of its function its cells became vacuolated, as in these other cases, and
its lumen became filled with notochordal tissue. This evidence strongly
confirms the suggestion that the notochord was once a digestive tube, but
by no means signifies that such tissue, wherever found, indicates the
presence of a notochord.

In order to resemble a notochord, this tissue must possess not only a
definite structure but a definite position, and this position is a
remarkably striking and suggestive one. The notochordal tube is
unsegmented, although the vertebrate is markedly segmented. But in all
segmented animals the only unsegmented tube which extends the whole length
of the body, from mouth to anus, is invariably the gut. In the vertebrate
there are three such tubes: (1) the gut itself, (2) the central canal of
the nervous system, and (3) the notochordal tube.

The first is the present gut, the second the gut of the invertebrate
ancestor, and the third the tube in question.

These three unsegmented tubes, extending along the whole length {440}of the
segmented animal, constitute the great peculiarity of the vertebrate group;
it is not the unsegmented notochord alone which requires explanation, but
the presence of three such tubes in the same animal. Any one of them might
be the unsegmented gut of the segmented animal. The most ventral tube is
the actual gut of the present vertebrate; the most dorsal--the neural
canal--was, according to my view, the original gut of the invertebrate
ancestor; the middle one--the notochordal tube--was, in all probability,
also once a gut, formed at the time when the exigencies of the situation
made it difficult for food to pass along the original gut.

[Illustration: FIG. 166.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN
SUCH A VERTEBRATE AS THE LAMPREY.

_Nc._, neural canal with its infundibular termination; _Nch._, notochord;
_Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial
or nasal tube; _Or._, oral chamber closed by septum.]

Yet another circumstance in favour of this suggestion is the very striking
position of the anterior termination of the notochord. It terminates at the
point of convergence of three structures:--

(1) The tube of the hypophysis or nasal tube.

(2) The infundibulum or old mouth-termination.

(3) The notochordal tube.

To these may be added, according to Kupffer, in the embryonic stage, the
anterior diverticulum of the gut (Fig. 166).

This is a very significant point. Here originally, in the invertebrate
stage, the olfactory passage opened into the old mouth and oesophagus.
Here, finally, in the completed vertebrate the same olfactory passage opens
into the new pharynx. In the stage between the two it may well have opened
into an intermediate gut, the notochordal tube, its separation from which
would leave the end of the {441}notochord blind, just as it had already
left the end of the infundibulum blind.

The whole evidence points to the derivation of the notochord from a ventral
groove on the surface of the animal, which closed to form a tube capable of
acting as an accessory gut at the critical period before the new gut was
fully formed. The essentials of a gut tube are absorption and digestion of
food; is it likely that a tube formed as I have suggested would be
efficient for such purposes?

As far as absorption is concerned, no difficulty would arise. The gut of
the arthropod is lined with a thin layer of chitin, which is traversed,
like all other chitinous surfaces, by fine canaliculi. Through these
canaliculi, absorption of fluid material takes place, from the gut to the
body. Similar canaliculi occur in the chitin covering the animal
externally, so that, if such external surface formed a tube, and food in
the right condition for absorption passed along it, absorption could easily
take place through the chitinous surface. The evidence of Apus proves that
food does pass along such a tube in the open condition, and in the
trilobites the chitinous surface lining a similar groove was apparently
very thin, a condition still more favourable to such an absorption process.

At first sight the second essential of a gut-tube--the power of
digestion--appears to present an insuperable difficulty to this method of
forming an accessory gut-tube, for it necessitates the formation of a
secretion capable of digesting proteid material by the external cells of
the body, whereas until recently it was supposed that such a function was
confined to cells belonging to the so-called hypoblastic layer. Experiments
were made now years ago of turning a Hydra inside out so that its internal
layer should become external, and _vice versâ_, and they were said to have
been successful. Such an animal could go on living and absorbing and
digesting food, although its epiblastic surface was now its digestive
internal surface. More recent observations have shown that these
experiments were fallacious. At night-time, when the observer was not
looking, the hydra reinverted itself, so that again its original digestive
surface was inside and it lived and prospered as before.

Another piece of evidence of somewhat similar kind, which has not as yet
been discredited, is seen in the Tunicata. In many of these, new
individuals are formed from the parent by a process of budding, and it has
been proved that frequently the gut of the new {442}individual thus budded
off arises not from the gut or hypoblastic layer of the parent, but from
the surface or epiblastic layer. Such gut so formed possesses as efficient
digestive powers as the gut of the parent.

The most remarkable evidence of all has been afforded by Miss Alcock's
experiments. She examined the different tissues of Ammocoetes for the
express purpose of finding out their power of digesting fibrin, with the
result that the most active cells were those of the liver. Next in activity
came the extract of the lining cells of the respiratory chamber and of the
skin. The intestine itself when freed from the liver-secretion had very
little digestive power; extracts of muscle, nervous system, and thyroid
gland had no power whatever, but the extract of the skin-cells possessed a
powerful digesting action.

Furthermore, it is not necessary to make an extract of the skin in order to
obtain this digestive fluid, for under the influence of chloroform the skin
of Ammocoetes secretes copiously, and this fluid thus secreted was found to
possess strong digestive powers. So, also, Miss Alcock has demonstrated the
power of digesting fibrin in a similar secretion of the epithelial cells
lining the carapace of the crayfish. In both cases a very plausible reason
for the presence of a digestive ferment in a skin-secretion is found in the
necessity of preventing the growth of parasites, fungoid, or otherwise,
especially in those parts where the animal cannot keep itself clean by
'preening.' Thus in a crayfish, in which the oesophageal commissures had
been cut, fungus was found to grow on the ventral side, but not on the
dorsal carapace. The animal was accustomed to keep its ventral surface
clean by preening; owing to the paralysis it could not do so, and
consequently the fungus grew there. In the lamprey I found that wherever
there was a removal of the surface-epithelium, from whatever cause, that
spot was immediately covered with a fungoid growth, although in the intact
lamprey the skin was invariably smooth and clean.

I imagine, then, that this digestive power of the skin arose as a
protective mechanism against parasitic attacks; it is self-evident how a
tube formed of such material must _ab initio_ act as a digestive tube.

In yet another respect this skin secretion of Ammocoetes is most
instructive. The surface of Ammocoetes is absolutely smooth, no scales
{443}of any kind exist; this smoothness is due to the presence of a very
well-defined cuticular layer secreted by the underlying epithelial cells.
This cuticle is very much thicker than is usually found in vertebrates,
and, strangely enough, has been thought to contain chitin. Whether it
really contains chitin or not I am unable to say, but it certainly
resembles a chitinous layer in one respect; it is perforated by innumerable
very fine tubes or canaliculi, along which, by appropriate staining, it is
easy to see the secretion of the underlying cell pass to the exterior (Fig.
140). This marked digestive power of the skin of Ammocoetes, together with
the easy passage of the secretion through the thin cuticular layer, renders
it almost certain that a tube formed from the deep ventral groove of the
trilobite would, from the very first, act as a digestive as well as an
absorbent tube; in other words, the notochord as soon as formed was able to
act as an accessory digestive tube.

This suggested origin of the notochord from a groove along the mid-ventral
surface of the body not only indicates a starting-point from a markedly
segmented portion of the body, but also points to its formation at a stage
previous to the formation of the operculum by the fusion of the two
foremost mesosomatic appendages--indicates therefore its formation at a
stage more nearly allied to the trilobite than to the sea-scorpion. The
chance of ever finding any direct evidence of such a chordate trilobite
stage appears to me exceedingly improbable, and I greatly fear that this
conception of the mode of formation of the notochord can never be put to
direct proof, but must always remain guesswork.

On the other hand, evidence of a kind in favour of its origin from a
segmented part of the body does exist, and that evidence has this special
value, that it is found only in that most primitive animal, Amphioxus.

This evidence is as follows:--

At fairly regular intervals, the sheath of the notochord is interrupted on
each side of the mid-dorsal line by a series of holes, which penetrate the
whole thickness of the sheath. This dorsal part is pressed closely against
the spinal cord, and through these holes fibres appear to pass from the
spinal cord to the interior of the notochord. So greatly do these fibres
present the appearance of ventral roots to the notochord, that Miss Platt
looks upon them as paired motor roots to the notochord, or at all events as
once having been such motor {444}roots. Lwoff and Rolph both describe a
direct communication between the spinal cord and the notochord by means of
fibres passing through these holes, without however looking upon this
connection as a nervous one. Joseph alone asserts that no absolute
connection exists, for the internal elastic layer of the notochord,
according to him, is not interrupted at these holes, and forms, therefore,
a barrier between the fibres from the spinal cord and those from the
interior of the notochord. Still, whatever is the ultimate verdict as to
these fibres, the suggestive fact remains of the spaces in the notochordal
sheath and of the corresponding projecting root-like fibres from the spinal
cord. The whole appearance gives the impression of some former connection,
or rather series of connections, between the spinal cord and the notochord,
such as would have occurred if nerves had once passed into the notochord.
On the other hand, such nerves were not arranged segmentally with the
myotomes, for, according to Joseph, in the middle of the animal ten to
twelve such holes occur in one body-segment. In Apus the appendages are
more numerous than the body-segments, so that it is not necessary for a
segmental arrangement to coincide with that of the body-segments.


THE ORIGIN OF THE ALIMENTARY CANAL.

In close connection with the notochord is the alimentary canal. Any
explanation of the one must be of assistance in explaining the other.

According to the prevalent embryological teaching, the body is formed of
three layers, epiblast, hypoblast, and mesoblast, and the gastræa theory of
the origin of all Metazoa implies of necessity that the formation of every
individual commences with the formation of the gut. For this reason the
alimentary canal must in every case be regarded as the earliest formed
organ, however late in the development it may attain its finished
appearance. Hence the notochord is spoken of as developed from the
mid-dorsal wall of the alimentary canal. It is possible to look at the
question the other way round, and suppose that the organ whose development
is finished first is older than the one still in process of making. In this
case it would be more right to say a ventral extension of the tissue, which
gives rise to the notochord, takes place and forms the alimentary canal. It
is, to my mind, perfectly possible, and indeed probable, that {445}the
formation of the vertebrate alimentary canal was a repetition of the same
process which had already led to the formation of the notochordal tube. The
formation of the anterior part of the alimentary canal in Ammocoetes at the
time of transformation strongly suggests the marked similarity of the two
processes.

Of all the startling surprises which occur at transformation, this
formation of a new anterior gut is the most startling. From the oral
chamber of Petromyzon two tubes start: the one leads into the
gill-chambers, is known as the bronchus, and is entirely concerned with
respiration; the other leads without a break from the mouth to the anus,
has no connection with respiration, and is the alimentary canal of the
animal. Any one looking at Petromyzon would say that its alimentary canal
was absolutely non-respiratory in character. Before transformation, this
kind of alimentary canal commences at the end of the respiratory chamber;
from here to the anus it is of the same character as in Petromyzon, but in
Ammocoetes the non-respiratory anterior part simply does not exist: the
whole anterior chamber is both respiratory and affords passage to food.
This part of the alimentary canal of the adult is formed anew. We see,
then, here the formation of a part of the alimentary canal taking place,
not in an embryo full of yolk, but in a free-living, independent, grown-up
larval form in which all yolk has long since disappeared: a condition
absolutely unique in the vertebrate kingdom, but one which more than any
other may be expected to give a clue to the method of formation of a
vertebrate gut.

The formation of this new gut can be easily followed at transformation, and
was originally described by Schneider. His statement has been confirmed by
Nestler, and its absolute truth has been demonstrated to me again and again
by Miss Alcock, in her specimens illustrative of the transformation
process. First, in the mid-dorsal line of the respiratory chamber a
distinct groove is formed, the edges of which come together and form a
solid rod. This solid rod blocks the opening of the respiratory chamber
into the mid-gut, so that during this period of the transformation no food
can pass out of the pharyngeal chamber. A lumen then begins to appear in
this solid rod at the posterior end, which steadily advances mouthwards
until it opens into the oral chamber and thus forms an open tube connecting
the mouth with the gut.

Here, then, is the foundation of a new gut on very similar lines {446}to
that of the notochord, by the conversion of a groove into a tube. Still
more suggestive is it to find that the tube so formed has no appearance
whatever of segmentation; it is as unsegmented as the rest of the gut,
although, as is seen in Fig. 62, the dorsal wall of the respiratory chamber
from which it arose is as markedly segmented as any part of the animal.
Here under our very eyes, in the course of a few days or weeks, an
object-lesson in the process of the manufacture of an alimentary canal is
carried out and completed, and the teaching of that lesson is that a
gut-tube may be formed in the same way as the notochordal tube, by the
conversion of a grooved surface into a canal, and that gut-tube so formed,
like the notochord, loses all sign of segmentation, even although the
original grooved surface was markedly segmented.

The suggestion then is, that the new gut may have been formed by a
repetition of the same process which had already given origin to the
notochord.

Such a method of formation is not, in my opinion, opposed to the evidence
given by embryology, but in accordance with it; the discussion of this
point will come best in the next chapter, which treats of the embryological
evidence as a whole, and will therefore be left till then.


THE EVIDENCE GIVEN BY THE INNERVATION OF THE VERTEBRATE ALIMENTARY CANAL.

Throughout this investigation the one fixed landmark to which all other
comparisons must be referred, is the central nervous system, and the
innervation of every organ has given the clue to the meaning of that organ.
So also it must be with the new alimentary canal; by its innervation we
ought to obtain some insight into the manner of its origination. In any
organ the nerves which are specially of value in determining its
innervation, are of necessity the efferent or motor nerves, for the limits
of their distribution in the organ are much more easily determined than
those of the afferent or sensory nerves. The question therefore of primary
importance in endeavouring to determine the nature of the origin of the
alimentary canal from its innervation is the determination of the efferent
supply to the musculature of its walls.

Already in previous chapters a commencement has been made in {447}this
direction; thus the musculature of the oral chamber has been derived
directly from the musculature of the prosomatic appendages; the muscles
which move the eyes from the prosomatic and mesosomatic dorso-ventral
somatic muscles; the longitudinal body-muscles from the dorsal longitudinal
somatic muscles of the arthropod; the muscles of respiration from the
dorso-ventral muscles of the mesosomatic appendages.

In all these cases we have been dealing with striated musculature and
consequently with only the motor nerves of the muscle; but the gut
posterior to the pharyngeal or respiratory chamber contains unstriped
instead of striped muscle, and is innervated by two sets of nerves, those
which cause contraction and are motor, and those which cause relaxation and
are inhibitory. It is by no means certain that these two sets of nerves
possess equal value from a morphological point of view. The meaning of an
inhibitory nerve is at present difficult to understand, and in this
instance, is rendered still more doubtful owing to the presence of
Auerbach's plexus along the whole length of the intestine--an elaborate
system of nerve-cells and nerve-fibres situated between the layers of
longitudinal and circular muscles surrounding the gut-walls, which has been
shown by the recent experiments of Magnus, to constitute a special enteric
nervous system.

One of the strangest facts known about the system of inhibitory nerves is
their marked tendency to leave the central nervous system at a different
level to the corresponding motor nerves, as is well known in the case of
the heart, where the inhibitory nerve--the vagus--arises from the medulla
oblongata, while the motor nerve--the augmentor or accelerator--leaves the
spinal cord in the upper thoracic region. It is very difficult to obtain
any idea of the origin of such a peculiarity; I know of only one suggestive
fact, which concerns the innervation of the muscles which open and close
the chela of the crayfish, lobster, etc. These muscles are antagonistic to
each other, and both possess inhibitory as well as motor nerves. The
central nervous system arrangements are of such a character that the
contraction of the one muscle is accompanied by the inhibition of its
opposer, and the nerves which inhibit the contraction of the one, leave the
central nervous system with the nerves which cause the other to contract.
Thus the inhibitory and motor nerves of either the abductor (opener) or
adductor (closer) muscles of the crayfish claw do not leave the central
nervous system together, but in separate nerves.

{448}If now for some cause the one set of muscles either disappeared, or
were so altered as no longer to present any appearance of antagonism, then
there would be left a single set of muscles, the inhibitory and motor
nerves of which would leave the central nervous system at different levels,
and the older such systems might be, the greater would be the modification
in the shape and arrangements of parts in the animal, so that the two sets
of fibres might ultimately arise from very different levels.

As mentioned in the introductory chapter, the whole of this investigation
into the origin of vertebrates arose from my work on the system of efferent
nerves which innervate the vascular and visceral systems. One of the main
points of that investigation was the proof that such nerves did not leave
the central nervous system uniformly along the whole length of it, but in
three great outflows, cranial, thoracico-lumbar, and sacral; there being
two marked gaps separating the three outflows, caused by the interpolation
of the plexuses for the innervation of the anterior and posterior limbs
respectively. All these nerves are characterized by the presence of
ganglion-cells in their course to the periphery, they are, therefore,
distinguished from ordinary motor nerves to striated muscle in that their
impulses pass through a ganglion-cell before they reach the muscle.

The ganglia of the large middle thoracico-lumbar outflow constitute the
ganglia of the sympathetic system.

The functions of the nerves constituting these three outflows are very
different, as I pointed out in my original papers. Since then a large
amount of further information has been obtained by various observers,
especially Langley and Anderson, which enable the following statements to
be made:--

All the nerves which cause contraction of the unstriped muscles of the
skin, whether pilomotor or not, all the nerves which cause secretion of
sweat glands wherever situated, all the nerves which cause contraction or
augmentation of the action of muscles belonging to the vascular system, all
the nerves which are motor to the muscles belonging to all organs derived
from the Wolffian and Müllerian ducts, _e.g._ the uterus, ureters, urethra,
arise from the thoracico-lumbar outflow, never from the cranial or sacral
outflows. It is essentially an efferent skin-system.

On the other hand, the latter two sets of nerves are concerned {449}with
the supply of motor nerves to the alimentary canal; they form essentially
an efferent gut-system in contradistinction to the sympathetic or
skin-system.

A marked distinction exists between these cranial and sacral nerves. The
vagus never supplies the large intestine, the sacral nerves never supply
the small intestine. Associated with the large intestine is the bladder,
the whole system arising from the original cloacal region; the vagus never
supplies the bladder, its motor nerves belong to the sacral outflow. The
motor nerves to the ureters, to the urethra, and to the trigonal portion of
the bladder between the ureters and the urethra, do not arise from the
sacral outflow, but from the thoracico-lumbar. These muscles belong really
to the muscles in connection with the Müllerian and Wolffian ducts and
skin, not to the cloacal region.

The motor innervation then of the alimentary canal reveals this striking
and suggestive state of affairs. The motor innervation of the whole of the
small intestine arises from the cranial region, and is immediately followed
by an innervation from the sacral region for the whole of the muscles of
the cloaca. It thus indicates a head-region and a tail-region in close
contiguity, the whole of the spinal cord region between these two extremes
being apparently unrepresented. Not, however, quite unrepresented, for
Elliott has shown recently that the ileo-colic valve at the junction of the
small and large intestine is in reality an ileo-colic sphincter muscle, and
that this muscle receives its motor nerves neither from the vagus nor from
the sacral nerves, but from the thoracico-lumbar outflow or sympathetic
system. This may mean one of two things, either that a band of fibres
belonging to the skin-system has been added to the gut-musculature, for the
purpose of forming a sphincter at this spot, or that the region between the
vagus territory and the cloaca is represented by this small band of muscle.
The second explanation seems to me the more probable of the two. Between
the mesosomatic region represented by the vagus, and the cloacal region,
there existed a small metasomatic region, represented by the pronephros,
with its segmental duct, as already discussed in Chapter XII. That part of
the new alimentary canal which belonged to this region is the short piece
indicated by the ileo-colic sphincter, and innervated, therefore, from the
same region as the organs derived from the segmental duct.

Such innervation seems to me to suggest that originally the {450}vertebrate
consisted, as far as its gut was concerned, of a prosomatic and mesosomatic
(branchial) region, close behind which came the cloaca and anus. Between
the two there was a short metasomatic region (possibly pronephric), so that
the respiratory chamber did not open directly into the cloaca.

Such an interpretation is, I think, borne out by the study of the most
ancient forms of fish. In Bothriolepis, according to Patten, and in
Drepanaspis, according to Traquair, the cloacal region and anus follow
immediately upon the posterior end of the head-shield, _i.e._ immediately
after that region which presumably contained the branchiæ. Similarly, on
the invertebrate side, all those forms which resembled Limulus must have
possessed a very short region between the branchial and cloacal parts of
the body. The original cloacal part of the vertebrate gut may well have
been the original cloaca of the arthropod, into which its intestine emptied
itself, especially when we see the tendency of the scorpion group of
animals to form an accessory cloacal pouch known as the stercoral pouch or
pocket.

Again, it is striking to see how, in certain of the scorpion group, _e.g._
Thelyphonus and Phrynus, there is a caudal massing of the central
nerve-cells as well as a cephalic massing, so that their central nervous
system is composed of a cephalic and caudal brain. These two brains are
connected together by commissures extending the whole length of the body,
in which I have been unable to find any sign of ganglion-cells. What this
caudal brain innervates I do not know; it is, I think, a matter worth
further investigation, especially as there are many indications in the
vertebrate that the lumbo-sacral region of the cord possesses higher
functions than the thoracic region.

The method of formation of the alimentary canal as indicated by its
innervation is as follows:--

In front an oral chamber, formed, as already pointed out, by the
modification of the prosomatic appendages, followed by a respiratory
chamber, the muscles and branchiæ of which were the muscles and branchiæ of
the mesosomatic appendages. This mesosomatic, or branchial, part was in
close contiguity to the cloaca and anus, being separated from it only by a
short tube formed in the metasomatic or pronephric region.

I imagine that this connection was originally in the form of an {451}open
groove, as already explained for both notochord and the anterior part of
the gut itself in Ammocoetes; an open groove formed from the mid-ventral
surface of the body, on each side of which were the remnants of the
pronephric appendages. By the closure of this groove ventrally, and the
growing round of the pleural folds, as already suggested, the remains of
the pronephric appendages are indicated by the segmental duct and the form
of the vertebrate body is attained.

Even in the branchial region the same kind of thing must, I think, have
occurred. The grooved ventral surface became a tube, on each side of which
were lying in regular order the in-sunk branchial appendages, the whole
being subsequently covered by the pleural folds to form an atrial chamber.
A tube thus formed from the grooved ventral surface would carry with it to
the new ventral surface the longitudinal venous sinuses, and thus form, in
the way already suggested, the heart and ventral aorta. Posterior to the
heart in the pronephric region, the same process would give rise to the
sub-intestinal vein.

The evidence of comparative anatomy bears out most conclusively the
suggestion that in the original vertebrate the gut was mainly a respiratory
chamber. In man and all mammals the oral chamber opens into a small
pharynx, followed by the oesophagus, stomach and small intestine. Of this
whole length, a very small part is taken up by the pharynx, in which, in
the embryo, the branchial arches are found, showing that this represents
the original respiratory part of the gut. In the ordinary fish this
branchial part is much more conspicuous, occupies a large proportion of the
gut, and in the lowest fishes, such as Ammocoetes and Amphioxus, the
branchial region extends over a large portion of the animal, while the
intestine proper is a straight tube, the length of which is insignificant
in comparison with its length in the higher vertebrates.

Such a tube was able to act as a digestive tube, owing, as already pointed
out, to the digestive powers of the skin-epithelium, and I imagine at first
the respiratory chamber, seeing that it composed very nearly the whole of
the gut, was at the same time the main digestive chamber; even in
Ammocoetes its digestive power is superior to that of the intestine itself.

Just posterior to the branchial part a diverticulum of the gut was formed
at an early stage, as seen in Amphioxus, and provided the {452}commencement
of the liver. This simple liver-diverticulum became the tubular liver of
Ammocoetes, and formed, curiously enough, not a glandular organ of the same
character as the liver of the higher vertebrates, but a hepato-pancreas,
like the so-called liver of the arthropods, which also is a special
diverticulum of the gut, or rather the main true gut of the animal. In both
cases the liver is the chief agent in digestion, for in Ammocoetes the
liver-extract is very much more powerful in the digestion of proteids than
the extract of any other organ tried by Miss Alcock. Subsequently in the
vertebrate the gastric and pancreatic glands arise and relieve the liver of
the burden of proteid digestion.

It is, to my mind, somewhat significant that the liver on its first
formation in the vertebrate should have arisen as a digestive organ of the
same character as the so-called liver in the arthropods; whether it
originally belonged to any separate segment is in our present state of
knowledge difficult to say.


CONCLUSION.

In conclusion, I will endeavour to illustrate crudely the way in which, on
my theory, the notochord and vertebrate gut may have been formed, the
agencies at work being in the main two, viz. the dwindling of appendages as
mere organs of locomotion, and the conversion of a ventral groove into a
tube.

I imagine that, among the Protostraca, forms were found somewhat resembling
trilobites with markedly polychætan affinities; which, like Apus, possessed
a deep ventral groove from one end of the body to the other, and also
pleural fringes, as in many trilobites. This might be called the Trilobite
stage (Fig. 167, A).

This groove became converted into a tube and so gave rise to the notochord,
while the appendages were still free and the pleuræ had not met to form a
new ventral surface. This might be called the Chordate Trilobite stage
(Fig. 167, B).

Then, passing from the protostracan to the palæostracan stage, the oral and
respiratory chambers were formed, not communicating with each other, in the
manner described in previous chapters, a ventral groove in the metasomatic
region being the only connection between respiratory chamber and cloaca.
This might be called the Chordate Palæostracan stage (Fig. 167, C).

{453}[Illustration: FIG. 167.--A, DIAGRAM OF SECTION THROUGH A
TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE THE SUGGESTED FORMATION OF
THE NOTOCHORD FROM A VENTRAL GROOVE; C, DIAGRAM TO ILLUSTRATE THE SUGGESTED
FORMATION OF THE POST-BRANCHIAL GUT BY THE CONTINUATION OF THE SAME PROCESS
OF VENTRAL GROOVE-FORMATION, COMBINED WITH OBLITERATION OF APPENDAGES AND
GROWTH OF PLEURAL FOLDS; D, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE
VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE
WITH THE OBLITERATION OF THE ATRIAL CAVITY AND THE CONVERSION OF THE
VENTRAL GROOVE INTO THE CLOSED ALIMENTARY CANAL.

_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._,
pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele; _Sd._,
segmental duct; _Mes._, mesonephros; _At._, atrial chamber; _Nc._,
notochord; _H._, heart; _F._, fat body; _Ng._, notochordal groove. (These
diagrams are intended to complete the diagrams on p. 413, which, as stated
there, were purposely left incomplete.)]

{454}Finally, with the conversion of this groove into a tube, the opening
of the oral into the respiratory chamber, and the formation of an atrium by
the ventralwards growth of the pleural folds, the formation of a Vertebrate
was completed (Fig. 167, D).

In my own mind I picture to myself an animal which possessed eurypterid and
trilobite characters combined, in which a notochordal tube had been formed
in the way suggested, and a respiratory chamber which communicated with the
cloaca by means of a grooved channel along the mid-ventral line of the
metasomatic portion of the body. On each side of this channel were the
remains of the metasomatic appendages (pronephric). The whole was enveloped
in the pleural folds, which probably at this time did not yet meet in the
middle line to form a new ventral surface. This respiratory chamber, owing
to the digestive power of the epidermis, assisted in the process of
alimentation to such an extent as to supersede the temporary notochordal
tube, with the effect of bringing about the conversion of the metasomatic
groove into a closed canal, and so the formation of an alimentary tube
continuous with the respiratory chamber. The amalgamation of the pleural
folds ventrally completed the process, and so formed an animal resembling
the Cephalaspidæ, Ammocoetes, or Amphioxus.

I have endeavoured in this chapter to make some suggestions upon the origin
of the notochord and of the vertebrate gut in accordance with my theory of
the origin of vertebrates. I feel, however, strongly that these suggestions
are much more speculative than those put forward in the previous chapters,
and of necessity cannot give the same feeling of soundness as those based
directly upon comparative anatomy and histology. Still, the fact remains
that the origin of the notochord is at present absolutely unknown, and that
my speculation that it may have originated as an accessory digestive tube
is at all events in accordance with the most widely spread opinion that it
arises in close connection with an alimentary canal.




{455}CHAPTER XIV

_THE PRINCIPLES OF EMBRYOLOGY_

  The law of recapitulation.--Vindication of this law by the theory
  advanced in this book.--The germ-layer theory.--Its present position.--A
  physiological not a morphological conception.--New fundamental law
  required.--Composition of adult body.--Neuro-epithelial 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
  Amphioxus and of Lucifer.--Summary.


In a discussion upon this theory of mine, which took place at Cambridge on
November 25 and December 2, 1895, it was said that such a theory was
absolutely and definitely put out of court, because it contravened the
principles of embryology, was opposed, therefore, to our surest guide in
such matters; and the law was laid down with great assurance that no claim
for genetic relationship between two groups of animals can be allowed which
is based upon topographical and structural coincidences revealed by the
study of the anatomy of two adult animals, however numerous and striking
they may be, if there are fundamental differences in the embryology of the
members of these two groups.

According to my theory the old gut of the arthropod still exists in the
vertebrate as the tubular lining of the central nervous system, and the
vertebrate has formed a new gut. According to the principles of embryology
as held up to the present, in all animals above the Protozoa, the different
structures of the body arise from three definite embryonic layers, the
epiblast, mesoblast, and hypoblast, and in all cases the gut arises from
the hypoblastic layer. In the vertebrate the gut also arises from the
hypoblast, while the neural canal is epiblastic. My theory, then, makes the
impossible assertion that what was hypoblast in the arthropod has become
epiblast in the vertebrate, and what was epiblast in the arthropod has
become hypoblast in the vertebrate. Such a conception is supposed to be so
{456}absolutely impossible that it only requires to be stated to be
dismissed as an absurdity.

Against this opinion I claim boldly that my theory is not only not contrary
to the principles of embryology, but is mainly based upon the teachings of
embryology. I wish here not to be misunderstood. The great value of the
study of embryology for questions of the sequence of the evolution of
animals is to be found in what is known as the Law of Recapitulation, which
asserts that every animal gives some indication in the stages of its
individual development of its ancestral history. Naturally enough it cannot
pass through all the stages of its past history with equal clearness, for
what has taken millions of years to be evolved has to be compressed into an
evolution lasting only a few months or weeks, or even less.

When in the highest vertebrate a vestigial organ, such as the pineal gland,
can be traced back without leaving the vertebrate kingdom to a distinct
median eye, such as is found in the lamprey, that rudimentary organ is
evidence of an organ which was functional in the earliest vertebrates or
their immediate ancestors. So it is generally with well defined vestigial
organs found in the adult animal; they always indicate an organ which was
functional in the near ancestor.

Passing from the adult to the embryo we still find the same law. Here,
also, vestigial organs are met with, which may leave no trace in the adult,
but indicate organs which were functional in the near ancestor. Thus, but
for embryology, we should have no certainty that the air-breathing
vertebrates had been derived from water-breathing fishes; the indication is
not given by any close resemblance between the formation of the embryos in
their earliest stages, but by the formation of vestigial gill-arches even
in the embryos of the highest mammal.

For all questions of evolution the presence of vestigial organs in the
embryo is the important consideration, for they give an indication of near
ancestry; the early formation of the embryo concerns a much more remote
ancestral period, all vestigial organs of which may well have been lost and
obscured by coenogenetic changes. Let us, then, consider the two
things--the vestigial organs and the early formation of the
embryo--separately, and see how far my opponents are justified in their
statement that my theory contravenes the principles of embryology.

{457}First, I will take the teachings of vestigial organs and the
arrangement of organs found in the vertebrate embryo. Here it is impossible
to say that my theory is contrary to the teaching of embryology, for as the
previous chapters have shown again and again, the argument is based very
largely upon the facts of embryology. In the first place, the comparison
which I have chiefly made is a comparison between the larval form of a very
low vertebrate and the arthropod group, a comparison which exists only for
the larval form, and not for the adult. The whole theory, then, is based
upon a developmental stage of the vertebrate, and not upon the anatomy of
the adult.

Throughout the whole history it seems to me perfectly marvellous how
completely the law of recapitulation is vindicated by my theory of the
origin of the vertebrate. The theory asserts that the clue to the origin of
vertebrates is to be found in the tubular nature of the central nervous
system of the vertebrate; in that the vertebrate central nervous system is
in reality formed of two things: (1) a central nervous system of the
arthropod type, and (2) an epithelial tube in the position of the
alimentary canal of the arthropod.

Is it possible for embryology to recapitulate such a phylogenetic history
more clearly than is here the case? In order to avoid all possibility of
our mistaking the clue, the nerve-tube in the embryo always opens into the
anus at its posterior end, while in the larval Amphioxus it is actually
still open to the exterior at the anterior end. The separateness of the
tube from the nervous system at its first origin is shown especially well
in the frog, where, as Assheton has pointed out, owing to the pigment in
the cells of the external layer of epithelium, a pigmented tube is formed,
on the outside of which the nervous tissue is lying, and step by step the
gradual intermingling of the nerve-cells and the pigmented lining cells can
be followed out.

Consider the shape of the nerve-tube when first formed in the vertebrate.
At the cephalic end a simple bulged-out tube with two simple anterior
diverticula, which passes into a narrow straight spinal tube; from this
large cephalic bulging a narrow diverticulum, the infundibulum, passes to
the ventral surface of the forming brain. This tube is the embryological
expression of the simple dilated cephalic stomach, with its ventral
oesophagus and two anterior diverticula, which opens into the straight
intestine of the arthropod. Nay, more, by its very shape, and the
invariable presence of two anterior {458}diverticula, it points not only to
an arthropod ancestry, but to a descent from a particular group of
primitive arthropods. Then comes the formation of the cerebral vesicles,
and the formation of the optic cup, telling us as plainly as can be how the
invasion of nervous material over this simple cephalic stomach and its
diverticula has altered the shape of the original tube, and more and more
enclosed it with nervous elements.

So, too, in the spinal cord region. When the tube is first formed, it is a
large tube, the latero-ventral part of which presents two marked bulgings;
connecting these two bulgings is the anterior commissure. These two lateral
bulgings, with their transverse commissure, represent, with marked
fidelity, the ventral ganglion-masses of the arthropod with their
transverse commissure, and occupy the same position with respect to the
spinal tube, as the ganglion-masses do with respect to the intestine in the
arthropod. Then the further development shows how, by the subsequent growth
of the nervous material, the calibre of the tube is diminished in size, and
the spinal cord is formed.

Again, I say, is it possible to conceive that embryology should indicate
the nature of the origin of the vertebrate nervous system more clearly than
it does?

It is the same with all the other organs. Take, for instance, the skeletal
tissues. The study of the vertebrate embryo asserts that the cartilaginous
skeleton arose as simple branchial bars and a simple cranio-facial
skeleton, and also that the parenchymatous variety of cartilage represents
the embryonic form. Word for word, the early embryonic stage of the
vertebrate skeleton closely resembles the stage reached in the arthropod,
as shown by Limulus, and again records, unmistakably, the past history of
the vertebrate.

So, too, with the whole of the prosomatic region; the situation of the old
mouth, the manner in which the nose of the cephalaspidian fishes arose from
the palæostracan, are all shown with vivid clearness by Kupffer's
investigations of the early stage of Ammocoetes, while at the same time the
closure of the oral cavity by the septum shows how the oral chamber was
originally bounded by the operculum. Nay, further, the very formation of
this chamber embryologically was brought about by the forward growth of the
lower lip, just as it must have been if the chilaria grew forward to form
the metastoma.

So, too, the study of the embryo teaches that the branchiæ arise as
{459}ingrowths, that the heart arises as two longitudinal veins, just as
the theory supposes from the facts provided by Limulus and the scorpions.
No indication of the origin of the thyroid gland is given by the study of
its structure in any adult vertebrate, but in the larval form of the
lamprey there is still preserved for us a most graphic record of its past
history.

The close comparisons which it is possible to make between the eye-muscles
of the vertebrate and the recti muscles of the scorpion group on the one
hand, and between the pituitary and coxal glands on the other, are based
upon, or at all events are strikingly confirmed by, the study of the
coelomic cavities and the origin of these muscles in the two groups. In
fact the embryological evidence of the double segmentation in the head and
the whole nature of the cranial segments is one of the main
foundation-stones on which the whole of my theory rests.

So it is throughout. Turn to the excretory organs--it is not the kidney of
the adult animal which leads direct to the excretory organs of the
primitive arthropod, but the early embryonic origin of that kidney.

So far from having put forward a theory which runs counter to the
principles of embryology, I claim to have vindicated the great Law of
Recapitulation which is the foundation-stone of embryological principles.
My theory is largely based upon embryological facts, and its strength
consists in the manner in which it links together into one harmonious
whole, the facts of Embryology, Palæontology, Anatomy, and Physiology. Why,
then, is it possible to assert that my theory disregards the principles of
embryology, when, as we have seen, embryology is proclaiming as loudly as
possible how the vertebrate arose? In my opinion, it is because the
embryologists have to a large extent gone wrong in their fundamental
principles, and have attached more weight to these faulty fundamental
principles than to the obvious facts which, looked at thoughtfully, could
not have failed to suggest a doubt as to the correctness of these
'principles.'

The current laws of embryology upon which such weight is laid are based on
the homology of the germinal layers in all Metazoa, and state that in all
cases after segmentation is finished a blastula is formed, from which there
arises a gastrula, formed of an internal layer, the hypoblast, and an
external layer, the epiblast; subsequently {460}between these arises a
third layer, the mesoblast. These layers are strictly morphological
conceptions, and are stated to be homologous in all cases, so that the
hypoblast of one animal must be homologous to the hypoblast of another. In
order, therefore, to compare two adult animals for the purpose of finding
kinship between them, it is necessary to find whether parts such as the
gut, which in both cases have the same function, arise from the same
germinal layer in the embryo. We can, in fact, have no certainty of
kinship, even although the two animals are built up as far as the adult
state is concerned on a remarkably similar plan, unless we can study their
respective embryos and find out what parts arise from the hypoblast and
what from the epiblast. The homology of the germinal layers constitutes in
all cases of disputed relationship the court of final appeal. A new gut,
therefore, in any animal can only be formed from hypoblast, and any theory,
such as that advocated in this book, which deals with the formation of a
new gut, and does not form that gut from pre-existing hypoblast, must of
necessity be wrong and needs no further consideration.

Such is the result of current conceptions--conceptions which to be valid
must be based upon an absolutely clear morphological definition of the
formation of the germinal layers, a definition not based on their
subsequent history and function, but determined solely by the uniformity of
the manner of their origin.

What, then, is a germinal layer? How can we identify it when it first
arises? What is the morphological criterion by which hypoblast can be
distinguished from epiblast, or mesoblast from either?

This is the question put by Braem, in an admirable series of articles in
the _Biologisches Centralblatt_, and is one that must be answered by every
worker who bases his views of the process of evolution upon embryological
investigation. As Braem points out, the germinal layers are definable
either from a morphological or physiological standpoint. In the one case
they must arise throughout on the same plan, and whatever be their fate in
the adult, they must form at an early stage structures strictly homologous
in all animals. In the other case the criterion is based on function, and
the hypoblast, for instance, is that layer which is found afterwards to
form the definitive alimentary canal. There is no longer any morphological
homology; such layers are analogous; they may be, but are not necessarily,
homologous. Braem gives a sketch of the history of the views held on
{461}the germinal layers, and shows how they were originally a purely
physiological conception, and how gradually such conception changed into a
morphological one, with the result that what had up to that time been
looked upon as analogous structures became strictly homologous and of
fundamental importance in deciding the position of any animal in the whole
animal series.

This change of opinion was especially due to the lively imagination of
Haeckel, who taught that the germinal layers of all Metazoa must be
strictly homologous, because they were all derived from a common ancestral
stock, represented by a hypothetical animal to which he gave the name
Gastræa; an animal which was formed by the simple invagination of a part of
the blastula, thus giving rise to the original hypoblast and epiblast, and
he taught that throughout the animal kingdom the germinal layers were
formed by such an invagination of a part of the blastula to form a simple
gastrula. If further investigation had borne out Haeckel's idea, if
therefore the hypoblast was in all cases formed as the invagination of a
part of a single-layered blastula, then indeed the dogma of the homology of
the germinal layers would be on so firm a foundation that no speculation
which ran counter to it could be expected to receive acceptance; but that
is just what has not taken place. The formation of the gastrula by simple
invagination of the single-layered blastula is the exception, not the rule,
and, as pointed out by Braem, is significantly absent in the earliest
Metazoa; in those very places where, on the Gastræa theory, it ought to be
most conspicuous.

Braem discusses the question most ably, and shows again and again that in
every case the true criterion upon which it is decided whether certain
cells are hypoblastic or not is not morphological but physiological. The
decision does not rest upon the answer to the question, Are these cells in
reality the invaginated cells of a single-celled blastula? but to the
question, Do these cells ultimately form the definitive alimentary canal?
The decision is always based on the function of the cells, not on their
morphological position. Not only in Braem's paper, but elsewhere, we see
that in recent years the physiological criterion is becoming more and more
accepted by morphologists. Thus Graham Kerr, in his paper on the
development of Lepidosiren, says: "It seems to me quite impossible to
define a layer as hypoblastic except by asking one or other of the two
questions: (1) Does it form the lining of an archenteric cavity? and (2)
{462}Does it become a certain part of the definitive epithelial lining of
the gut?"

The appearance of Braem's paper was followed by a criticism from the pen of
Samassa, who agrees largely with Braem, but thinks that he presses the
physiological argument too far. He considers that morphological laws must
exist for the individual development as well as for the phylogenetic, and
finishes his article with the following sentence, a sentence in which it
appears to me he expresses what is fast becoming the prevailing view: "Mit
dem Satz, den man mitunter lesen kann: 'es muss doch auch für die Ontogenie
allgemeine Gesetze geben' kann leicht Missbrauch getrieben werden; diese
allgemeinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand
und bis zu ihrer Erkenntnis hat es noch gute Wege; das eine kann man aber
wohl heute schon sagen, die Keimblätterlehre gehört zu diesen allgemeinen
Gesetzen nicht."

I conclude, then, that we ought to go back to a time previous to that of
Haeckel and ask ourselves seriously the question, When we lay stress on the
germinal layers and speak of this or that organ arising from this or that
germinal layer, are we thereby adding anything to the knowledge that we
already possess from the study of the anatomy and physiology of the adult
body? If by hypoblast we only mean the internal surface or alimentary canal
and its glands, etc., and by epiblast we mean the external surface or skin
and its glands, etc., while mesoblast indicates the middle structures
between the other two, then I fail to see what advantages we obtain by
using Greek terms to express in the embryo what we express in English in
the adult.

The evidence given by Braem, and it could be strengthened considerably, is
conclusive against the morphological importance of the theory of the
germinal layers, and transfers the fundamental importance of the early
embryonic formation, from that of a three-layered embryo to that of a
single-layered embryo--the blastula--from which, in various ways, the adult
animal has arisen.

The derivation of both arthropod and vertebrate from such a single-layered
animal is perfectly conceivable, even though the gut of the latter is not
homologous with the gut of the former. We have seen that the teachings of
embryology, as far as its later stages are concerned, afford one of the
main supports upon which this theory rests. What, therefore, is required to
complete the story is the way {463}in which these later stages arise from
the blastula stage; here, as in all cases, the ontogenetic laws must be in
harmony with the phylogenetic; of the latter the most important is the
steady development of the central nervous system for the upward progress of
the animal race. The study of comparative anatomy indicates the central
nervous system, not the gut, as the keystone of the edifice. So, also, it
must be with ontogeny; here also the central factor in the formation of the
adult from the blastula ought to be the formation of the central nervous
system, not that of the gut.

Such, it appears to me, is the case, as may be seen from the following
considerations.

The study of the development of any animal can be treated in two ways:
either we can trace back from the adult to the very beginning in the ovum,
or we can trace forward from the fertilized egg to the adult. Both methods
ought to lead to the same result; the difference is, that in the first case
we are passing from the more known to the less known, and are expressing
the unknown in terms of the known. In the second case we are passing from
the less known to the more known, and are expressing the known in
speculative terms, invented to explain the unknown. What has just been said
with respect to the germinal layers means that, however much we may study
the embryo and try to express the adult in terms of it, we finally come
back to the first way of looking at the question, and, starting with the
adult, trace the continuity of function back to the first formation of
cells having a separate function.

Let us, then, apply this throughout, and see what are the logical results
of tracing back the various organs and tissues from the adult to the
embryo.

The adult body is built up of different kinds of tissues, which fall
naturally, from the standpoint of physiology, into groups. Such groups are,
in the first place--

  1. All those tissues which are connected with the central nervous system,
  including in that group the nervous system itself.

  2. All those tissues which have no connection with the nervous system.

In the second group the physiologist places all germinal cells, all blood-
and lymph-corpuscles, all plasma-cells and connective tissue and its
derivatives--in fact, all free-living cells, whether in a free state or in
a quiescent, so to speak encysted, condition, such as is {464}found in
connective tissue. In the first group the physiologist recognizes that the
central nervous system is connected with all muscular tissues, whether
striped or unstriped, somatic or splanchnic, and that such connection is of
an intimate character. Further, all epithelial cells, either of the outer
or inner surfaces, whether forming special sense-organs and glands, such as
the digestive and sweat-glands, or not, are connected with the nervous
system. Besides these structures, there is another set of organs as to
which we cannot speak definitely at present, which must be considered
separately, viz. all the cells, together with their derived organs, which
line the body-spaces. Whatever may be the ultimate decision as to this
group of cells, it must fall into one or other of the two main groups.

The members of these two groups are so interwoven with one another that
either, if taken alone, would still give the form of the body, so that, in
a certain sense, we can speak of the body as formed of two syncytia,
separate from each other, but interlaced, of which the one forms a
continuous whole by means of cells connected together by a fluid medium or
by solid threads formed in such fluid medium, while the other does not form
a syncytium in the sense that any cell of one kind may be connected with
any cell of another kind, but a syncytium of which all the different
elements are connected together only through the medium of the nervous
system.

If we choose to speak of the body as made up of two syncytia in this way,
we must at the same time recognize the fundamental difference in character
between them. In the one case the elements are connected together only by
what may be called non-living material; there is no direct metabolic
activity caused by the action of one cell over a more distant cell in
consequence of such connection, it is not a true syncytium; in the second
case there is a living connection, the metabolism of one part is directly
influenced by the activity of another, and the whole utility of the system
depends upon such functional connection.

The tissues composing this second syncytium may be spoken of as the
master-tissues of the body, and we may express this conception of the
building up of the body of the higher Metazoa by saying that it is composed
of a syncytial host formed of the master-tissues, which contains within its
meshes a system of free-living cells, none of which have any connection
with the nervous system. This syncytial {465}host is in the adult composed
of a number of double elements, a nerve-cell element, and an epithelial
element, such as muscle-cell, gland-cell, etc., connected together by
nerves; and if such connection is always present as we pass from the adult
to the embryo, if there is no period when, for example, the neural element
exists alone free from the muscle-cell, no period when the two can be seen
to come together and join, then it follows that when the single-layered
blastula stage is reached, muscle-cell and nerve-cell must have fused
together to form a neuro-muscular cell. Similarly with all the other
neuro-epithelial organs; however far apart their two components may be in
the adult, they must come together and fuse in the embryo to form a
neuro-epithelial element.

The close connection between muscle and nerve which has always been
recognized by physiologists, together with the origin of muscle from a
myo-epithelial cell in Hydra and other Coelenterata, led the older
physiologists to accept thoroughly Hensen's views of the neuro-epithelial
origin of all tissues connected with the central nervous system. Of late
years this conception has been largely given up owing to the statement of
His that the nervous system arises from a number of neuroblasts, which are
entirely separate cells, and have at first no connection with muscle-cells
or any peripheral epithelial cells, but subsequently, by the outgrowing of
an axial fibre, find their way to the muscle, etc., and connect with it. I
do not think that His' statement by itself would have induced any
physiologist to give up the conception of the intimate connection of muscle
and nerve, if the work of Golgi, Ramón y Cajal, and others had not brought
into prominence the neurone theory, _i.e._ that each element of the central
nervous system is an independent element, without real connection with any
other element and capable of influencing other cells by contact only. These
two statements, emanating as they did from embryological and anatomical
studies respectively, have done much to put into the background Hensen's
conceptions of the syncytial nature of the motor, neural, and sensory
elements, which make up the master-tissues of the body, and have led to the
view that all the elements of the body are alike, in so far as they are
formed of separate cells each leading an independent existence, without any
real intimate connection with each other.

The further progress of investigation is, it seems to me, bringing us back
to the older conception, for not only has the neuroblast theory {466}proved
very difficult for physiologists to accept, but also Graham Kerr, in his
latest papers on the development of Lepidosiren, has shown that there is
continuity between the nerve-cell and the muscle-cell from the very first
separation of the two sets of elements; in fact, Hensen is right and His
wrong in their respective interpretation of the earliest stages of the
connection between muscle and nerve. So also, it seems to me, the intimate
connection between the metabolism of the gland-cell, as seen in the
submaxillary gland, and the integrity of its nervous connection implies
that the connection between nerve-cell and gland-cell is of the same order
as that between nerve-cell and muscle-cell. Graham Kerr also states in his
paper that from the very commencement there is, he believes, continuity
between nerve-cell and epithelial cell, but so far he has not obtained
sufficiently clear evidence to enable him to speak positively on this
point.

Further, according to the researches of Anderson, the cells of the superior
cervical ganglion in a new-born animal will continue to grow healthily as
long as they remain connected with the periphery, even though entirely
separated from the central nervous system by section of the cervical
sympathetic nerve, and conversely, when separated from the periphery, will
atrophy, even though still connected with the central nervous system. So,
also, on the sensory side, Anderson has shown that the ganglion-cells of
the posterior root-ganglion will grow and remain healthy after separation
of the posterior roots in a new-born animal, but will atrophy if the
peripheral nerve is cut, even though they are still in connection with the
central nervous system. Further, although section of a posterior root in
the new-born animal does not affect the development of the nerve-cells in
the spinal ganglion, and of the nerve-fibres connecting the posterior
root-ganglion with the periphery, it does hinder the development of that
part of the posterior root connected with the spinal ganglion.

These experiments of Anderson are of enormous importance, and force us, it
seems to me, to the same conclusion as that to which he has already
arrived. His words are (p. 511): "I suggest, therefore, that the section of
peripheral nerves checked the development of motor and sensory neurones,
not because it blocked the passage of efferent impulses in the first case
and the reception of stimuli from the periphery in the second, but for the
same reason in both cases, {467}viz. that the lesion disturbed the
chemico-physical equilibrium of an anatomically continuous (neuro-muscular
or neuro-epithelial) chain of cells, by separating the non-nervous from the
nervous, and that the changes occurring in denervated muscle, which I shall
describe later (and possibly those in denervated skin), are in part due to
the reciprocal chemico-physical disturbance effected in these tissues by
their separation from the nervous tissues; also that the section of the
posterior roots checked the development of those portions of them still
attached to the spinal ganglia, because the chemico-physical equilibrium in
those processes is maintained not only by the spinal ganglion-cells, but
also by the intra-spinal cells with which these processes are anatomically
continuous."

What is seen so strikingly in the new-born animal can be seen also in the
adult, and in Anderson's paper references are given to the papers of Lugaro
and others which lead to the same conclusion.

These experiments seem to me distinctly to prove that the connection
between the elements of the peripheral organ and the proximate neurone is
more than one of contact.

We can, however, go further than this, for, apart from the observations of
Apathy, there is direct physiological evidence that the vitality of other
neurones besides the terminal neurones is dependent upon their connection
with the peripheral organ, even though their only connection with the
periphery is by way of the terminal neurone. Thus, as is seen from
Anderson's experiments, section of the cervical sympathetic nerve in a very
young animal causes atrophy of many of the cells in the corresponding
intermedio-lateral tract, cells which I supposed gave origin to all the
vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking
experiment given by Anderson is the effect of the removal of the periphery
upon the medullation of those efferent fibres which arise from these same
spinal cells, for, as he has shown, section of the nerves from the superior
cervical ganglion to the periphery in a very young animal delays the
medullation in the fibres of the cervical sympathetic--that is, in
preganglionic fibres which are not directly connected with the periphery
but with the terminal neurones in the superior cervical ganglion. So also
on the afferent side a sufficiently extensive removal of sensory field will
cause atrophy of the cells of Clarke's column, so that, just as in the case
of the primary neurones, {468}the secondary neurones show by their
degenerative changes the importance of their connection with the peripheral
organs.

In this way I can conceive the formation of a series of both efferent and
afferent relays in the nervous system by proliferation of the original
neural moiety of the neuro-epithelial elements, every one of which is
dependent upon its connection with the peripheral epithelial elements for
its due vitality, the whole system being a scheme for co-ordination of a
larger and larger number of peripheral elements. Thus the cells of the
vasomotor centre are in connection with the whole system of segmental
vaso-constrictor centres in the lateral horns of the thoracic region of the
cord, so that to cause atrophy of these cells a very extensive removal of
the vascular system would be required. Each of the segmental centres in the
cord supplies a number of sympathetic segments, the connection with all of
which would have to be cut in order to ensure complete removal of the
connection of each of its cells with the periphery, and finally each of the
cells in the sympathetic ganglia supplies a number of peripheral elements,
all of which must be removed to ensure complete severance.

Thus, if we take any arbitrary number, such as 4, to represent the number
of peripheral organ-elements with which each terminal neurone is connected,
and suppose that each neurone has proliferated into sets of 4, then a cell
of the third order, such as a cell of the vasomotor centre, would require
the removal of 64 peripheral elements to cause its complete separation from
the periphery, one of the second order (a cell of the thoracic lateral
horn) 16 elements, one of the first order (a cell of a sympathetic
ganglion) 4 elements.

Such intimate inter-relationship between the neurones, both afferent and
efferent, and their corresponding peripheral organs does not imply that all
nerve-cells are necessarily as closely dependent upon some connection with
the periphery, for just as the proliferation of epithelial or muscle-cells
forms an epithelial or muscular sheet, the elements of which are so
loosely, if at all, connected together that their metabolism is in no way
dependent upon such connection, so also a similar proliferation of the
neural elements may form connections between nerve-cell and nerve-cell of a
similarly loose nature.

It is this kind of proliferation which, in my opinion, would bind together
the separate relays of efferent and afferent neurones, and {469}so give
origin to reflex actions at different levels. Such neurones would not be in
the direct chain of either the afferent or efferent neurones, and so not
directly connected with the periphery, and could therefore be removed
without affecting the vitality of either the efferent or afferent chain of
neurones. In other words, the vitality of the cells on the efferent side
ought not to be dependent on the integrity of the reflex arc. With regard
to the development of the anterior roots, Anderson has shown that this is
the case, for section of all the posterior roots conveying afferent
impulses from the lower limb in a new-born animal does not hinder the
normal development of the anterior roots supplying that limb. Also Mott,
who originally thought that section of all the posterior roots to a limb
caused atrophy of the corresponding anterior roots, has now come to the
same conclusion as other observers, and can find no degeneration on the
efferent side due to removal of afferent impulses.

Again, the process of regeneration after section of a nerve is not in
favour of the neuroblast theory. There is no evidence that the cut end of a
nerve can grow down and attach itself to a muscular or epithelial element
without the assistance of a nerve tube down which to grow. When the cut
nerves connected with the periphery degenerate, that applies only to the
axis-cylinder and the medullary sheath, not to the neurilemma; the
connective tissue elements remain alive and form a tube into which the
growing axon finds its way, and so is conducted to the end-plate or
end-organ of the peripheral structure.

Possibly, as suggested by Mott and Halliburton, the products of
degeneration of the axis-cylinder and medullary sheath stimulate these
connective tissue sheath-cells into active proliferation, and so bring
about the great multiplication of cells arranged as cell-chains, which are
so often erroneously spoken of as forming the young nerves. These
sheath-cells are then supposed to re-form and secrete a pabulum which is
important for the process of regeneration of the down-growing axis-cylinder
and medullary sheath. Without such pabulum regeneration does not take
place, as is seen in the central nervous system, where the sheath of
Schwann is absent.

Again, it is becoming more and more doubtful whether the peripheral
terminations of nerves are ever really free. As far as efferent nerves are
concerned the nervous element may entirely {470}predominate over the
muscular or glandular, as in the formation of the electric organs of the
Torpedo and Malapterurus, but still the final effect is produced by the
alteration of the muscle or gland-cell. On the afferent side especially
free nerve-terminations are largely recognized, or, as in Barker's book,
nerves are spoken of as arising in connective tissue. Thus the numerous
kinds of special sense-organs, such as Pacinian bodies, tendon-organs,
genital corpuscles, etc., are all referred to by Barker under the heading
of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these
organs has been known for a long time in the shape of Grandry's corpuscles
or the tactile corpuscles in the duck's bill, where it has been proved that
the nerve terminates in special large tactile cells derived from the
surface-epithelium.

So also with all the others, further investigation tends to put them all in
the same category, all special sensory organs originating from a localized
patch of surface-epithelium. Thus Anderson has shown me in his specimens
how the young Pacinian body is composed of rows of epithelial cells, into
each of which a twig from the nerve passes. He has also shown me how, in
the case of the tendon-organ, each nerve-fibre passes towards the
attachment of the tendon and then bends back to supply the tendon-organ,
thus indicating, as he suggests, how the nest of epithelial cells has
wandered inwards from the surface to form the tendon-organ. Again,
Meissner's corpuscles and Herbst's corpuscles are evidently referable to
the same class as those of Grandry and Pacini.

Yet another instance of the same kind is to be found in the chromatophores
of the frog and other animals which are under the influence of the central
nervous system and yet have been supposed by various observers to be
pigmented connective tissue cells. The most recent work of Leo Loeb and
others has conclusively shown that such cells are invariably derived from
the surface-epithelium.

Finally, in fishes we find the special sense-organs of the lateral line and
other accessory sensory organs, all of which are indisputably formed from
modified surface epithelial cells.

The whole of this evidence seems to me directly against Barker's
classification of sensory nerve-beginnings in mesoblastic tissues; in none
of these cases are we really dealing with free nervous tissue alone, the
starting point is always a neuro-epithelial couple.

We may then, I would suggest, look upon the adult as formed of {471}a
neural syncytium, which we may call the host, which carries with it in its
meshes a number of free cells not connected with the nervous system. If,
then, we confine our attention to the host and trace back this neural
syncytium to its beginnings in the embryo, we see that, from the very
nature of the neuro-epithelial couple, each epithelial moiety must approach
nearer and nearer to its neural moiety, until at last it merges with it;
the original neuro-epithelial cell results, and we must obtain, as far as
the host is concerned, a single-layered blastula as the origin of all
Metazoa. It follows, further, that there must always be continuity of
growth in the formation of the host, _i.e._ in the formation of the
neuro-epithelial syncytium; that therefore cells which have been previously
free cannot settle down and take part in its formation, as, for instance,
in the case of the formation of any part of the gut-epithelium or of
muscle-cells from free-living cells.

Further, since the neural moiety is the one element common to all the
different factors which constitute the host, it follows that the
convergence of each epithelial moiety to the neural moiety, as we pass from
the adult to the embryo, is a convergence of all outlying parts to the
neural moiety, _i.e._ to the central nervous system, if there is a
concentrated nervous system. Conversely, in the commencing embryo the place
from which the spreading out of cells takes place, _i.e._ from which growth
proceeds, must be the position of the central nervous system, if the
nervous system is concentrated. If the nervous system is diffuse, and forms
a general sub-epithelial layer, then the growth of the embryo would take
place over the whole surface of the blastula.

Turning now to the consideration of the second group of tissues, those that
are not connected with the central nervous system, we find that they
include among them such special cells as the germinal cells, free cells of
markedly phagocytic nature, and cells which were originally free and
phagocytic, but have settled down to form a supporting framework of
connective tissue, and are known as plasma-cells. In the embryo we find
also in many cases free cells in the yolk, forming more or less of a layer,
which function as phagocytes and prepare the pabulum for the fixed cells of
the growing embryo; these cells are known by the name of vitellophags, and
in meroblastic vertebrate eggs form somewhat of a layer known by the name
of periblast. Such cells must be included in the second group, and,
{472}indeed, have been said again and again to give origin to the
free-living blood-corpuscles of the adult. In other cases they are said to
disintegrate after their work is done.

In the adult the free-living lymphocytes and hæmocytes reproduce themselves
from already existing free-living cells, but as we pass back to the embryo
there comes a time, comparatively late in the history of the embryo, when
such free-living cells are not found in the fluids of the body, and they
are said to arise from the proliferation and setting free of cells which
form a lining epithelium. Such formation of leucocytes has been especially
described in connection with the lining epithelium of the coelomic
cavities, as stated in Chapter XII., so that anatomists look upon the
origin of these free cells as being largely from the coelomic epithelium,
or mesothelium, as Minot calls it.

Then, again, the free cells which form the germinal cells can be traced
back to a germinal epithelium, which also is part of the coelom. Thus the
suggestion arises that in the embryo a cellular lining is formed to a
coelomic cavity (mesothelium) composed of cells which have no communication
with the nervous system, and are capable of a separate existence as free
individuals, either in the form of germinal cells or of lymphocytes,
hæmocytes, and plasma-cells, so that these latter free cells may be
considered as living an independent existence in the body, and ministering
to it in the same sense as the germ-cells live an independent existence in
the body. Again, the function of this mesothelium apart from the germ-cell
is essentially excretory and phagocytic. It is the cells of the excretory
organs as well as the lymphocytes which pick up carmine-grains when
injected. It is the cells of the modified excretory organs, as mentioned in
Chapter XII., which, according to Kowalewsky and others, give origin to the
free leucocytes.

We see, then, that the conception of a syncytial neuro-epithelial host
holding in its meshes a number of free cells leads directly to the
questions: What is the coelom? To which category does its lining membrane
belong? and further, also, What is the origin of these free cells?

The Metazoa have been divided into two great groups--those which possess a
coelom (the Coelomata; Lankester's Coelomocoela) and those which do not
(Coelenterata; Lankester's Enterocoela). As an example of the latter we may
take Hydra, because it is a very {473}primitive form, and because its
development has been carefully worked out recently by Brauer.

In Hydra we find a dermal layer of cells and an inner layer of cells
separated by a gelatinous mass known as mesogloea; in this mass between the
dermal and inner layers scattered cells are found, the interstitial cells.
Now, according to Brauer the position of the germ in Hydra is the
interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the
others have their substance changed into yolk-grains, forming the so-called
pseudo-cells, and as such afford pabulum to the growing egg-cell. Thus we
see that in between the dermal and gastral layer of cells a third layer of
cells is found, composed of free living germ-cells, some of which, by the
formation of yolk-granules, become degraded into pabulum for their more
favoured kinsfolk. These interstitial cells are said to arise from the
dermal layer, or ectoderm, but clearly, as in other cases, germ-cells
constitute a class by themselves and cannot be spoken of as originating
from ectoderm-cells or from hypoderm-cells.

So also in Porifera, Minchin states: "In addition to the collared cells of
the gastral layer, and the various cell-elements of the dermal layer, the
body-wall contains numerous wandering cells or amoebocytes, which occur
everywhere among the cells and tissues. Though lodged principally in the
dermal layer, they are not to be regarded as belonging to it, but as
constituting a distinct class of cells by themselves. They are concerned
probably with the functions of nutrition and excretion, and from them arise
the genital products." Further (p. 31): "At certain seasons some of these
cells become germ-cells; hence the wandering cells and the reproductive
cells may be included together under the general term archæocytes." Also
(p. 51): "The mesogloea is the first portion to appear as a structureless
layer between the dermal and gastral epithelia, and is probably a secretion
of the former."

He also points out that in these, the very lowest of the Metazoa, the
separate origin of these archæocytes can be traced back to a very early
period of embryonic life. Thus in _Clathrina blanca_ the ovum undergoes a
regular and total cleavage, resulting in the formation of a hollow ciliated
blastula of oval form. At one point, the future posterior pole of the
larva, are a pair of very large granular cells with vesicular nuclei, which
represent undifferentiated blastomeres and are destined to give rise to the
archæocytes, and, therefore, also to the {474}sexual cells of the adult.
Thus, as he says, from the very earliest period a distinction is made
between the "tissue-forming" cells (my syncytial host) and the archæocytes.

We see, then, that the origin of all these free-living cells can be traced
back to the very earliest of the Metazoa. Here between the dermal and
gastral layers a gelatinous material, the mesogloea is secreted by these
layers. This material is non-living, non-cellular. In it live free cells
which may either be germ-cells, amoebocytes, or 'collencytes' (connective
tissue cells). If this mesogloea were a fluid secretion, then we should
have a tissue of the nature of blood or lymph; if it were solid, then we
should have the foundation of connective tissue, cartilage, and bone.

From this primitive tissue it is easy to see how the special elements of
the vascular, lymphatic, and skeletal tissues gradually arose, the matrix
being provided by the cells of the syncytial host and the cellular elements
by the archæocytes. In fact, we have no right to speak of these lowest
members of the Metazoa as not being triploblastic, as possessing nothing
corresponding to mesoblast, for in these free cells in the mesogloea we
have the origin of the mesenchyme of the higher groups. Thus Lankester,
talking of mesenchyme, says: "I think we are bound to bring into
consideration here the existence in many Coelentera of a tissue resembling
the mesenchyme of Coelomocoela. In Scyphomedusæ, in Ctenophora, and in
Anthozoa, branched fixed and wandering cells are found in the mesogloea
which seem to be the same thing as a good deal of what is distinguished as
mesenchyme in Coelomocoela. These appear to be derived from both the
primitive layers; some produce spicules, others fibrous substance, others
again seem to be amoebocytes with various functions. It appears to be
probable that, though it may be necessary to distinguish other elements in
it, the mesenchyme of Coelomocoela is largely constituted by cells, which
are the mother-cells of the skeletotrophic group of tissues, and are
destined to form connective tissues, blood-vessels, and blood."

Thus we see that the earliest Metazoa were composed of a dermal and gastral
epithelium, with a sub-epithelial nervous system connecting the parts
together, which formed, as it were, a host, carrying around free living
cells of varying function, all of which may be looked on as derived from
archæocytes, _i.e._ germ-cells. From these the coelomatous animals arose,
and here also we find, according to {475}present-day opinion, that the
coelom arose in the first place in the very closest connection with the
germ-cells or gonads. Thus Lankester, in his review of the history of the
coelom, states:--

"The numerous embryological and anatomical researches of the past twenty
years seem to me to definitely establish the conclusion that the coelom is
primarily the cavity, from the walls of which the gonad cells (ova or
spermata) develop, or which forms around those cells. We may suppose the
first coelom to have originated by a closing or shutting off of that
portion of the general archenteron of Enterocoela (Coelentera), in which
the gonads developed as in Aurelia or as in Ctenophora. Or we may suppose
that groups of gonad mother cells, having proliferated from the endoderm,
took up a position between it and the ectoderm, and there acquired a
vesicular arrangement, the cells surrounding the cavity in which liquid
accumulated.

"The coelom is thus essentially and primarily (as first clearly formulated
by Hatschek) the perigonadial cavity or gonocoel, and the lining cells of
gonadial chambers are coelomic epithelium. In some few groups of
Coelomocoela the coeloms have remained small and limited to the character
of gonocoels. This seems to be the case in the Nemertina, the Planarians,
and other Platyhelmia. In some Planarians they are limited in number, and
of individually large size; in others they are numerous."

When Lankester says that "the lining cells of gonadial chambers are
coelomic epithelium," that is equivalent to saying that the lining cells of
the coelom form an epithelium which was originally gonadial, provided that,
as seems to me most probable, his second suggestion, of the coelom being
formed from gonadial mother-cells which have taken up an intermediate
position between endoderm and ectoderm and there acquired a vesicular
arrangement, is the true one. It does not seem to me possible to conceive
of the gonads arising from cells of the epiblast or of the hypoblast, in
the sense that such cells are differentiated cells belonging to a layer
with a definite meaning. When we consider that the gonad gives origin to
the whole of a new individual, that in the protozoan ancestors of the
Metazoa their ultimate aim and object was the formation of gonads, it seems
a wrong conception to speak of the gonads as formed from cells belonging
either to the gut-wall or to the external epithelium. The gonads must stand
in a category by themselves; they represent a whole, {476}while the other
cells represent only a part; they cannot therefore be derived from the
latter. They may, and indeed do, give rise to cells of a subordinate
character, but they cannot rightly be spoken of as derived from such cells.
The very fact mentioned by Lankester, that in the lowest coelomatous
Metazoa, the Platyhelminthes, the coeloms are limited to the character of
simple gonocoels, strongly points to the conclusion that all the coelomic
cells were originally of the nature of gonadial cells, and therefore
free-living and independent of the rest of the cells of the body. Whether
the germ-cells appear, as in Hydra, to be derived from the ectoblast, or,
as is usually stated, from the endoblast, in neither case ought they to be
classed with the internal or external epithelium; they are germ-cells, and
the epithelium which they form is neither epiblastic nor hypoblastic, but
germinal, forming originally a simple gonocoele, afterwards, in the higher
forms, the coelom with its cells of various function. Thus, to quote again
from Lankester, "The coelomic fluid and the coelomic epithelium, as well as
the floating corpuscles derived from that epithelium, acquire special
properties and importance over and above the original functions subservient
to the maturation of the gonadial cells ... the most important developments
of the coelom are in connection with the establishment of an exit for the
generative products through the body-wall to the outer world, and further
in the specialization of parts of its lining epithelium for renal excretory
functions."

Such exits led very early to the formation of coelomoducts, which are true
outgrowths of the coelom itself (p. 14): "The coelomoducts and the
gonocoels of which they are a part, frequently acquire a renal excretory
function, and may retain both the function of genital conduits and of renal
organs, or may, where several pairs are present (metamerized or segmented
animals), subserve the one function in some segments of the body, and the
other function in other segments."

The origin of the coelom and its derivatives from a germinal membrane, as
suggested by Lankester, appears to me most probable, and, if true, it
carries with it conclusions of far-reaching importance, for it necessitates
that all the cells which line true coelomic cavities, and their
derivatives, belong to the category of free-living cells, and are not
connected with the nervous system. The cells in question are essentially
those which line serous cavities and those which form excretory glands such
as the kidneys. In the latter organ we ought especially to be able to
obtain a clear answer to this question, for is {477}it not a gland which
secretes into a duct and might therefore be expected to be innervated in
the same way as other secretory glands? Although there is a strong _primâ
facie_ presumption in favour of the existence of renal secretory nerves,
yet according to the universal opinion of physiologists no evidence in
favour of such nerves has hitherto been found; all the phenomena of
excretion of urine consequent on nerve stimulation are explicable by the
action of nerves on the renal vessels, not on the renal cells. Not only is
the physiological evidence negative up to the present time, but also, I
think, the histological. On the one hand, Retzius has failed to find
nerve-connections with kidney-cells; on the other, Berkley has obtained
such evidence with the Golgi method, but failed entirely with methylene
blue. I do not myself think that the evidence of the Golgi method alone is
sufficient without corroboration by other methods, and, in any case,
Berkley's evidence does not show the nerve-fibres terminating in the
kidney-cells, in the same way as can be shown by modern methods to exist in
the case of epithelial cells of the surface, etc. Quite recently another
paper on this subject has appeared by Smirnow, who appears to have obtained
better results than those given by Berkley.

Apart from these physiological and histological considerations, this
question is also dependent upon the nature of the development of the
excretory organs, for, according to Lankester, all excretory organs may be
divided into the two classes of nephridial organs and coelomostomes, of
which the former are largely derived from epiblast. We should, therefore,
expect to find secretory nerves to nephridial organs, though possibly not
to coelomostomes. The kidneys of the Mammalia are supposed to be true
coelomostomes, although, according to Goodrich's researches, the excretory
organs in Amphioxus are solenocytes, _i.e._ true nephridia.

As to the lining epithelium of the peritoneal, pleural, and pericardial
cavities--_i.e._ the mesothelium--there is no definite evidence that these
cells are provided with nerves. Such surfaces are remarkably insensitive in
the healthy condition, and the pain in such cavities is essentially a
pressure phenomenon and referable to special sense-organs, such as Pacinian
bodies, etc., rather than to the mesothelium itself.

These sense-organs are identical in structure with those in the skin, and,
as Anderson has shown, the nerves of these organs {478}medullate at the
same time as those in the skin, and both obtain their medullary sheaths
earlier than any other nerves, whether afferent or efferent. However
difficult it may be to explain this fact, only one conclusion seems to me
possible--these Pacinian bodies, like the skin Pacinians, originate from a
nest of surface epithelial cells, a conclusion which is extremely probable
on my theory of the origin of vertebrates, but not, as far as I can see, on
any other.

At the present moment the weight of evidence is, to my mind, in favour of
the lining endothelium of the coelomic cavities being composed of free
cells, unconnected with the nervous system rather than the reverse, but I
must confess that the question is undecided. If it be true that the
coelomic lining is partly enterocoelic and partly gonocoelic, as Lankester
teaches, then it would be natural that its cells should be in connection
with the nervous system, to some extent at all events. This view is,
however, based on very slender foundations. If the mesothelium is composed
of cells capable of becoming free, it cannot give rise to the skeletal
muscles, and it cannot therefore be right to speak of the skeletal muscles
as derived from the lining cells of a part of the primary coelom. The
phylogenetic history of the musculature of the different animals points
strongly to its intimate connection with and derivation from surface
epithelial cells rather than from coelomic mesothelial cells. Thus in the
coelenterates, as seen in Hydra, the muscular layer arises directly from a
modification of the surface epithelial cells; and right up to the annelids,
even to the highest form in the Polychæta, we still see it stated that the
musculature, both circular and longitudinal, arises from the ectoderm. In
the Oligochæta and Hirudinea, according to Bergh, there are five rows of
teloblasts on each side, of which four are ectodermic and give rise to the
nerve-ganglia and the circular muscles, while one is mesoblastic and forms
the nephridial organs and the longitudinal muscles. (The latter statement
is, according to Bergh, well known, and is not particularly shown by him.
These longitudinal muscle-bands always lie close against the nervous system
at their first formation, and may well have been derived in connection with
it.)

It is apparently only in the Vertebrata that the lining cells of the
coelomic cavity are definitely stated to give origin to the
body-musculature, and taking into account on the one hand the evidence of
Graham Kerr as to the intimate connection between nerve-cell and
{479}muscle-cell from the very beginning, and on the other the manner in
which all the skeletal muscles of the adult are lined with a lymphatic
endothelium, I am strongly inclined to believe that at the closing up of
the myocoele, when the myomere separates from the mesomere, the lining
cells remain scattered in among the forming muscle-cells and form the
ultimate lymphatic tissue of the muscles. If this is really so, then the
evidence in favour of the mesothelium being composed of free cells not
connected with the nervous system would be much strengthened, for, on the
one hand, an intimate relation exists between the connective tissue cells
and the endothelium of the roots of the lymphatic vessels, a relation
which, according to Virchow, has rendered it impossible to draw any sharp
line of distinction between the two; and, on the other, the lymphatic
endothelium merges into the lining cells of the great serous cavities of
the body.

It is impossible to conceive of an animal possessing a nervous system which
is not in connection with sensory and muscular tissues; an isolated
nerve-cell is a meaningless possession; but it is equally natural to
conceive of a germ-cell being isolated, capable of living an independent
existence. Such a difference between the two kinds of tissues must have
existed from the very commencement of the Metazoa, so that we must, it
seems to me, imagine that in the formation of the Metazoa from the Protozoa
the whole of the body of the latter did not break up into a mass of
separate gonads, each capable of becoming a free-living protozoan similar
to its parent, but that a portion proliferated into a multinucleated
syncytium while the remainder formed the free-living gonads. This
multinucleated syncytium, or host, as it might be called, would still
continue to exist for the purpose of carrying further afield the immortal
gonads, which need no longer be all shed at one time.

In such an animal as _Volvox globator_ we have an indication of the very
kind of animal postulated as connecting the single-celled Protozoa and the
multi-cellular Metazoa, for it consists of a many-celled case which forms a
hollow sphere, each of the cells being provided with flagella for the
purpose of locomotion of the sphere, except a certain number which are not
flagellated; the latter leave the case to swim freely in the fluid
contained within the sphere, and forming spermaries and ovaries, conjugate,
maturate, and then are set free by the rupture of the encircling locomotor
host.

{480}This conception of the predecessors of the Metazoa being composed of a
mortal host, holding within itself the immortal sexual products, leads
naturally to the idea of the separate development of the host from that of
the germ-cells _ab initio_, so that the study of the development of the
Metazoa means the study of two separate constituents of the metazoan
individual--on the one hand, the elaboration of the elements forming the
syncytial host, on the other, of those derived from the free-living
independent germ-cells. The elaboration of the host means the
differentiation of the protoplasm into epithelial, muscular, and nervous
elements, by means of which the gonads were carried further afield and
their nourishment as well as that of the host ensured.

The _rôle_ of the nervous system as the middleman between internal and
external muscular and epithelial surfaces was, I imagine, initiated from
the very earliest time. The further evolution of the host consisted in a
greater and greater differentiation and elaboration of this
neuro-epithelial syncytium, with the result of a steadily increasing
concentration and departmental centralization of the main factor of the
syncytium; in other words, it led to the origin and elaboration of a
central nervous system. In the interstices of this syncytium the gonads
were placed, and at first, doubtless, the life of the host ended when all
the germ-cells had been set free. 'Reproduce and die' was, I imagine, the
law of the Metazoa at its earliest origin, and throughout the ages, during
all the changes of evolution, the reminiscence of such law still manifests
itself even up to the highest forms as yet reached. With the
differentiation of the syncytial host there came also differentiation of
the free-living gonads, so that only some of them attained to the
perfection of independent existence, capable of continuing the species;
while others became subordinate to the first and provided them with
pabulum, manufacturing within themselves yolk-spherules, and thus in the
shape of yolk-cells ministered to the developing egg-cell. Thus arose a
germinal epithelium of which only a few of the elements passed out of the
host as perfect individuals, the remainder being utilized for the nutrition
of these few. Such yolk-cells of the germinal epithelium would still,
however, retain their character as free cells totally independent of the
syncytial host, and, situated as they were between the internal and
external epithelium, capable of amoeboid movement, would naturally have
their phagocytic action {481}utilized either as yolk-cells for the
providing of pabulum to the egg-cell, or as excretory cells for the removal
and rendering harmless of deleterious products of all kinds. Thus the free
cells of the body would become differentiated into the three classes of
germ-cells, yolk-cells, and excretory cells.

Further, the mass of gonads, which originally occupied so large a space
within the interior of the host, necessarily, as the tissues of the host
differentiated more and more, took up less and less space in proportion to
the whole bulk of the host and formed a germinal mass of cells between the
outer and inner epithelial layers. This germinal mass formed an epithelium,
some of the members of which acted as scavengers for the inner and outer
layers of the host, with the result that fluid accumulated between the two
parts of the germinal epithelium in connection respectively with the
external and internal epithelial surfaces of the host, and thus led to the
formation of a gonocoele, which, by obtaining an external opening, a
coelomostome, gave origin to the coelom.

Again, with the longer life of the host, the setting free of the gonads no
longer necessitating the destruction of the host, and also the gonads
themselves requiring a longer and longer time to be fed up to maturity, the
bulk and complexity of the whole organism increased and special supporting
structures became a necessity. The host itself could and did provide these
to a certain extent by secretions from its epithelial elements, but the
intermediate supports were provided by the system of phagocytic cells
utilizing the fluids of the body, at first in the shape of plasma-cells
able to move from place to place, then settling down to form a connective
tissue framework, and, later on, cartilage and bone.

So also were gradually evolved the whole of the endothelial structures; the
lymph-cells, blood-cells, etc., all having their origin from the free cells
of the body, which themselves originated in the extension of a germinal
epithelium. Just as in a bee-hive the egg-cells may form the fully
developed sexual animal, whether drone or queen bee, or the asexual host of
workers, so in the body of the Metazoa the free cells may form either male
or female germ-cells spermatozoa, or ova, or a host of workers, scavengers,
repairers, food-providers, all useful to the community, all showing their
common origin by their absolute independence of the nervous system.

Two points of great importance follow from this method of looking {482}at
the problem. First, the evolution of the animal kingdom means essentially
the evolution of the host, for that is what forms the individual; secondly,
as the host is composed of a syncytium, the common factor of whose elements
is the neural moiety, it follows that the tissue of central importance for
the evolution of the host must be, as indeed it is, the nervous system.
Further, seeing that the growth of the individual means the orderly
spreading out of the epithelial moiety away from the neural moiety, it
follows that the germ-band or germ-area from which growth starts must be in
the position of the nervous system. If then, the nervous system in the
animal is a concentrated one, then the growth will emanate from the
position of such nervous system. If, on the other hand, the nervous system
is diffused, then the growth will also be diffused.

In this book I have throughout argued that the ancestors of vertebrates
belonged to a great group of animals which gave origin also to Limulus and
scorpion-like animals; it is therefore instructive to see what is the
nature of the development of such animals. For this purpose I will take the
development of the scorpion, as given by Brauer, for he has worked out its
development with great thoroughness and care. His papers show that the
segmentation is discoidal, and results in an oval blastodermic area lying
on a large mass of yolk. Very early there separates out in this area
genital cells and yolk-cells, which latter move freely into the yolk and
prepare it into a fluid pabulum for the nutrition of the cells of the
embryonic shield or germ-band. These free yolk-cells do not take part in
the formation of the germinal layers, nor does the endoderm when formed
give origin to free yolk-cells.

The cells of the germ-band form a small compact area, in which by continual
mitosis the cells become more than one-layered, and soon it is found that
those cells which lie close against the fluid pabulum form a continuous
layer and absorb the nutritious material for themselves and the rest of the
embryo. While this area is thus increasing in thickness by continuous
development, the group of genital cells remains always apart, increasing in
number, but being always in a state of isolation from the cells of the rest
of the growing area. Thus from the very first Brauer's observations on the
development of the scorpion point to the formation of a syncytial host
containing separate genital cells. The continuous layer of cells against
the fluid pabulum, which is already functioning as a gut, and may
{483}therefore be called hypoblast, spreads continuously over the yolk, as
also does the surface epithelial layer, or epiblast. Such spreading is
always a continuous one for both surfaces, so that the yolk is gradually
enclosed by a continuous orderly growth from the germ-band, and not by the
settling down of free cells in the yolk here and there to form the
gut-lining. This steady orderly development proceeds owing to the
nourishment afforded by the activity of the free cells or vitellophags and
the absorbing power of the hypoblast, a steady growth round the yolk which
results in the formation of the gut-tube, the outer covering and all the
muscular and excretory organs. Where, then, is this starting-point, this
germ-band from which the whole embryo grows? It forms the mid ventral area
of the adult animal, it corresponds exactly to the position of the central
nervous system. The whole phenomenon of embryonic growth in the scorpion is
exactly what must take place on the argument deduced from the study of the
adult that the animal arises as a neuro-epithelial syncytium, and we see
that that layer of cells which is situated next to the food-material forms
the alimentary tube. It is not a question whether such layer is ventral or
dorsal to the neural cells, but whether it is contiguous to or removed from
the food-material.

Take, again, a meroblastic vertebrate egg as of the bird. Again we find
free cells passing into the yolk to act as vitellophags, the so-called
periblast cells; again we see that the embryo starts from a germ-band or
embryonic shield, and spreads from there continuously and steadily; again
we see that the layer of cells which lies against the yolk absorbs the
fluid pabulum for the growing cells; again we see that the area from which
the whole process of growth starts is that of the central nervous system,
and again we see that those cells which are contiguous to the food form the
commencing gut, and are therefore called hypoblast, though in this case
they are ventral not dorsal to the neural layer.

The comparison of these two processes shows that there is one common
factor, one thing comparable in the two, one thing that is homologous and
is the essential in the formation of that part of the animal which I have
called the host, and that is the central nervous system. Whether the
epithelial layer which lies ventrally to it or the one that is dorsal forms
the gut depends upon the position of the food-mass. Where the food is,
there will be the absorbing layer. {484}Where the food is not, there will
be no gut formation, whatever may have been the previous history of that
layer. If, then, we suppose, as I do, that the vertebrate arose from a
scorpion-like animal without any reversal of dorsal and ventral surfaces,
and that the central nervous system remained the same in the two animals,
then the comparison of the development of the two embryos shows that the
one would be derived from the other if the yolk-mass shifted from the
dorsal to the ventral side of the nervous system. This would leave the
dorsal epithelial layer of the original syncytium free from pabulum; it
would no longer form the definite gut, _but it would still tend to form
itself in the same manner as before, would still grow from a ventrally
situated germ-band dorsalwards to form a tube, would recapitulate its past
history, and show how the alimentary canal of the arthropod became the
neural canal of the vertebrate_. Although this alimentary canal is formed
in the same way as before, it is no longer recognized as homologous with
the scorpion's alimentary canal, but because it no longer absorbs pabulum,
and does not therefore form the definite gut, it is called an epiblastic
tube, and, in the words of Ray Lankester, has no developmental importance.

All the arthropods are built up on the same type, and in all the
development may in its broad outlines be referred to the type just
mentioned. So also with the vertebrate group; in both cases the position of
the central nervous system determines the starting area of embryonic
growth. In both cases the absorbing layer shows the position of the
definite gut. A concentrated nervous system of this type is common to all
the segmented animals from the annelids to the vertebrates, and in all
cases the germ-band which indicates the first formation of the embryo is in
the position of this nervous system.

As far as the embryo is concerned, there is no great difficulty in the
conception that the yolk-mass may have shifted from one side to the other
in passing from the arthropod to the vertebrate, for in the arthropod the
embryo at first is surrounded by yolk and then passes to the periphery of
the egg. If it is permissible to speak of a dorsal and ventral surface to
an egg, and we may imagine the egg held with such dorsal surface uppermost,
then the yolk would be situated ventrally to the embryo, as in the
vertebrate, if the protoplasmic cells of the embryo rose from their central
position to the surface through the yolk, while if they sank through the
yolk, the yolk would be situated dorsally to the embryo, as in the
arthropod.

{485}In cases where there is no yolk, or very little, as in Lucifer and
Amphioxus respectively, the embryo is compelled to feed itself at a very
early age; such embryos form a free-swimming pelagic ciliated blastula, the
invagination of which, for the purpose of collecting food material out of
the open sea, is the simplest method of obtaining nutriment. Here, as in
other cases, it is the physiological necessity which determines the method
of formation of the gut, and such similarity of appearance as exists
between the gastrula of Lucifer and that of Amphioxus, by no means implies
that the gut of the adult Lucifer is homologous with the gut of Amphioxus.

I have compared two meroblastic eggs of the two classes respectively,
because the scorpion's egg is meroblastic. I imagine that no real
difficulty arises with respect to holoblastic eggs, for the experiments of
O. Hertwig and Samassa show that by centrifugalizing, stimulating, and
breaking down of large spheres the holoblastic amphibian egg may be
converted into a meroblastic one, and then development will proceed
regularly, _i.e._ in this case also the growth proceeds from the animal
pole; the large cells of the vegetal pole, like the yolk-cells of the
meroblastic egg, manufacture pabulum for the growing syncytial host.


SUMMARY.

  Any attempt to discover how vertebrates arose from invertebrates must be
  based upon the study of Comparative Anatomy, of Palæontology, and of
  Embryology. The arguments and evidence put forward in the preceding
  chapters show most clearly how the theory of the origin of vertebrates
  from palæostracans is supported by the geological evidence, by the
  anatomical evidence, and by the embryological evidence. Of the three the
  latter is the strongest and most conclusive, if it be taken to include
  the evidence given by the larval stage of the lamprey.

  The stronghold of embryology for questions of this sort is the Law of
  Recapitulation, which asserts that the history of the race is
  recapitulated to a greater or less extent in the development of the
  individual. In the previous chapters such recapitulation has been shown
  for all the organs of the vertebrate body. In this respect, then,
  embryology has proved of the greatest value in confirming the evidence of
  relationship between the palæostracan and the vertebrate, given by
  anatomical and geological study.

  There is, however, another side to embryology, which claims that the
  tissues of all the Metazoa are built up on the same plan; that in all
  cases in the very early stage of the embryo three layers are formed, the
  epiblast, mesoblast, and hypoblast; that in all animals above the
  Protozoa these three layers are {486}homologous, the epiblast in all
  cases forming the external or skin-layer, the hypoblast the internal or
  gut-layer.

  Such a theory, therefore, as is advocated in this book, which turns the
  gut of the arthropod into the neural canal of the vertebrate, and makes a
  new gut for the vertebrate from the external surface must be wrong, as it
  flatly contradicts the fundamental germ-layer theory.

  Of recent years grave doubts have been thrown upon the validity of this
  theory, doubts which have increased in force year by year as more and
  more facts have been discovered which are not in agreement with the
  theory. So much is it now discredited that any criticism against my
  theory, which is based upon it, weighs nothing in the balance against the
  positive evidence of recapitulation already stated. If the germ-layer
  theory is no longer credited, upon what fundamental laws is embryology
  based?

  In this chapter I have ventured to suggest a reply to this question,
  based on the uniformity of the laws of growth throughout the existence of
  the individual.

  In the adult animal the body is composed of two kinds of tissues, those
  which are connected with or at all events are under the control of the
  nervous system, and those which are capable of leading a free life
  independent of the nervous system. These two kinds of tissues can be
  traced back from the adult to the embryo, and it is the task of
  embryology to find out how these two kinds of tissue originate.

  The following out of this line of thought leads to the conception that,
  throughout the Metazoa, the body is composed of a host which consists of
  the master-tissues of the body, and takes the form of a neuro-epithelial
  syncytium, within the meshes of which free living independent organisms
  or cells live, so to speak, a symbiotic existence.

  The evidence points to the origin of all these free cells from
  germ-cells, and thus leads to the conception that the blastula stage of
  every embryo represents two kinds of cells, the one which will form the
  mortal host being the locomotor neuro-epithelial cell, the other the
  independent immortal symbiotic germ-cell. Such conception leads directly
  to the conclusion that the blastula stage of every member of the Metazoa
  is the embryonic representation of a Protozoan ancestor of the Metazoa;
  an ancestor, whose nature may be illustrated by such a living form as
  _Volvox globator_, which, like a blastula, is composed of a layer of
  cells forming a hollow sphere. These cells partly bear cilia, and so form
  a locomotor host, partly are of a different character, and form male and
  female germ-cells. The latter leave the surface of the sphere, pass as
  free individuals into its fluid contents, form spermaries and ovaries,
  and then by the rupture of the mortal locomotor host pass out into the
  external medium, as free swimming young Volvox.

  It is of interest to note that such members of the Protozoa are among the
  most highly developed of the members of this great group.

  From such a beginning arose in orderly evolution, on the one hand, all
  the neuro-muscular and neuro-epithelial structures of the body--the
  so-called master-tissues; on the other, the germ-cells, the
  blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective
  tissue cells, cartilage and bone-cells, etc., all of them independent of
  the central nervous system, all traceable to a modification of the
  original germ-cells.

  {487}Such a view of the processes of embryology brings embryology into
  harmony with comparative anatomy and phylogeny, for it makes the central
  nervous system and not the alimentary canal the most important factor in
  the development of the host.

  The growth of the individual, whether arthropod or vertebrate, spreads
  from the position of the central nervous system, regardless of whether
  that position is a ventral or dorsal one with respect to the yolk-mass.
  Where the pabulum is, there is the definite gut, the lining walls of
  which are called in the embryo, hypoblast; but when the pabulum is no
  longer there, although a tube is formed in the same manner as the
  alimentary canal of the arthropod, it is now called an epiblastic tube,
  and is known as the neural tube of the vertebrate.

  This is the great fallacy of the germ-layer theory, a fallacy which
  consists of an argument in a vicious circle: thus the alimentary canal is
  homologous in all of the Metazoa, because it is formed of hypoblast, but
  there is no definition of hypoblast, except that it is always that layer
  which forms the definitive alimentary canal.

  When, after the process of segmentation has been completed, a free
  swimming blastula results, unprovided with any store of pabulum in the
  shape of yolk, then the same physiological necessity causes such a form
  to obtain its nutriment from the surrounding medium. The simplest way to
  do this is by a process of invagination, in consequence of which food
  particles are swept into the invaginated part and then absorbed. For this
  reason in such cases true gastrulæ are formed, as in the case of
  Amphioxus among the vertebrates, and Lucifer among the crustaceans; such
  a formation does not in the least imply that the gut of the arthropod is
  homologous with that of the vertebrate. The resemblance between the two
  is not a morphological one, but due to the same physiological necessity.
  They are analogous formations, not homologous.

  The muscular tissues are found to be formed in close connection with the
  nervous tissues, and in very many cases are described as formed from
  epiblast, so that there are strong reasons for placing them in a special
  category of the so-called mesoblastic tissues. If they be separated out,
  then it seems to me, the rest of the mesoblast would consist of the
  free-living cells of the body, which are not connected with the central
  nervous system. In watching, then, the formation of mesoblast, defined in
  this way, we are watching the separation out from the master-tissues of
  the body of the independent skeletal and excretory cells.




{488}CHAPTER XV

_FINAL REMARKS_

  Problems requiring investigation--

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

  Criticism of Balanoglossus theory.--Theory of parallel
  development.--Importance of the theory advocated in this book for all
  problems of Evolution.


The discussion in the last chapter on the "Principles of Embryology"
completes the evidence which I am able to offer up to the present time in
favour of my theory of the "Origin of Vertebrates." There are various
questions which I have left untouched, but still are well worth discussion,
and may be mentioned here. The first of these is the significance of the
giant nerve-cells and giant nerve-fibres so characteristic of the
brain-region of the lower vertebrates. In most fishes two very large cells
are most conspicuous objects in any transverse section of the _medulla
oblongata_ at the level of entrance of the auditory nerves. Each of these
cells gives off a number of processes, some of which pass in the direction
of the auditory nerves and one very large axis-cylinder process which forms
a giant-fibre, known by the name of a Mauthnerian fibre. Each Mauthnerian
fibre crosses the middle line soon after its origin from the giant-cell,
and passes down the spinal cord on the opposite side right to the tail.
Here, near the end of the spinal cord, it breaks up into smaller fibres,
which are believed by Fritsch and others to pass out directly into the
ventral roots to supply the muscles of the tail. Thus Bela Haller says:
"The Mauthnerian fibres are known to give origin to certain fibres which
supply the ventral roots of the last three spinal nerves, so that their
terminal branches serve, in all probability, for the innervation of the
muscles of the tail-fin." They do not occur in the eel, according to
Haller, or in Silurus, according to Kölliker. {489}Their absence in those
fishes, in which a well-developed tail-fin is also absent, increases the
probability of the truth of Fritsch's original conclusion that these
giant-fibres are associated axis-cylinders for certain definite
co-ordinated movements of the fish, especially for the lateral movement of
the tail.

In Ammocoetes, instead of two Mauthnerian fibres, a number of giant-fibres
are found. They are called Müllerian fibres, and arise from giant-cells
which are divisible into two groups. The first group consists of three
pairs situated headwards of the level of exit of the trigeminal nerves. Two
of these lie in front of the level of exit of the oculomotor nerves, and
one pair is situated at the same level as the origin of the oculomotor
nerves. The second group consists of a number of cells on each side at the
level of the entrance of the fibres of the auditory nerves.

The Müllerian fibres largely decussate, as described by Ahlborn, and then
become the most anterior portion of the white matter of the spinal cord,
forming a group of about eight fibres on each side (Fig. 73). A few fibres
are also found laterally, and slightly dorsally, to the grey matter. These
giant-fibres pass down the spinal cord right to the anal region; their
ultimate destination is unknown. Mayer considers that in the first part of
their course they correspond to those tracts of fibres known as the
"posterior longitudinal bundles" in other vertebrates. I imagine,
therefore, that the spinal part of their course represents the two
antero-lateral descending tracts. The second group of giant-cells, which
appears to have some connection with the auditory nerves, may represent
"Deiter's nucleus." The whole system is probably the central nervous part
of a co-ordination mechanism, which arises entirely in the pro-otic or
prosomatic region of the brain--the great co-ordinating and equilibrating
region _par excellence_.

If we turn now to the arthropod it is a striking coincidence that in the
crayfish and in the lobster the work of Retzius, of Celesia, of Allen, and
of many others demonstrates the existence of an equilibration-mechanism for
the swimming movements of the tail-muscles, which is carried out by means
of giant-fibres. These giant-fibres are the axis-cylinder processes of
giant-cells, situated exclusively in the brain-region, and they run through
the whole ventral ganglionic chain in order to supply the muscles of the
tail. In the ventral nerve-cord of the crayfish, according to Retzius, two
specially large {490}giant-fibres exist, each of which breaks up, at the
last abdominal ganglion, into smaller fibres, which pass directly out with
the nerves to the tail-fin. Allen has shown that, in addition to these two
specially large giant-fibres, there are a number of others, some of which,
similarly to the Müllerian fibres of Ammocoetes, cross the middle line,
while some do not. Each of these arises from a large nerve-cell and passes
to one or other of the last pair of abdominal ganglia. The latter fibres,
he says, send off collaterals, while the two specially large giant-fibres
do not. The cells which give origin to all these large, long fibres are
situated in or in front of the prosomatic region of the brain, similarly to
the giant-cells, which give rise to the corresponding Müllerian fibres of
Ammocoetes. I do not know how far this system is represented in Limulus or
Scorpio.

It is, to my mind, improbable that the Mauthnerian fibres pass out directly
as motor fibres to the muscles of the tail-fin; it is more likely that they
are conducting paths between the equilibration-mechanism in connection with
the VIIIth nerve and the spinal centres for the movements of the tail.
Similarly, with respect to the arthropod, it is difficult to believe that
the motor fibres for the tail-muscles arise in the brain-region. In either
case, the striking coincidence remains that the movements of the tail-end
of the body are regulated by means of giant-fibres which arise from
giant-cells in the head-region of the body in both the Arthropoda and the
lowest members of the Vertebrata.

The meaning of this system of giant-cells and giant-fibres in both classes
of animals is well worthy of further investigation.

Another important piece of comparative work which ought to help in the
elucidation of this problem is the comparison of the blood- and
lymph-corpuscles of the vertebrate with those of the invertebrate groups.
As yet, I have not myself made any observations in this direction, and feel
that it is inadvisable to discuss the results of others until I know more
about the facts from personal observation.

The large and important question of the manner of formation of the
vertebrate skin has only been considered to a slight extent. A much more
thorough investigation requires to be made into the nature of the skin of
the oldest fishes in comparison with the skin of Ammocoetes on the one
side, and of Limulus and the Palæostraca on the other.

The muscular system requires further investigation, not so much {491}the
different systems of the striated voluntary musculature--for these have
been for the most part compared in the two groups of animals in previous
chapters--as the involuntary unstriped musculature, about which no word has
been said. The origin of the different systems of unstriped muscles in the
vertebrate is bound up with the origin of the sympathetic system and its
relation to the cranial and sacral visceral systems. The reason why I have
not included in this book the consideration of the sympathetic nervous
system is on account of the difficulty in finding any such system in
Ammocoetes. Also, so far as I know, the distribution of unstriped muscle in
Ammocoetes has not been worked out.

One clue has arisen quite recently which is of great importance, and must
be worked out in the future, viz. the extraordinary connection which exists
between the action of the sympathetic nervous system and the action of
adrenalin. This substance, which is obtained from the medullary part of the
adrenal or suprarenal glands, when injected into an animal produces the
same effects as stimulation of the nerves, which belong to the
lumbo-thoracic outflow of visceral nerves, _i.e._ the system known as the
sympathetic nervous system, which is distinct from both the cranial and
sacral outflows of visceral nerves. The similarity of its action to
stimulation of nerves is entirely confined to the nerves of this
sympathetic system, and never resembles that of either the cranial or
sacral visceral nerves.

Another most striking fact which confirms the great importance of this
connection between the adrenals and the sympathetic nervous system from the
point of view of the evolution of the latter system is that the extract of
the adrenals always produces the same effect as that of stimulation of the
nerves of the sympathetic system, whatever may be the animal from which the
extract is obtained. Thus adrenalin obtained from the elasmobranch fishes
will produce in the highest mammal all the effects known to occur upon
stimulation of the nerves of its sympathetic system.

Further, the cells, which are always associated with the presence of this
peculiar substance--adrenalin--stain in a characteristic manner in the
presence of chromic salts. In Ammocoetes patches of cells which stain in
this manner have been described in connection with blood-vessels in certain
parts, so that, although I know of no definite evidence of the existence of
cell-groups in Ammocoetes corresponding to the ganglia of the sympathetic
system in other vertebrates, it is {492}possible that further investigation
into the nature and connection of these "chromaffine" cells may afford a
clue to the origin of the sympathetic nervous system. At present it is
premature to discuss the question further.

Finally, another test as to the kinship of two animals of different species
must be considered more fully than I have been able to do up to the present
time. This test is of a totally different nature to any put forth in
previous pages. It is known as the "biological test" of relationship, and
is the outcome of pathological rather than of physiological or anatomical
research. It is possible that this test may prove the most valuable of all.
At present we do not know sufficiently its limitations and its sources of
error, especially in the case of cold-blooded animals, to be able to look
upon it as decisive in a problem of the kind considered in this book.

The nature of this test is as follows: It has been found that the serum of
the blood of another animal, when injected in sufficient quantity into a
rabbit, will cause such a change in the serum of that rabbit's blood that
when it is added to the serum of the other animal a copious precipitate is
formed, although the serum of normal rabbit's blood when mixed with that of
another animal will cause no precipitate whatever. This extraordinary
production of a precipitate in the one case and not in the other indicates
the production of some new substance in the rabbit's serum in consequence
of the introduction of the foreign serum into the rabbit, which brings
about a precipitate when the rabbit's serum containing it is mixed with the
serum originally injected. The barbarous name "antibody" has been used to
express this supposed substance in accordance with the meaning of such a
word as "antitoxin," which has been a long time in use in connection with
preventive remedies against pathogenic bacteria. Now, it is found that the
rabbit's serum containing a particular "antibody" will cause a precipitate
only when added to the serum of the blood of the animal from which the
"antibody" was produced or to the serum of the blood of a nearly related
animal.

Further, if that animal is closely related a precipitate will be formed
nearly as copious as with the original serum, if more distantly related a
cloudiness will occur rather than a precipitate, and if the relationship is
still more distant the mixture of the two sera will remain absolutely
clear. Thus this test demonstrates the close relationship of man to the
anthropoid apes and his more distant {493}relationship to monkeys in
general. By this method very evident blood-relationships have been
demonstrated, especially between members of the Mammalia.

I therefore started upon an investigation into the possibility of proving
relationship in this way between Limulus and Ammocoetes, with the kind
assistance of Mr. Graham Smith. I must confess I was not sanguine of
success, as I thought the distance between Limulus and Ammocoetes was too
great. Dr. Lee, of New York, kindly provided me with most excellent serum
of Limulus, and the first experiments showed that the anti-serum of Limulus
gave a most powerful precipitate with its own serum. Graham Smith then
tried this anti-serum of Limulus with the serum of Ammocoetes, and to his
surprise, and mine, he obtained a distinct cloudiness, indicative of a
relationship between the two animals. This, however, is not considered
sufficient, the reverse experiment must also succeed. I therefore, with
Graham Smith, obtained a considerable amount of blood from the adult
lampreys at Brandon, and produced an anti-serum of Petromyzon, which gave
some precipitate with its own serum, but not a very powerful one. This
anti-serum tried with Limulus gave no result whatever, but at the same time
it gave no result with serum from Ammocoetes, so that the experiment not
only showed that Petromyzon was not related to Limulus, but also was not
related to its own larval form, which is absurd.

Considerable difficulties were encountered in preparing the Petromyzon
anti-serum owing to the extreme toxic character of the lamprey's serum to
the rabbit; in this respect it resembled that of the eel. It is possible
that the failure of the lamprey's anti-serum was due to the necessity of
heating the serum sufficiently to do away with its toxicity before
injecting it into the rabbit. At this point the experiments have been at
present left. It will require a long and careful investigation before it is
possible to speak decisively one way or the other. At present the
experiment is positive to a certain extent, and also negative; but the
latter proves too much, for it proves that the larva is not related to the
adult.

Some day I hope this "biological test" will be of use for determining the
relationships of the Tunicata, the Enteropneusta, Amphioxus, etc., as well
as of Limulus and Ammocoetes.

The origin of Vertebrates from a Palæostracan stock, as put forward in this
book, gives no indication of the systematic position {494}of the Tunicata
or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility
be on the direct line of ascent from the invertebrate to the vertebrate.
They must both be looked upon as persistent failures, relics of the time
when the great change to the vertebrate took place. The Enteropneusta are
on a different footing; in their case any evidence of affinity with
vertebrates is very much more doubtful.

The observer Spengel, who has made the most exhaustive study of these
strange forms, rejects _in toto_ any connection with vertebrates, and
considers them rather as aberrant annelids. The so-called evidence of the
tubular central nervous system is worth nothing. There is not the slightest
sign of any tubular nervous system in the least resembling that of the
vertebrate. It is simply that in one place of the collar-region the piece
of skin containing the dorsal nerve of the animal, owing to the formation
of the collar, is folded, and thus forms just at this region a short tube.
My theory explains in a natural manner every portion of the elaborate and
complicated tube of the vertebrate central nervous system. In the
Balanoglossus theory the evolution of the vertebrate tube in all its
details from this collar-fold is simple guesswork, without any reasonable
standpoint. Similarly, the small closed diverticulum of the gut in
Balanoglossus, which is dignified with the name of "notochord," has no
right to the name. As I have already said, it may help to understand why
the notochord has such a peculiar structure, but it gives no help to
understanding the peculiar position of the notochord. The only really
striking resemblance is between the gill-slits of Amphioxus and of the
Enteropneusta. In this comparison there is a very great difficulty, very
similar to that of the original attempts to derive vertebrates from
annelids--the gill-slits open ventrally in the one animal and dorsally in
the other. In both animals an atrial cavity exists which is formed by
pleural folds, and in these pleural folds the gonads are situated so that
the similarity of the two branchial chambers seems at first sight very
complete. In the Enteropneusta, however, there are certain
forms--Ptychodera--in which these pleural folds have not met in the
mid-line in this branchial region, and in these it is plainly visible that
these folds, with their gonads, spring from the ventral mid-line and arch
over the dorsal region of the body. Equally clearly Amphioxus shows that
its pleural folds, with the gonads, spring from the dorsal side of the
animal, {495}and grow ventralwards until they fuse in the ventral mid-line
(_cf._ Fig. 168).

As far, then, as this one single striking similarity between Amphioxus and
the Enteropneusta is concerned it necessitates the reversal of dorsal and
ventral surfaces to bring the two branchial chambers into harmony.

[Illustration: FIG. 168.--DIAGRAM ILLUSTRATING THE POSITION OF THE PLEURAL
FOLDS AND GONADS IN PTYCHODERA (A) AND AMPHIOXUS (B) RESPECTIVELY.

_Al._, alimentary canal; _D.A._, dorsal vessel; _V.A._, ventral vessel;
_g._, gonads; _NC._, notochord; _C.N.S._, central nervous system.]

In a mud-dwelling animal, like Balanoglossus, which possesses no
appendages, no special sense-organs, it seems likely enough that ventral
and dorsal may be terms of no particular meaning, and consequently what is
called ventral in Balanoglossus may correspond to what is dorsal in
Amphioxus; in this way the branchial regions of the two animals may be
closely compared. Such comparison, however, immediately upsets the whole
argument of the vertebrate nature of Balanoglossus based on the relative
position of the central nervous system and gut, for now that part of its
nervous system which is looked upon as the central nervous system in
Balanoglossus is ventral to the gut, just as in a worm-like animal, and not
dorsal to it as in a vertebrate.

There is absolutely no possibility whatever of making such a detailed
comparison between Balanoglossus and any vertebrate, as I have done between
a particular kind of arthropod and Ammocoetes. In the latter case not only
the topographical anatomy of the organs in the two animals is the same, but
the comparison is valid even to microscopical structure. In the former case
the origin of almost all {496}the vertebrate organs is absolutely
hypothetical, no clue is given in Balanoglossus, not even to the segmented
nature of the vertebrate. The same holds good with the evidence from
Embryology and from Palæontology. I have pointed out how strongly the
evidence in both cases confirms that of Comparative Anatomy. In neither
case is the strength of the evidence for Balanoglossus in the slightest
degree comparable. In Embryology an attempt has been made to compare the
origin of the coelom in Amphioxus and in Balanoglossus. In Palæontology
there is nothing, only an assumption that in the Cambrian and Lower
Silurian times a whole series of animals were evolved between Balanoglossus
and the earliest armoured fishes, which have left no trace, although they
were able to hold their own against the dominant Palæostracan race. The
strangeness of this conception is that, when they do appear, they are fully
armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck
for the believers in the Balanoglossus theory that no intermediate less
armoured forms have been found, especially in consideration of the fact
that the theory of the origin from the Palæostracan does not require such
intermediate forms, but finds that those already discovered exactly fulfil
its requirements.

One difficulty in the way of accepting the theory which I have advocated is
perhaps the existence of the Tunicata. I cannot see that they show any
affinities to the Arthropoda, and yet they are looked upon as allied to the
Vertebrata. I can only conclude that both they and Amphioxus arose late,
after the vertebrate stock had become well established, so that in their
degenerated condition they give indications of their vertebrate ancestry
and not of their more remote arthropod ancestry.

In conclusion, the way in which vertebrates arose on the earth as suggested
in this book carries with it many important far-reaching conclusions with
respect to the whole problem of Evolution.

When the study of Embryology began, great hopes were entertained that by
its means it would be possible to discover the pedigree of every group of
animals, and for this end all the stages of development in all groups of
animals were sought for and, as far as possible, studied. It was soon
found, however, that the interpretation of what was seen was so difficult,
as to give rise to all manner of views, depending upon the idiosyncrasy of
the observer. At his will he decided whether any appearance was
coenogenetic or palingenetic, {497}with the result that, in the minds of
many, embryology has failed to afford the desired clue.

At the same time, the geological record was looked upon as too imperfect to
afford any real help; it was said, and is said, that the Cambrian and
pre-Cambrian periods were so immense, and the animals discovered in the
lower Silurian so highly organized, as to compel us to ascribe the
origination of all the present-day groups to this immense early period, the
animals of which have left no trace of their existence as fossils.

In consequence of, or at all events following upon, the supposed failure of
embryology and of geology to solve the problem of the sequence of evolution
of animal life, a new theory has arisen, which goes very near to the denial
of evolution altogether. This is the theory of parallel development. It
discards the old picture of a genealogical tree with main branches arising
at different heights, these again branching and branching into smaller and
smaller twigs, and substitutes instead the picture of the ribs of a fan,
every rib running independently of every other, each group represented by a
rib reaching its highest development on the circumference of the fan and
coming nearer and nearer to a common point at the handle of the fan. This
point of convergence, where all the groups ultimately meet, is so far back
as to reach to the lowest living organisms.

This, in my opinion, unscientific and inconceivable suggestion has arisen
largely in consequence of a conception which has become firmly fixed in the
minds of very many writers on this subject--the conception that in the
evolution of every group, the higher members of the group are the most
specialized in the peculiarities of that group, and it is impossible to
obtain a new group with different peculiarities from such specialized
members. If, then, a higher group is to arise from a lower, it must arise
from the generalized members of that lower group, in other words, from the
lowest members or those nearly akin to the next lower group.

Similarly, the highest members of this latter group are too specialized,
and again we must go to the more generalized members of the group. In this
way each separate specialized group is put on one side, and so the
conception of parallel development comes into being.

The evidence given in this book dealing with the origin of vertebrates
strikes at the foundations of this belief, for it presents an {498}image of
the sequence of evolution of animal forms in orderly upward progress,
caused by the struggle for existence among the members of the race dominant
at the time, which brought about the origin of the next higher group not
from the lowest members of the dominant group, but from some one of the
higher members of that group.

The great factor in evolution has been throughout the growth of the central
nervous system; from that group of animals which possessed the highest
nervous system evolved up to that time the next higher group must have
arisen.

In this way we can trace without a break, always following out the same
law, the evolution of man from the mammal, the mammal from the reptile, the
reptile from the amphibian, the amphibian from the fish, the fish from the
arthropod, the arthropod from the annelid, and we may be hopeful that the
same law will enable us to arrange in orderly sequence all the groups in
the animal kingdom.

This very same law of the paramount importance of the development of the
central nervous system for all upward progress will, I firmly believe, lead
to the establishment of a new and more fruitful embryology, the leading
feature of which will be, as suggested in the last chapter, not the attempt
to derive from the blastula three germ-layers common to all animals, but
rather two sets of organs--those which are governed by the nervous system
and those which are not--and thus by means of the development of the
central nervous system obtain from embryology surer indications of
relationship than are given at present.

The great law of recapitulation, which asserts that the past history of the
race is indicated more or less in the development of each individual, a law
which of late years has fallen somewhat into disrepute, owing especially to
the difficulty of interpreting the embryological history of the vertebrate,
is triumphantly vindicated by the theory put forward in this book. Each
separate vertebrate organ, one after the other, as shown in the last
chapter, indicates in its development the manner in which it arose from the
corresponding organ of the arthropod. There is no failure in the evidence
of embryology, the failure is in the interpretation thereof.

So, too, my theory vindicates the geological method. There is no failure
here; on the contrary, the record of the rocks proclaims with startling
clearness not only the sequence of evolution in the {499}vertebrate kingdom
itself, but the origin of the vertebrate from the most highly-developed
invertebrate race.

The study of the comparative anatomy of organs down to the finest details
has always been a most important aid in finding out relationship between
animals or groups of animals. My theory endorses this view to the
uttermost, and especially indicates the study of the central nervous system
and its outgoing nerves as that comparative study which is most likely to
afford valuable results.

As for the individual, so for the nation; as for the nation, so for the
race; the law of evolution teaches that in all cases brain-power wins.
Throughout, from the dawn of animal life up to the present day, the
evidence given in this book suggests that the same law has always held. In
all cases, upward progress is associated with a development of the central
nervous system.

The law for the whole animal kingdom is the same as for the individual.
"Success in this world depends upon brains."




{501}BIBLIOGRAPHY AND INDEX OF AUTHORS

  --------------+------------------------------------------+---------------
  Author's name.|            Title of Paper.               |    Pages of
                |                                          |   reference.
  --------------+------------------------------------------+---------------
  AHLBORN       |"Untersuchungen über das Gehirn der       | 210, 489
                | Petromyzonten"                           |
                |  _Zeitsch. f. wiss. Zool._ Vol. 39. 1883 |
                |                                          |
                |"Ueber die Segmentation des               | 260
                | Wirbelthierkörpers"                      |
                |  _Zeitsch. f. wiss. Zool._ Vol. 40. 1884 |
                |                                          |
  AICHEL        |"Vergleichende Entwicklungsgeschichte     | 424, 428
                | und Stammesgeschichte der Nebennieren"   |
                |     _Arch. f. Mikr. Anat._ Vol. 56. 1900 |
                |                                          |
  ALCOCK        |                                          | 135, 287, 288,
                |                                          | 289, 304, 307,
                |                                          | 347, 445
                |                                          |
                |"The Peripheral Distribution of the       | 164, 171, 177,
                | Cranial Nerves of Ammocoetes"            | 188, 202, 297,
                |           _Journ. of Anat. and Physiol._ | 300, 310, 311,
                |                            Vol. 33. 1898 | 316
                |                                          |
                |"On Proteid Digestion in Ammocoetes"      | 58, 213, 442,
                |           _Journ. of Anat. and Physiol._ |
                |                            Vol. 33. 1898 | 452
                |                                          |
  ALLEN         |"Studies on the Nervous System of         | 489
                | Crustacea"                               |
                |         _Q. J. Micr. Sci._ Vol. 36. 1894 |
                |                                          |
  ANDERSON,     |                                          | 448, 470
   H. K.        |                                          |
                |"The Nature of the Lesions which hinder   | 466, 467, 469
                | the Development of Nerve-cells and their |
                | Processes                                |
                |       _Journ. of Physiol._ Vol. 28. 1902 |
                |                                          |
                |"On the Myelination of Nerve-fibres"      | 467, 477
                |         _Report of the Brit. Assn._ 1898 |
                |                                          |
  APATHY        |"Das leitende Element des Nervensystems   | 467
                | und seine topographischen Beziehung zu   |
                | den Zellen"                              |
                |    _Mitth. a. d. Zool. Stat. zu Neapel._ |
                |                            Vol. 12. 1896 |
                |                                          |
  ASSHETON      |"On the Phenomenon of the Fusion of the   | 42
                | Epiblastic Layers in the Rabbit and in   |
                | the Frog"                                |
                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
                |                                          |
                |"An Experimental Examination into the     | 154
                | Growth of the Blastoderm of the Chick"   |
                |       _Proc. of Roy. Soc._ Vol. 60. 1896 |
                |                                          |
  ASSHETON      |"On the Growth in Length of the Frog      | 154
                | Embryo"                                  |
                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
                |                                          |
                |"A Re-investigation into the Early Stages | 154
                | of the Development of the Rabbit"        |
                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
                |                                          |
                |"The Primitive Streak of the Rabbit: the  | 154
                | Causes which may determine its Shape,    |
                | and the part of the Embryo formed by its |
                | Activity"                                |
                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
                |                                          |
  BALFOUR       |'Comparative Embryology.' Vol. 2          | 73, 74, 94,
                |            London. 1881. Macmillan & Co. | 103, 104, 120,
                |                                          | 181, 259, 424
                |                                          |
                |"On the Origin and History of the         | 390, 392
                | Urino-genital Organs of Vertebrates"     |
                |           _Journ. of Anat. and Physiol._ |
                |                            Vol. 10. 1876 |
                |                                          |
                |"On the Nature of the Organ in Adult      | 420
                | Teleosteans and Ganoids, which is usually|
                | regarded as the Head-kidney or           |
                | Pronephros"                              |
                |         _Q. J. Micr. Sci._ Vol. 22. 1882 |
                |                                          |
  BARKER        |'The Nervous System'                      | 470
                |                             London. 1901 |
                |                                          |
  BATESON       |"The Ancestry of the Chordata"            | 11
                |         _Q. J. Micr. Sci._ Vol. 26. 1886 |
                |                                          |
                |'Materials for the Study of Variation'    | 387
                |                             London. 1894 |
                |                                          |
  BEARD         |"The System of Branchial Sense Organs and | 262, 281, 283
                | their Associated Ganglia in Ichthyopsida"|
                |         _Q. J. Micr. Sci._ Vol. 26. 1885 |
                |                                          |
                |"The Development of the Peripheral        | 262, 281, 283
                | Nervous System in Vertebrates"           |
                |         _Q. J. Micr. Sci._ Vol. 29. 1888 |
                |                                          |
                |"The Old Mouth and the New"               | 318
                |                   _Anat. Anzeiger._ 1888 |
                |                                          |
                |"The Source of Leucocytes and the True    | 425, 426
                | Function of the Thymus"                  |
                |          _Anat. Anzeiger._ Vol. 18. 1900 |
                |                                          |
                |"The Parietal Eye of the Cyclostome       | 84
                | Fishes"                                  |
                |         _Q. J. Micr. Sci._ Vol. 29. 1882 |
                |                                          |
  BECK AND      |"On the Muscular and Endo-skeletal        | 171, 222, 224,
   LANKESTER    | Tissues of Scorpio"                      | 247, 268-277
                |        _Trans. Zool. Soc._ Vol. 11. 1885 |
                |                                          |
  BEECHER       |"Natural Classification of the Trilobites"| 283, 351, 436,
                |                   _Amer. Journ. of Sci._ | 437
                |                     Ser. 4. Vol. 3. 1897 |
                |                                          |
  BELL, C.      |'The Nervous System of the Human Body'    | 155, 156, 183
                |                             London. 1830 |
                |                                          |
  BELLONCI      |"Système Nerveux et Organes des sens du   | 62, 90, 92,
                | _Sphæroma serratum_"                     | 101
                |    _Archiv. Ital. de Biol._ Vol. 1. 1882 |
                |                                          |
                |"Sur la structure et les rapports des     | 221, 225
                | lobes olfactives dans les Arthropods     |
                | superieurs et les Vertébrés"             |
                |    _Archiv. Ital. de Biol._ Vol. 3. 1883 |
                |                                          |
  BENHAM AND    |"On the Muscular and Endo-skeletal        | 143, 171, 176,
   LANKESTER    | Systems of Limulus"                      | 177, 247
                |        _Trans. Zool. Soc._ Vol. 11. 1885 |
                |                                          |
  BERGER        |"Untersuchungen über den Bau des Gehirns  | 88-92, 97,
                | und der Retina der Arthropoden"          | 100, 101
                |      _Arbeit. a. d. Zool. Instit. Wien._ |
                |                             Vol. 1. 1878 |
                |                                          |
  BERGH         |"Neue Beiträge zur Embryologie der        | 478
                | Anneliden"                               |
                |  _Zeitsch. f. wiss. Zool._ Vol. 50. 1890 |
                |                                          |
  BERKLEY       |"The Intrinsic Nerves of the Kidney"      | 477
                | _Bulletin of the Johns Hopkins Hospital._|
                |                                   Vol. 4 |
                |                                          |
  BERNARD       |'The Apodidæ: a Morphological Study'      | 284
                |                    _Nature Series._ 1892 |
                |                                          |
  BERTKAU       |"Beiträge zur Kenntniss der Sinnesorgane  | 369
                | der Spinnen. 1. Die Augen der Spinnen"   |
                |   _Archiv. f. mikr. Anat._ Vol. 27. 1886 |
                |                                          |
  BIEDERMANN    |'Electro-physiology'                      | 20
                |  Translated by F. A. Welby. London. 1896 |
                |                                          |
  BLANCHARD     | Quoted by Huxley                         | 225
                |                                          |
                |'L'Organisation du Règne Animal.          | 109, 177, 190,
                | Arachnides'                              | 206, 313, 315
                |                              Paris. 1852 |
                |                                          |
  BLES          |"The Correlated Distribution of Abdominal | 431
                | Pores and Nephrostomes in Fishes"        |
                |           _Journ. of Anat. and Physiol._ |
                |                            Vol. 32. 1898 |
                |                                          |
  BOBRETSKY     |'Development of Astacus and Palæmon'      | 74
                |                               Kiew. 1873 |
                |                                          |
  BOURNE AND    | _See_ Lankester and Bourne.              |
   LANKESTER    |                                          |
                |                                          |
  BOVERI        |"Die Nieren Canälchen des Amphioxus"      | 392, 395, 402,
                |           _Zool. Jahrbuch._ Vol. 5. 1892 | 407, 412, 426,
                |                                          | 427
                |                                          |
  BRAEM         |"Was ist ein Keimblatt"                   | 460, 461, 462
                |      _Biol. Centralblatt._ Vol. 15. 1895 |
                |                                          |
  BRAUER        |"Beiträge zur Kenntniss der               |
                | Entwicklungsgeschichte des Skorpions"    | 62, 167, 222,
                |                   _Zeit. f. wiss. Zool._ | 237, 281, 482
                |                    Part I. Vol. 57. 1894 |
                |                   Part II. Vol. 59. 1895 |
                |                                          |
                |"Beiträge zur Kenntniss der Entwicklung   | 393, 394, 400,
                | und Anatomie der Gymnophionen." III.     | 402
                | "Die Entwicklung der Excretionsorgane"   |
                |          _Zool. Jahrbuch._ Vol. 16. 1902 |
                |                                          |
                |"Ueber die Entwicklung von Hydra"         | 473
                |     _Zeit. f. wiss. Zool._ Vol. 52. 1891 |
                |                                          |
  BÜTSCHLI      |"Notiz zur Morphologie des Auges der      | 114
                | Muscheln"                                |
                |         _Festschrift des Natur-hist-med. |
                |             Vereins zu Heidelberg._ 1886 |
                |                                          |
  BUJOR         |"Contribution a l'étude de la métamorphose| 135, 304
                | de _l'Ammocoetes branchialis_ en         |
                | _Petromyzon Planeri_"                    |
                | _Revue Biologique du Nord de la France._ |
                |                             Vol. 3. 1891 |
                |                                          |
  CARLSON       |                                          | 177, 315, 316
                |                                          |
  CELESIA       |'Differenziamento della proprietà         | 489
                | inibitoria e dei funzioni coordinatrici  |
                | nella catena gangliare dei crustacei     |
                | decapodi'                                |
                |                              Genoa. 1897 |
                |                                          |
  CLAUS         |"Untersuchungen über den Organismus und   | 90-92, 97,
                | Entwicklung von Branchipus und Artemia"  | 100, 396
                |      _Arbeit a.d. Zool. Institut. Wien._ |
                |                             Vol. 6. 1886 |
                |                                          |
  COPE          |"On the Phylogeny of the Vertebrata"      | 343
                | _Proc. Amer. Philos. Soc._ Vol. 30. 1892 |
                |                                          |
  CRONEBERG     |"Ueber die Mundtheile der Arachniden"     | 221-224, 241
                |       _Archiv. f. Naturgeschichte._ 1880 |
                |                                          |
  CUÉNOT        |"Études sur le sang et les glandes        | 422
                | lymphatiques dans la série animale;      |
                | 2nd partie; invertébrés"                 |
                |             _Arch. d. Zool. exper. gen._ |
                |                    2nd Ser. Vol. 9. 1891 |
                |                                          |
  CUNNINGHAM,   |"The Significance of Kupffer's Vesicle,   | 318
   J. T.        | with Remarks on other Questions of       |
                | Vertebrate Morphology"                   |
                |         _Q. J. Micr. Sci._ Vol. 25. 1885 |
                |                                          |
                |"The Nephridia of _Lanice conchilega_"    | 403
                |                  _Nature._ Vol. 36. 1887 |
                |                                          |
  DANA          |"On Cephalization"                        | 53
                |                _Mag. of Nat. Hist._ 1863 |
                |                                          |
  DEAN-BASHFORD |'Fishes, Living and Fossil'               | 344
                |                           New York. 1895 |
                |                                          |
                |"On the Embryology of _Bdellostoma        | 405
                | Stouti_"                                 |
                |  _Festschr. z. siebenzigsten Geburtstag. |
                |           von C. v. Kupffer._ Jena. 1899 |
                |                                          |
  DENDY         |"On the Parietal Sense-organs and         | 80, 82
                | Associated Structures in the New Zealand |
                | Lamprey (_Geotria australis_)"           |
                |         _Q. J. Micr. Sci._ Vol. 51. 1907 |
                |                                          |
  DIETL         |"Die Organisation des Arthropoden Gehirns"| 101
                |  _Zeitsch. f. wiss. Zool._ Vol. 27. 1876 |
                |                                          |
  DOHRN         |'Der Ursprung der Wirbelthiere und das    | 14, 60, 185,
                | Princip des Functionswechsels'           | 186, 317, 318
                |                            Leipzig. 1875 |
                |                                          |
                | Studien zur Urgeschichte des Wirbelthiere| 188, 195-198,
                | Körpers. VIII. "Die Thyroidea bei        | 199, 212, 213
                | Petromyzon, Amphioxus, und Tunicaten"    |
                |          _Mitth. Zool. Stat. z. Neapel._ |
                |                             Vol. 6. 1886 |
                |                                          |
                |"Neue Grundlagen zur Beurtheilung der     | 262, 263, 279
                | Metamerie des Kopfes"                    |
                |          _Mitth. Zool. Stat. z. Neapel._ |
                |                             Vol. 9. 1890 |
                |                                          |
                | Studien zur Urgeschichte des Wirbelthiere| 167, 314, 337
                | Gefässe Körpers. XIII. "Ueber Nerven     |
                | und bei Ammocoetes und _Petromyzon       |
                | Planeri_"                                |
                |          _Mitth. Zool. Stat. z. Neapel._ |
                |                             Vol. 8. 1888 |
                |                                          |
  DREVERMANN    |"Ueber _Pteraspis dunensis_"              | 29, 30
                |             _Zeitschr. d. Deutsch. Geol. |
                |             Gesellschaft._ Vol. 56. 1904 |
                |                                          |
  EDGEWORTH     |"The Development of the Head-muscles in   | 266
                | _Gallus domesticus_, and the Morphology  |
                | of the Head-muscles in the Sauropsida"   |
                |         _Q. J. Micr. Sci._ Vol. 51. 1907 |
                |                                          |
  EDINGER       |'Anatomy of Central Nervous System in Man | 17, 264
                | and in Vertebrates'                      |
                |                 Translated by Hall. 1899 |
                |                                          |
  v. EICHWALD   |"Die Thier- und Pflanzenreste des alten   | 327
                | rothen Sandsteins und Bergkalks im       |
                | Nowgorodschen Gouvernement"              |
                |            _Bull. Sci. de l'Acad. Impér. |
                |                d. St. Petersbourg._ 1840 |
                |                                          |
  EISIG         |"Die Seiten-organe und becherförmigen     | 357
                | Organe der Capitelliden"                 |
                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
                |                             Vol. 1. 1879 |
                |                                          |
                |"Capitelliden"                            | 357
                |    _Faun. u. Flor. d. Golfes v. Neapel._ |
                |                            Vol. 16. 1887 |
                |                                          |
  ELLIOTT       |"On the Innervation of the Ileo-colic     | 449
                | Sphincter"                               |
                |       _Journ. of Physiol._ Vol. 31. 1904 |
                |                                          |
  EMERY         | Quoted by Weldon                         | 420
                |                                          |
  FOSTER, M.    | Text-book of Physiology                  | 108
                |                                          |
  FREUND        |"Die Beziehungen der Schilddrüse zu den   | 215
                | weiblichen Geschlechtsorganen"           |
                |         _Deutsch. Zeitsch. f. Chirugie._ |
                |                            Vol. 18. 1883 |
                |                                          |
  FRITSCH, G.   |'Untersuchungen über den feineren Bau des | 488, 489
                | Fischgehirns'                            |
                |                             Berlin. 1878 |
                |                                          |
  FRORIEP       |"Ueber Anlagen von Sinnesorganen am       | 261, 262, 281,
                | Facialis, Glossopharyngeus und Vagus,    | 283
                | über die genetische Stellung des Vagus   |
                | zum Hypoglossus, und über die Herkunft   |
                | der Zungenmusculatur"                    |
                |              _Arch. f. Anat. u. Physiol; |
                |                     Anat. Abtheil._ 1885 |
                |                                          |
  FÜRBRINGER,   |'Ueber die Spino-occipetalen Nerven der   |
   M.           | Selachier und Holocephalen'              | 276-278, 409
                |    Fest-schrift für Carl Gegenbaur. 1897 |
                |                                          |
  GAUBERT       |'Recherches sur les organes des sens et   | 364, 368-375
                | sur les systèmes tegumentaire,           |
                | glandulaire et musculaire des appendices |
                | des arachnides'                          |
                |                              Paris. 1892 |
                |                                          |
  GEGENBAUR     |"Anatomische Untersuchung eines Limulus"  | 20, 358-360
                |     _Abhandl. d. Naturforsch. Gesellsch. |
                |                  z. Halle._ Vol. 4. 1858 |
                |                                          |
                |"Ueber die Skeletgewebe der Cyclostomen"  | 181
                |         _Jen. Zeitschrift._ Vol. 5. 1870 |
                |                                          |
                | Untersuchungen zur vergleichende Anatomie| 151, 259, 261
                | der Wirbelthiere III. Heft. 'Das         |
                | Kopfskeletder Selachiern'                |
                |                            Leipzig. 1872 |
                |                                          |
                |'Grundriss der vergleichenden Anatomie'   | 392
                |                            Leipzig. 1878 |
                |                                          |
  v. GEHUCHTEN  |"De l'origine du pathétique et de la      | 264
                | racine supérieure du trijumeau"          |
                |          _Acad. d. Sci. Belg. Bulletin._ |
                |                   3rd Ser. Vol. 29. 1895 |
                |                                          |
  GOETHE        |                                          | 258
                |                                          |
  GÖTTE         |'Entwicklungsgeschichte der Unke'         | 101, 102, 114
                |                            Leipzig. 1875 |
                |                                          |
  GOLGI         |                                          | 72, 465, 477
                |                                          |
  GOODRICH      |"On the Structure of the Excretory Organs |
                | of Amphioxus"                            |
                |         _Q. J. Micr. Sci._ Vol. 45. 1902 | 395, 396, 477
                |                                          |
                |"On the Nephridia of the Polychæta."      | 395
                | Parts I., II., III.                      |
                |      _Q. J. Micr. Sci._ Vols. 40, 41, 43 |
                |                                          |
                |"On the Excretory Organs of Amphioxus"    | 477
                |          _Proc. Roy. Soc._ Vol. 69. 1902 |
                |                                          |
  GRABER        |"Die Chordo-tonalen Sinnesorgane und das  | 364, 369-371
                | Gehör der Insecten"                      |
                |                 _Archiv. f. Mikr. Anat._ |
                |                    Vols. 20 and 21. 1882 |
                |                                          |
  GRENACHER     |'Untersuchungen über das Sehorgan der     | 76, 100
                | Arthropoden'                             |
                |                          Göttingen. 1879 |
                |                                          |
  GUDDEN        | Quoted in Obersteiner                    | 264
                |                                          |
  HAECKEL       |                                          | 461, 462
                |                                          |
  HALLER, BELA  |"Untersuchungen über die Hypophyse und    | 320, 321
                | die Infundibulärorgane"                  |
                |         _Morph. Jahrbuch._ Vol. 25. 1898 |
                |                                          |
                |"Untersuchungen über das Rückenmark der   | 488
                | Teleostier"                              |
                |         _Morph. Jahrbuch._ Vol. 23. 1895 |
                |                                          |
  HARDY         |"On the Histological Features and         | 110, 159
                | Physiological Properties of the          |
                | Post-oesophageal Nerve-cord of the       |
                | Crustacea"                               |
                |        _Phil. Trans. Roy. Soc._ 1894. B. |
                |                                          |
  HARDY AND     |"On the Structure and Functions of the    | 112, 206
   MACDOUGALL   | Alimentary Canal of Daphnia"             |
                |    _Proc. Camb. Phil. Soc._ Vol. 8. 1893 |
                |                                          |
  HATSCHEK      |"Die Metamerie des Amphioxus und des      | 289, 300, 337
                | Ammocoetes"                              |
                |       _Anat. Anzeig._, 7 Jahrgang, 1892. |
                |    _Verhandl. d. Anat. Gesell. in Wien_, |
                |                                   p. 136 |
                |                                          |
                |"Studien über Entwicklung des Amphioxus"  |  407
                |        _Arbeit. d. Zool. Inst. z. Wien._ |
                |                             Vol. 4. 1881 |
                |                                          |
                | Quoted by Lankester                      | 475
                |                                          |
  HAZEN         | _See_ Patten and Hazen.                  |
                |                                          |
  HEIDENHAIN    |                                          | 258, 259
                |                                          |
  HEIDER        | _See_ Korschelt and Heider.              |
                |                                          |
  HENSEN        |"Zur Entwicklung des Nervensystem"        | 465, 466
                |         _Virchows Archiv._ Vol. 30. 1864 |
                |                                          |
  HENSEN AND    | _Archiv. f. Opthalmol._ Vol. 24. 1878    | 265, 266
   VÖLCKERS     |                                          |
                |                                          |
  HERTWIG, O.,  | Quoted in Zeigler's 'Lehrbuch der        |  485
   AND SAMASSA  | vergleichenden Entwicklungsgeschichte    |
                | der niederen Wirbelthiere.' 1902         |
                |                                          |
  HIS           |"Die Neuroblasten und deren Entstehung    | 465, 466
                | im embryonalen Mark"                     |
                |            _Archiv. f. Anat. u. Physiol. |
                |                        Anat. Abth._ 1889 |
                |                                          |
  HOFFMANN      |"Ueber die Metamerie des Nachhirns und    | 276
                | Hinterhirns, und ihre Beziehung zu den   |
                | segmentalen Kopfnerven bei Reptilien     |
                | embryonen"                               |
                |          _Zool. Anzeiger._ Vol. 12. 1889 |
                |                                          |
  HOLM          |"Ueber die Organisation des _Eurypterus   | 192, 240, 241,
                | Fischeri_"                               | 306
                |         _Mem. d. l'Acad. Imp. d. Sci. d. |
                |           St. Petersbourg._ Vol. 8. 1898 |
                |                                          |
  HOYER         |"Ueber den Nachweis des Mucins in Geweben | 131
                | Mittelst der Färbe-Methode"              |
                |   _Archiv. f. Mikr. Anat._ Vol. 36. 1890 |
                |                                          |
  HUXLEY        |"Hunterian Lectures." 1869                | 124, 258, 259
                |                                          |
                |"On the Structure of the Mouth and Pharynx| 222, 225, 271
                | of the Scorpion"                         |
                |          _Q. J. Micr. Sci._ Vol. 8. 1860 |
                |                                          |
                |"On the Anatomy and Affinities of the     | 238
                | Genus Pterygotus"                        |
                |              _Mem. of the Geol. Survey._ |
                |                        Monograph I. 1859 |
                |                                          |
                |"On Cephalaspis and Pteraspis"            | 327
                |      _Q. J. of Geol. Soc._ Vol. 14. 1858 |
                |                                          |
  JAEKEL        |"Ueber Tremataspis und Patten's Ableitung | 329, 339, 340,
                | der Wirbelthiere von Arthropoden"        | 351
                |         _Protocoll der Deutschen Geolog. |
                |    Gesellschaft_, p. 84; in _Zeitsch. d. |
                |       Deutschen Geologischen Gesellsch._ |
                |                            Vol. 55. 1903 |
                |                                          |
                |"Ueber die Organisation und systematische | 345
                | Stellung der Asterolepiden"              |
                |                           _Ibid._, p. 41 |
                |                                          |
  JOHNSON       |"Contributions to the Comparative Anatomy | 70
                | of the Mammalian Eye, chiefly based on   |
                | Opthalmoscopic Examination"              |
                |              _Phil. Trans. Roy. Soc. B._ |
                |                           Vol. 194. 1901 |
                |                                          |
  JOSEPH        |"Ueber das Achsenskelett des Amphioxus"   | 444
                |  _Zeitsch. f. wiss. Zool._ Vol. 59. 1895 |
                |                                          |
  JULIN AND     | Recherches sur l'Organisation des        | 425
   VAN BENEDEN  | Ascidies simples. "Sur l'hypophyse," etc.|
                |     _Archives de Biologie._ Vol. 2. 1881 |
                |                                          |
  KAENSCHE      |"Beiträge zur Kenntniss der Metamorphose  | 135, 304
                | des _Ammocoetes branchialis_ in          |
                | _Petromyzon_"                            |
                |     _Schneider's Beiträge._ Vol. 2. 1890 |
                |                                          |
  v. KENNEL     |"Entwickelungsgeschichte von _Peripatus   | 398, 399, 411
                | Edwardsii_ und _Peripatus torquatus_."   |
                | II. Theil                                |
                |       _Arbeit. a. d. Zool. Zoot. Instit. |
                |                  Würzburg._ Vol. 8. 1888 |
                |                                          |
  KERR          |"On some Points in the Early Development  | 461, 466, 478
                | of Motor Nerve-trunks and Myotomes in    |
                | _Lepidosiren paradoxa_"                  |
                |   _Trans. Roy. Soc. Edin._ Vol. 41. 1904 |
                |                                          |
  KILLIAN       |"Zur Metamerie des Selachierkopfes"       | 262
                |              _Verhandl. d. Anat. Gesell. |
                |              Versamml. in München._ 1891 |
                |                                          |
  KISHINOUYE    |"On the Development of _Limulus           | 167, 238, 252,
                | longispina_"                             | 253, 273, 320,
                |        _Journ. of Coll. of Sci., Tokio._ | 382
                |                             Vol. 5. 1891 |
                |                                          |
  KLEINENBERG   | Quoted by Beard                          | 318
                |                                          |
  v. KÖLLIKER   |"Die obere Trigeminus-Wurzel"             | 280
                |     _Arch. f. Mikr. Anat._ Vol. 53. 1899 |
                |                                          |
  v. KÖLLIKER   |                                          |
   AND          | Handbuch der Gewebe-Lehre. 6th Auflage.  | 264, 425, 488
   TERTERJANZ   | 1893                                     |
                |                                          |
  KOHL          |"Rudimentäre Wirbelthieraugen"            | 94, 96, 99,
                |      _Bibliotheca Zoologica. Leukart und | 101
                |                 Chun._ Vol. 4 and Vol. 5 |
                |                                          |
  KOHN          |"Ueber den Bau und die Entwicklung der    | 428
                | sogenannten Carotis-drüse"               |
                |   _Archiv. f. Mikr. Anat._ Vol. 56. 1900 |
                |                                          |
  KORSCHELT AND |'Text-book of the Embryology of the       | 27, 73, 88,
   HEIDER       | Invertebrates.' Translated by M. Bernard.| 114-116, 397,
                | 1900. Part III. and Part IV.             | 429, 431
                |                                          |
  KOWALEWSKY    |"Ein Beitrag zur Kenntniss der            | 421
                | Excretionsorgane der Pantopoden"         |
                |  _Mem. d. l'Acad. d. Imp. d. Sci. d. St. |
                |    Petersbourg._ Ser. VII. Vol. 38. 1890 |
                |                                          |
                |"Une nouvelle glande lymphatique chez le  | 423
                | scorpion d'Europe"                       |
                |          _Ibid._ Ser. VIII. Vol. 5. 1897 |
                |                                          |
                |"Étude Biologique sur les Clepsines"      | 421
                |          _Ibid._ Ser. VIII. Vol. 5. 1897 |
                |                                          |
                |"Ein Beitrag zur Kenntniss der            | 420, 422, 472
                | Excretionsorgane"                        |
                |        _Biologisches Centralblatt._ 1889 |
                |                                          |
                |"Weitere Studien über die                 | 409, 410
                | Entwicklungsgeschichte des               |
                | _Amphioxus lanceolatus_"                 |
                |   _Archiv. f. Mikr. Anat._ Vol. 13. 1877 |
                |                                          |
  KRIEGER       |"Ueber das Centralnervensystem des        | 101
                | Flusskrebses                             |
                |  _Zeitsch. f. wiss. Zool._ Vol. 33. 1880 |
                |                                          |
  v. KUPFFER    |'Studien zur vergleichenden Entwicklungs- |
                | geschichte des Kopfes der Kranioten.'    |
                |   Heft. 1. 'Die Entwicklung des Kopfes   | 318, 319, 320,
                |   von _Acipenser_'                       | 440
                |                            München. 1893 |
                |                                          |
                |   Heft. 2. 'Die Entwicklung des Kopfes   | 300, 440
                |   von _Ammocoetes Planeri_'              |
                |                            München. 1894 |
                |                                          |
                |   Heft. 3. 'Die Entwicklung der          | 228, 263, 282,
                |   Kopfnerven von _Ammocoetes Planeri_.'  | 283, 405, 458
                |    Dritter Abschnitt. 'Die Metamorphose  |
                |    des larvalen Nervensystems des Kopfes'|
                |                            München. 1895 |
                |                                          |
  LANG          |'Text-book of Comparative Anatomy.'       | 357
                | Translated by H. M. and M. Bernard       |
                |                                          |
  LANGERHANS    |"Untersuchungen über _Petromyzon          | 94-101, 301,
                | Planeri_"                                |
                | _Bericht v. d. Verhandl. d. Naturforsch. | 405
                |            Gesellsch. z. Freiburg._ 1873 |
                |                                          |
  LANGLEY       | Schäfer's 'Text-book of Physiology.'     | 2, 3, 448
                | Vol. 2. 1900                             |
                |                                          |
  LANKESTER     | Article "Vertebrata" in the              | 484
                | 'Encyclopædia Britannica'                |
                |                                          |
                |"On the Skeleto-trophic Tissues and Coxal | 137, 139, 253,
                | Glands of Limulus, Scorpio, and Mygale   | 320, 321
                |         _Q. J. Micr. Sci._ Vol. 24. 1884 |
                |                                          |
                |"Limulus an Arachnid"                     | 62, 238, 241,
                |         _Q. J. Micr. Sci._ Vol. 21. 1881 | 306, 361, 366
                |                                          |
                |'Extinct Animals'                         | 22, 150, 345
                |             London. Constable & Co. 1906 |
                |                                          |
                | A treatise on Zoology. Edited by E. Ray  |
                | Lankester.                               |
                |   Part II. 'The Entero-coela and the     | 472-478
                |   Coelomocoela'                          |
                |                                          |
  LANKESTER AND |"A Monograph of the Fishes of the Old Red | 29, 275, 327,
   POWRIE       | Sandstone of Britain."                   | 339, 345
                |   Part I. "The Cephalaspidæ"             |
                |            _Palæontographical Soc._ 1868 |
                |                                          |
  LANKESTER,    |"On the Muscular and Endo-skeletal Systems| 177, 222, 224,
   BENHAM, AND  | of Limulus and Scorpio, with some Notes  | 313
   BECK         | on the Anatomy and Generic Characters of |
                | Scorpions"                               |
                |        _Trans. Zool. Soc._ Vol. 11. 1885 |
                |                                          |
  LANKESTER AND |"The Minute Structure of the Lateral and  | 74, 81-83
   BOURNE       | Central Eyes of Scorpio and Limulus"     |
                |               _Q. J. Micr. Sci._ Vol. 23 |
                |                                          |
  LANKESTER AND |"The Development of the Atrial Chamber of |
   WILLEY       | Amphioxus"                               | 409
                |         _Q. J. Micr. Sci._ Vol. 31. 1890 |
                |                                          |
  LANKESTER AND |"Evidence in Favour of the View that the  | 429
   GULLAND      | Coxal Gland of Limulus and of other      |
                | Arachnids is a Modified Nephridium"      |
                |         _Q. J. Micr. Sci._ Vol. 25. 1885 |
                |                                          |
  LATREILLE     |                                          | 221
                |                                          |
  LAURIE        |"The Anatomy and Relations of the         | 237
                | Eurypteridæ"                             |
                |   _Trans. Roy. Soc. Edin._ Vol. 37. 1893 |
                |                                          |
                |"On a Silurian Scorpion and some          | 238, 239
                | Additional Eurypterid Remains from the   |
                | Pentland Hills                           |
                |                    _Ibid._ Vol. 34. 1899 |
                |                                          |
  LEYDIG        |                                          | 91
                |                                          |
  LOCY          |"Contributions to the Structure and       | 179, 262
                | Development of the Vertebrate Head"      |
                |            _Journ. Morph._ Vol. 11. 1895 |
                |                                          |
  LOEB, LEO, AND|"On Regeneration in the Pigmented Skin of | 470
   R. M. STRONG | the Frog, and on the Character of the    |
                | Chromatophores"                          |
                |      _Amer. Jour. of Anat._ Vol. 3. 1904 |
                |                                          |
  LOWNE         |'The Anatomy, Physiology, Morphology, and | 369, 370, 375
                | Development of the Blow-fly'             |
                |                             London. 1895 |
                |                                          |
  LUGARO        | Quoted by Anderson                       | 467
                |                                          |
  LWOFF         |"Ueber den Zusammenhang von Markrohr und  | 444
                | Chorda beim Amphioxus und ähnliche       |
                | Verbältnisse bei Anneliden"              |
                |  _Zeitsch. f. wiss. Zool._ Vol. 56. 1893 |
                |                                          |
  MAAS          |"Ueber Entwicklungstadien der Vorniere und| 392, 402, 412,
                | Urniere bei Myxine"                      | 419
                |          _Zool. Jahrbuch._ Vol. 10. 1897 |
                |                                          |
  MACBRIDE      |"Further Remarks on the Development of    | 410
                | Amphioxus"                               |
                |         _Q. J. Micr. Sci._ Vol. 43. 1900 |
                |                                          |
  McDOUGALL     | _See_ Hardy and McDougall.               |
                |                                          |
  MACLEOD       |"Recherches sur la structure et la        | 169, 174
                | signification de l'appareil respiratoire |
                | des Arachnides"                          |
                |          _Archiv. de Biol._ Vol. 5. 1881 |
                |                                          |
  MAGNUS        |"Versuche am überlebenden Dünndarm von    | 447
                | Säugethieren"                            |
                |        _Archiv. f. d. Ges. Physiologie._ |
                |                     Vols. 102, 103. 1904 |
                |                                          |
  MARK          |                                          | 115
                |                                          |
  MARSHALL      |"On the Head-cavities and Associated      | 185, 186
                | Nerves of Elasmobranchs"                 |
                |         _Q. J. Micr. Sci._ Vol. 21. 1881 |
                |                                          |
                |"The Segmental Value of the Cranial       | 260
                | Nerves"                                  |
                |           _Journ. of Anat. and Physiol._ |
                |                            Vol. 16. 1882 |
                |                                          |
  MASTERMAN     |"On the Diplochorda"                      | 16
                |         _Q. J. Micr. Sci._ Vol. 43. 1900 |
                |                                          |
  MAURER        |"Die Schilddrüse, Thymus und andere       | 427, 428
                | Schlundspaltenderivate bei den Eidechse" |
                |         _Morph. Jahrbuch._ Vol. 27. 1899 |
                |                                          |
  MAYER, F.     |"Das Centralnervensystem von Ammocoetes"  | 489
                |            _Anat. Anzeig._ Vol. 13. 1897 |
                |                                          |
  MAYER, P.     |"Ueber die Entwicklung des Herzens und der| 179
                | grossen Gefässstämme bei den             |
                | Selachiern"                              |
                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
                |                             Vol. 7. 1887 |
                |                                          |
  METSCHNIKOW   | Quoted by Kowalewsky                     | 422
                |                                          |
  MEYER         |"Studien über den Körperbau der           | 403
                | Anneliden"                               |
                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
                |                             Vol. 7. 1887 |
                |                                          |
  MILNE-EDWARDS |"Anatomie des Limules"                    | 157, 159, 176,
                |       _Annales des Sciences Naturelles._ | 177, 313
                |                    Ser. 5. Vol. 17. 1872 |
                |                                          |
  MINCHIN       | A treatise on Zoology. Edited by Ray     | 473
                | Lankester. Part II. "The Porifera and    |
                | Coelenterata"                            |
                |                                          |
  MITSUKURI     |"On the Fate of the Blastopore, the       | 179
                | Relations of the Primitive Streak, and   |
                | the Formation of the Posterior End of the|
                | Embryo in Chelonia," etc.                |
                |               _Journ. Coll. Sci._ Tokyo. |
                |                            Vol. 10. 1896 |
                |                                          |
  MOTT          |"Croonian Lectures of the Roy. Coll. of   | 469
                | Physicians," 1900                        |
                |                                          |
  MOTT AND      |"On the Chemistry of Nerve-degeneration"  | 469
   HALLIBURTON  |              _Phil. Trans. Roy. Soc. B._ |
                |                           Vol. 194. 1901 |
                |                                          |
  MÜLLER, J.    |                                          | 1
                |                                          |
                |"Vergleichende Anatomie der Myxinoiden"   | 126
                |        _Abhandl. d. Kgl. Akad. d. Wiss._ |
                |                             Berlin. 1834 |
                |                                          |
  MÜLLER, W.    |"Ueber die Stammes Entwickelung des       | 96-100, 105,
                | Sehorgans der Wirbelthiere"              | 108
                |        Festgabe C. Ludwig. Leipzig. 1874 |
                |                                          |
  NEAL          |"The Segmentation of the Nervous System   | 179, 266, 300
                | in _Squalus acanthias_"                  |
                |              _Bull. of Mus. Comp. Zool._ |
                |                   Harvard. Vol. 31. 1898 |
                |                                          |
  NESTLER       |"Beiträge zur Anatomie und                | 168, 171, 175,
                | Entwicklungsgeschichte von _Petromyzon   | 445
                | Planeri_"                                |
                |   _Archiv. f. Naturgesch. Jahrgang_, 56. |
                |                             Vol. I. 1890 |
                |                                          |
  NIESKOWSKI    |"Der _Eurypterus Remipes_ aus den ober-   | 26, 239, 240
                | silurischen Schichten der Insel Oesel"   |
                |     _Arch. f. d. Naturkunde Liv-Ehst-und |
                |         Kurlands._ 1st Ser. Vol. 3. 1858 |
                |                                          |
  NUSBAUM, J.   |"Einige neue Thatsachen zur               | 320
                | Entwicklungsgeschichte des _Hypophysis   |
                | Cerebri_ bei Säugethieren"               |
                |          _Anat. Anzeiger._ Vol. 12. 1896 |
                |                                          |
  OBERSTEINER   |'Central Nervous System.' Translated by   | 264, 280
                | Hill. 1896                               |
                |                                          |
  OKEN          |                                          | 258
                |                                          |
  OWEN          |"Essays on the Conario-Hypophysial Tract, | 14
                | and the Aspects of the Body in Vertebrate|
                | and Invertebrate Animals"                |
                |                                          |
                |"On the Anatomy of the American King-crab | 211
                |(_Limulus polyphemus_)"                   |
                |        _Trans. Linn. Soc._ Vol. 28. 1873 |
                |                                          |
  PANDER        |'Monographie der fossilen Fische des      | 327
                | Silurischen Systems des russisch-        |
                | baltischen Gouvernements'                |
                |                    St. Petersbourg. 1856 |
                |                                          |
  PARKER, G. H. |"The Retina and Optic Ganglia in Decapods,| 91, 93, 97
                | especially in Astacus"                   |
                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
                |                            Vol. 12. 1895 |
                |                                          |
                |"The Compound Eyes in Crustaceans"        | 99, 100, 114
                |   _Bull. of Harvard Mus. of Comp. Zool._ |
                |                            Vol. 20. 1890 |
                |                                          |
                |"The Function of the Lateral-line Organs  | 357
                | in Fishes"                               |
                |         _Bull. of the Fisheries Bureau._ |
                |                Washington. Vol. 24. 1904 |
                |                                          |
                |"Studies on the Eyes of Arthropods"       | 73, 79, 83-85,
                |                  _Journ. of Morphology._ | 114
                |             Vols. 1 and 2. 1887 and 1889 |
                |                                          |
  PARKER, W. K. |"On the Skeleton of the Marsipobranch     | 120, 125, 126,
                | Fishes"                                  | 131
                |            _Phil. Trans. Roy. Soc._ 1883 |
                |                                          |
  PATTEN        |"On the Origin of Vertebrates from        | 352, 353
                | Arachnids"                               |
                |         _Q. J. Micr. Sci._ Vol. 31. 1890 |
                |                                          |
                |"On the Morphology and Physiology of the  | 358-367, 371
                | Brain and Sense-organs of Limulus"       |
                |         _Q. J. Micr. Sci._ Vol. 35. 1893 |
                |                                          |
                |"New Facts concerning Bothriolepis"       | 32, 351, 450
                |      _Biological Bulletin._ Vol. 7. 1904 |
                |                                          |
                |"On the Structure and Classification of   | 329
                | the Tremataspidæ"                        |
                |         _Mem. d. l'Acad. Imp. d. Sci. de |
                |          St. Petersbourg._ Vol. 13. 1903 |
                |                                          |
                |"On the Structure of the Pteraspidæ and   |
                | Cephalaspidæ"                            | 415
                | _The American Naturalist._ Vol. 37. 1903 |
                |                                          |
                |"On the Appendages of Tremataspis"        | 351
                | _The American Naturalist._ Vol. 37. 1903 |
                |                                          |
                |"On Structures Resembling Dermal Bones in | 346
                | Limulus"                                 |
                |             _Anat. Anzeig._ Vol. 9. 1894 |
                |                                          |
  PATTEN AND    |"The Development of the Coxal Gland, etc.,| 408
   HAZEN        | of _Limulus Polyphemus_"                 |
                |       _Journ. of Morphol._ Vol. 16. 1900 |
                |                                          |
  PATTEN AND    | Studies on Limulus. II. "The Nervous     | 314, 315, 381,
   REDENBAUGH   | System of _Limulus Polyphemus_"          | 382
                |       _Journ. of Morphol._ Vol. 16. 1900 |
                |                                          |
  PERLIA        | Quoted by Edinger                        | 264
                |                                          |
  PICK          |   "         "                            | 265
                |                                          |
  PLATT         |"A Contribution to the Morphology of the  | 253, 265-267,
                | Vertebrate Head, based on a Study of     | 273, 274, 279,
                | _Acanthias vulgaris_"                    | 284
                |           _Journ. Morphol._ Vol. 5. 1891 |
                |                                          |
                |"Fibres connecting the Central Nervous    | 443
                | System and Chorda in Amphioxus"          |
                |                     _Anat. Anzeig._ 1892 |
                |                                          |
  PRICE         |"Development of the Excretory Organs of   | 394
                | _Bdellostoma Stouti_"                    |
                |          _Zool. Jahrbuch._ Vol. 10. 1897 |
                |                                          |
  RABL          |"Ueber die Metamerie des Wirbel-          | 258, 262
                | thierkopfes"                             |
                | _Verhandl. der Anat. Gesellsch. Versamml.|
                |             in Wien. Anat. Anzeig._ 1892 |
                |                                          |
                |"Die Entwicklung und Structur der         | 424
                | Nebennieren bei den Vögeln"              |
                |     _Arch. f. mikr. Anat._ Vol. 38. 1891 |
                |                                          |
  RAMÓN Y. CAJAL|                                          | 72, 465
                |                                          |
  RATHKE        |"Anatomie des Querders"                   | 161, 169, 304
                |    _Naturforsch. Gesellsch. zu Dantzig._ |
                |                             Vol. 2. 1827 |
                |                                          |
  REDENBAUGH    | _See_ Patten and Redenbaugh.             |
                |                                          |
  REICHENBACH   |"Entwicklungs-geschichte des Flusskrebses"| 98-100, 114
                |           _Abhandl. d. Senckenbergischen |
                |  Naturforsch. Gesellsch._ Vol. 14. 1886. |
                |                                          |
  RETZIUS       |'Biologische Untersuchungen.' Vol. 1.     | 20, 489
                | 1890. "Zur Kenntniss des Nervensystem der|
                | Crustaceen"                              |
                |                                          |
  ROHON         | Die Obersilurischen Fische von Oesel. 1st| 32, 275, 276
                | Theil. "Thyestidæ und Tremataspidæ"      |
                |     _Mem. d. l'Acad. Imp. d. Sci. d. St. |
                |     Petersbourg._ 7th Ser. Vol. 38. 1892 |
                |                                          |
                |"Weitere Mittheilungen über die Gattung   | 327-330,
                | _Thyestes_"                              | 339-341, 382
                |   _Bull. d. l'Acad. d. St. Petersbourg._ |
                |                    5th Ser. Vol. 4. 1896 |
                |                                          |
  ROLPH         |"Untersuchungen über den Bau des          | 444
                | _Amphioxus lanceolatus_"                 |
                |        _Morphol. Jahrbuch._ Vol. 2. 1887 |
                |                                          |
  RÜCKERT, J.   |"Entwicklung der Excretionsorgane"        | 392, 393, 400
                |        _Merkel und Bonnet; Anat. Hefte._ |
                |                            Vol. 1. 1891. |
                |                                          |
                |"Ueber die Entstehung der Excretionsorgane| 403
                | bei Selachiern"                          |
                |              _Archiv. f. Anatomie._ 1888 |
                |                                          |
  ST. HILAIRE   |"Sur la Vertèbre"                         | 11
                |          _La Revue Encyclopédique._ 1822 |
                |                                          |
  SAMASSA       |"Bemerkungen über die Methode der         | 462
                | Vergleichenden Entwicklungsgeschichte"   |
                |      _Biol. Centralblatt._ Vol. 18. 1898 |
                |                                          |
  SCHAFFER      |"Ueber das Knorpelige Skelett von         | 126-135
                | Ammocoetes"                              |
                |  _Zeitsch. f. wiss. Zool._ Vol. 61. 1896 |
                |                                          |
                |"Ueber die Thymusanlage bei _Petromyzon   | 426-428
                | Planeri_"                                |
                |        _Sitzungsber. d. K. Akad d. Wiss. |
                |                 in Wien._ Vol. 103. 1894 |
                |                                          |
  SCHIMKÉWITSCH |"Sur la structure et sur la signification | 143-145, 342
                | de l'Endosternite des Arachnides"        |
                |                     _Zool. Anzeig._ 1893 |
                |                                          |
                |"Anatomie de l'Epeire"                    | 369
                |        _Ann. d. Sci. Nat._ Vol. 17. 1884 |
                |                                          |
  SCHMIDT       |"Die Crustaceen-fauna der Eurypteren-     | 190, 191, 236,
                | schichten von Rootziküll auf Oesel"      | 240, 329, 341
                |            _Mem. d'Acad. Imp. d. Sci. d. |
                |          St. Petersbourg._ Vol. 31. 1883 |
                |                                          |
  SCHMIEDEBERG  |"Ueber die chemische Zusammensetzung des  | 147
                | Knorpels"                                |
                |   _Arch. f. exper. Pathol. und Pharmak._ |
                |                            Vol. 28. 1891 |
                |                                          |
  SCHNEIDER, A. |'Beiträge zur Anatomie und                | 128, 130, 172,
                |Entwicklungsgeschichte der Wirbelthiere'  | 195, 197, 213,
                |                             Berlin. 1879 | 310, 445
                |                                          |
  SCHNEIDER, G. |"Ueber phagocytäre Organe und             | 421
                | Chloragogenzellen der Oligochæta"        |
                |  _Zeitsch. f. wiss. Zool._ Vol. 61. 1896 |
                |                                          |
  SCOTT         |"Notes on the Development of Petromyzon"  | 42, 78, 111,
                |        _Journ. of Morphol._ Vol. 1. 1887 | 112, 406
                |                                          |
  SEDGWICK      |"A Monograph of the Development of        | 397-400
                | _Peripatus capensis_"                    |
                |          _Studies from the Morphological |
                |     Laboratory, Cambridge._ Vol. 4. 1888 |
                |                                          |
                |"Development of the Kidney in its Relation| 390
                | to the Wolffian Body in the Chick"       |
                |         _Q. J. Micr. Sci._ Vol. 20. 1880 |
                |                                          |
                |"Early Development of the Wolffian Duct   | 393, 394, 400
                | and Anterior Wolffian Tubules in the     |
                | Chick; with some Remarks on the          |
                | Vertebrate Excretory System"             |
                |         _Q. J. Micr. Sci._ Vol. 21. 1881 |
                |                                          |
  SEMON         |"Das Excretionssystem der Myxinoiden"     | 400, 419
                | _Festschrift f. Gegenbaur._ Leipzig. 1897|
                |                                          |
  SEMPER        |"Die Stammesverwandschaft der Wirbelthiere| 390, 392
                | und Wirbellosen"                         |
                |         _Arbeit. a. d. Zool. Zoot. Inst. |
                |                  Würzburg._ Vol. 2. 1875 |
                |                                          |
                |"Das Urinogenitalsystem der Plagiostomen  | 390, 392
                | und seine Bedeutung für die übrigen      |
                | Wirbelthiere"                            |
                |                     _Ibid._ Vol. 2. 1875 |
                |                                          |
  SHELDON       |"On the Development of _Peripatus         | 400
                | Nova-Zealandiæ_"                         |
                |          _Studies from the Morphological |
                |     Laboratory, Cambridge._ Vol. 4. 1889 |
                |                                          |
  SHERRINGTON   |"On the Anatomical Constitution of the    | 267
                | Nerves of Muscles"                       |
                |      _Journ. of Physiol._ Vol. 17. 1894. |
                |         _Proc. of Physiol. Soc._ June 23 |
                |                                          |
  SHIPLEY       |                                          | 334
                |                                          |
                |"On some points in the Development of     | 167, 305, 378,
                | _Petromyzon fluviatilis_"                | 401, 405, 406
                |         _Q. J. Micr. Sci._ Vol. 27. 1887 |
                |                                          |
  v. SMIRNOW    |"Ueber die Nervenendigungen in den Nieren | 477
                | der Säugethiere"                         |
                |          _Anat. Anzeiger._ Vol. 19. 1901 |
                |                                          |
  SMITH, ELLIOT |                                          | 17
                |                                          |
  SPANGENBERG   |"Zur Kenntniss von _Branchipus stagnalis_"| 396
                |  _Zeitsch. f. wiss. Zool._ Vol. 25. 1875 |
                |                                          |
  SPENGEL       |'Die Enteropneusten'                      | 494
                |                             Berlin. 1893 |
                |                                          |
  STARR         | Quoted by Edinger                        | 265, 266
                |                                          |
  STUDNIÇKA     |"Sur les organes pariétaux de _Petromyzon | 80, 81, 86
                | Planeri_"                                |
                |           _Sitzungsber. d. K. Gesell. d. |
                |                     Wiss. in Prag._ 1893 |
                |                                          |
                |"Ueber den feineren Bau der               | 81, 86
                | Parietalorgane von _Petromyzon marinus_" |
                |   _Sitzungsber. d. K. böhmischen Gesell. |
                |                     d. Wiss. Prag._ 1899 |
                |                                          |
  TAKAMINE      |"The Isolation of the Active Principle    | 423
                | of the Supra-renal Gland"                |
                |            _Journ. of Physiol._ Vol. 27. |
                |  _Proc. of Physiol. Soc._, Dec. 14, 1901 |
                |                                          |
  TARNANI       |"On the Anatomy of the Thelyphonides"     | 190, 206-208
                |          _Revue des Sciences Naturelles, |
                |                   St. Petersbourg._ 1890 |
                |                                          |
                |"Die genitalen Organe der Thelyphonus"    | 190, 206-208
                |       _Biol. Centralblatt._ Vol. 9. 1889 |
                |                                          |
  TRAQUAIR      |"Report on Fossil Fishes collected by the | 343-345, 350
                | Geological Survey of Scotland in the     |
                | Silurian Rocks of the South of Scotland" |
                |  _Trans. Roy. Soc., Edin._ Vol. 39. 1899 |
                |                                          |
  VIALLANES     |"Contribution à l'histologie du système   | 100
                | nerveux des Invertébrés; la lame         |
                | ganglionnaire de la Langouste"           |
                |                 _Ann. Sci. Nat._ Vol. 13 |
                |                                          |
  VINCENT,      |"The Carotid Gland of Mammalia and its    | 424
   SWALE        | Relation to the Supra-renal Capsule, with|
                | some Remarks upon Internal Secretion and |
                | the Phylogeny of the latter Organ"       |
                |          _Anat. Anzeiger._ Vol. 18. 1900 |
                |                                          |
                |"Contributions to the Comparative Anatomy | 424
                | and Histology of the Supra-renal         |
                | Capsules"                                |
                |        _Trans. Zool. Soc._ Vol. 14. 1897 |
                |                                          |
  VIRCHOW       |"Transformation and Descent"              | 479
                |            _Journ. of Path. and Bacter._ |
                |                             Vol. 1. 1893 |
                |                                          |
  VOGT          |                                          | 258
                |                                          |
  VÖLCKERS      | _See_ Hensen and Völckers.               |
                |                                          |
  WAGNER        | Quoted by Gaubert                        |
                |                                          |
  WEISS         |"Excretory Tubules in _Amphioxus          | 426
                | Lanceolatus_"                            |
                |         _Q. J. Micr. Sci._ Vol. 31. 1890 |
                |                                          |
  WELDON        |"On the Supra-renal Bodies of Vertebrates"| 420, 424, 429
                |         _Q. J. Micr. Sci._ Vol. 25. 1885 |
                |                                          |
                |"Note on the Origin of the Supra-renal    | 424
                | Bodies in Vertebrates"                   |
                |          _Proc. Roy. Soc._ Vol. 37. 1884 |
                |                                          |
  WHEELER       |"Development of the Urino-genital         | 402, 405
                | Organs of the Lamprey"                   |
                |          _Zool. Jahrbuch._ Vol. 13. 1899 |
                |                                          |
  v. WIJHE      |"Ueber die Mesodermsegmente des Rumpfes   | 155-157, 172,
                | und die Entwicklung des Excretionsystems | 173, 188, 234,
                | bei Selachiern"                          | 258, 260, 262,
                |   _Archiv. f. Mikr. Anat._ Vol. 33. 1889 | 263, 266, 273,
                |                                          | 280, 308, 390-
                |                                          | 393, 397, 400,
                |                                          | 406-408, 412
                |                                          |
                |"Beiträge zur Anatomie der Kopfregion des | 410, 426-428
                | _Amphioxus lanceolatus_"                 |
                | _Petrus Camper. Deel._ 1; _Aflevering._ 2|
                |                                          |
  WILLEY        | _See_ Lankester and Willey.              |
                |                                          |
  WOLFF         |"Die Cuticula der Wirbelthierepidermis"   | 302
                |    _Jen. Zeitsch. f. Naturwissenschaft._ |
                |                            Vol. 23. 1889 |
                |                                          |
  WOODWARD, H.  |"A Monograph of the British Fossil        | 235-240, 249,
                | Crustacea, belonging to the order        | 251, 275
                | Merostomata"                             |
                |        _Palæontographical Society._ 1878 |
                |                                          |
  WOODWARD,     |                                          | 339
   SMITH        |                                          |
                |'Catalogue of Fossil Fishes in the British| 29, 326, 327,
                | Museum.' Part II.                        | 344, 349, 351
                |                             London. 1891 |
                |                                          |
  v. ZITTEL     | Handbuch der Palæontologie               | 190




GENERAL INDEX

[_The numbers in dark type refer to illustrations_]


  Acilius larva, eye of, 78, 83
  Acromegaly, 425
  Actinotrocha, 438
  Addison's disease, 423
  Adelopthalmus, 249
  Adrenalin, 423, 491
  Adrenals, 423, 491
  Agnathostomatous fishes, 29, 343
  Alimentary canal, 433
      "       "     Ammocoetes, 168, 405, 445
      "       "     invertebrate, compared to tube of central nervous
                      system of vertebrate, 43, 433
      "       "     innervation of, 447
      "       "     origin of, 444
      "       "     position of vertebrate and invertebrate, 10
      "       "     possibility of formation of new, 58
      "       "     relationship between notochord and, 434
  Ammocoetes, 32, 245
      "       an ancestral type, 35, 309
      "       alimentary canal, 168, 405, 445
      "       auditory organ, 378, 379
      "       brain, 39, 40, 41, 45, 46, 48, 54, 61
      "       branchial appendages, 161, 162, 163, 164
      "          "      basket-work, 126, 128, 296, 331, 335
      "          "      chamber, 161, 168, 162, 163
      "          "      circulation in Limulus and, 174
      "          "      diaphragms, 161, 167
      "          "      lamellæ, 175
      "          "      muscles, 171
      "          "      nerves, 164
      "          "      segments, 178, 312
      "       cartilage, hard, 133, 133, 293, 294, 377
      "           "      muco, 130, 131, 291, 293, 294, 296, 330, 331, 333,
                          334, 335, 338
      "           "      soft, 129, 130, 293, 294, 296, 335
      "       degeneracy, evidence of, 59, 94, 343
      "       development, 228, 458
      "       digestion, 58, 442
      "       epithelial cells of gills, 214
      "       epithelial cells of skin, 347
      "           "      pits, 173, 200
      "       eye, 93
      "        "   muscles, 267
      "        "   median or pineal, 63, 75, 76, 77, 78, 80, 85, 86
      "        "     "         "    left, 78, 79
      "       fat-column, 181, 182
      "        "  in degenerated muco-cartilage, 333, 334
      "       ganglia in embryo, 229, 283
      "       gland-tissue round the brain, 209, 210, 379
      "       head-region, 128, 162, 163, 193, 293, 294, 296, 298, 335
      "       head-shield, 329, 331, 338
      "       liver, 442, 452
      "       lymphatic glandular tissue, 426
      "       Müllerian fibres, 489
      "       muscles, eye, 173, 267
      "         "      lip, lower, 297
      "         "        "   upper, 305
      "         "       respiratory, 171
      "         "       somatic, 332, 336, 409
      "         "       tubular, 173, 298, 309
      "       nerves, cranial, 141
      "         "     facial, 186, 311
      "         "     glossopharyngeal, 186
      "         "     optic, 105
      "         "     trigeminal, 282, 288, 288
      "         "     vagus, 153, 173, 186
      "       nerve-fibres, medullation of, 20
      "       notochord, 182, 435
      "       olfactory tube, 219, 225, 227, 317
      "       oral chamber, 317, 243, 287, 458
      "       parasitism, 60, 286
      "       pituitary, 321
      "       prosomatic region, 243
      "       pronephric duct, 402, 405
      "       relationship to Ostracodermata, 326, 338, 344, 414, 416
      "       retina, 93, 111
      "       skin, 58, 346, 348, 442
      "       skeleton, 125, 126, 132, 291, 296, 335
      "       segments, comparison with segments of Eurypterus, 323
      "           "     facial, 201
      "           "     hyoid, 186, 201
      "           "     prosomatic, 286
      "       septa between myomeres, 416
      "       tentacles of upper lip, 303
      "       test, biological, to show relationship with Limulus, 493
      "       thyroid, 192, 194, 196, 205, 213, 430
      "       transformation, 18, 59, 125, 168, 193, 199, 200, 220, 227,
                228, 287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445
      "       velum, 228, 289, 298, 302
  Amoebocytes, 473
  Amphibia, 23, 345
  Amphioxus, 33, 407
       "     atrial cavity, 409
       "     branchial nephric glands, 426
       "     endostyle, 198, 212
       "     excretory organs, 389, 395, 477
       "     neuropore, 220, 457
       "     notochord, 435, 436, 443
       "     pleural folds, 495
       "     septa between myomeres, 416
       "     somatic muscles, 409
       "     yolk, 485
  Androctonus, 53, 54, 372, 423
  Annelids, lateral sense-organs, 357, 367
     "      nephric organs, 390
     "      rigin of Arthropods from, 395
     "      parapodal ganglia, 283
     "      phagocytic glands, 421
  Anthozoa, 474
  Antiarcha, 29, 326, 343
  Antibody, 492
  Antitoxin, 492
  Anus, 43, 457
  Aponeuroses, 327, 342, 414
  Apparatus, auditory, 355
      "      dioptric, 83
      "      respiratory, 148
      "      suctorial, of Petromyzon, 287
  Appendages, branchial, of Ammocoetes, 161, 162, 163, 164
      "          "          Limulus, 164
      "          "          internal, 149
      "       derivation of suctorial apparatus of Petromyzon from, 290
      "       disappearance of, in transformation of Arthropod into
                Vertebrate, 386, 413
      "       evidence of, in prosomatic region of ancient fishes, 342
      "       muscles, in Limulus and Scorpion, 247
      "       prosomatic, of Gigantostraca, 234
      "       Trilobites, 351
  Apus, 28, 137, 436, 437
  Arachnids, eyes, 75, 87
      "      diverticula of stomach, 109
      "      lyriform organs, 364, 368
      "      segmental excretory organs, 423
  Archæocytes, 473
  Artemia, _v._ Branchipus
  Arthropleura, 249
  Arthropoda, arrangement of organs, 10
       "      evolution, 11
       "      excretory organs, 396, 418
       "      eyes, 75, 89
       "      giant-fibres, 489
       "      musculature, 411
       "      olfactory organs, 220
       "      resemblance to ancient fishes, 29
  Astacus, brain, 54
     "     digestive ferment in cells lining the carapace, 442
     "     optic chiasma, 101
     "     optic stalk, 91
     "     etina, 98
  Asterolepis, 326, 342
  Atrium, 410
  Auchenaspis (Thyestes), 30, 31, 75, 275, 326, 327, 328, 338
  Auditory apparatus, 355
  Auerbach, plexus of, 447
  Aurelia, 475
  Autonomic nerves, 3

  Balanoglossus, 12, 12, 433, 438, 494
  Bdellostoma, 394, 405
  Belinurus, 24, 249, 351
  Bird, rhomboidal sinus, 46
  Bladder, 449
     "     swim, 148
  Blastula, 459, 471, 473
  Blood, 463, 472, 474
    "    circulation, in Ammocoetes and Limulus, 174
    "    secretion of ductless glands into, 418
  Bothriolepis, 29, 32, 239, 326, 351, 450
  Bone, 344, 474, 481
  Brain, Ammocoetes and Arthropod, 54, 61
    "    and brain-case of Ammocoetes, 40, 41, 46, 209
    "    caudal, of Thelyphonus, 450
    "    epithelial lining of, 38
    "    roof, 39
    "    Sphæroma serratum, 62, 90
    "    Thelyphonus, 56
    "    ventricles, 4
    "    vesicles, 48
  Branchial basket-work of Ammocoetes, 126, 128, 296, 331, 335
  Branchipus, 28
      "       brain, 51, 54
      "       eyes, lateral, 88
      "        "       "     retina of, 91, 97
      "        "    median, 75
      "       excretory organs, 396
      "       (Artemia) diverticula of gut and retinal ganglion, 110, 111,
                113
      "       nerves of appendages, 157
      "       segmentation, 159
      "       resemblance to Trilobite, 436
  Bunodes, 24, 30, 249, 341, 351, 414
  Bundle of Meynert, 48, 77
  Bundles, posterior longitudinal, 489
  Buthus, muscles, 270

  Calcification in aponeuroses of Cephalaspis, 414
         "      cartilage, 140,330
         "      successive layers of the skin, 348
  Camerostome, 221, 222, 223, 224, 241, 271
  Canal, alimentary, formation of vertebrate, 58, 433, 446
    "         "      innervation, 447
    "         "      relationships between notochord and, 434
    "         "      origin, 444
    "    Haversian, 329
    "    central, of spinal cord, 405, 439, 455
    "    spinal, 182
  Capsule, auditory, 377, 379
  Cartilage Ammocoetes, muco, 127, 130, 131, 200, 291, 303, 330, 333, 334,
                                344
      "         "       hard, 133, 133, 377
      "         "       soft, 126, 129, 130
      "         "       spinal cartilages, 414
      "     Hypoctonus, 133, 142
      "     Limulus, hard, 142
      "        "     muco, 139
      "        "     soft, 20, 130, 137
      "     origin, 474, 481
      "     staining reactions, 131, 133, 139, 330, 336
  Cavity, atrial, 409, 413
     "    coelomic, 167, 251, 266, 320, 389, 391, 408, 422, 430, 472
  Cells, free-living, 463
  Centre, vaso-motor, 468
  Cephalaspis, diverticula of gut, 109
      "        eyes, lateral, 75, 275
      "         "    median, 75
      "        head-shield, 327, 328, 330, 338
      "        muscles on head-shield, 269
      "        resemblance to Ammocoetes, 145, 291, 326, 329, 338, 348, 414
      "             "         Arthropod, 29
      "        segmentation, 339
  Ceratodus, 148
  Cephalization, 51
  Cephalodiscus, 438
  Cephalopod, 23
  Cerebellum, 47, 50
  Chætopoda, 395
  Chamber, oral, of Ammocoetes, 243, 287, 458
  Cheliceræ, 235
  Chiasma, optic, 101
  Chilaria, 235, 238, 291, 301, 458
  Chitin, 85, 119, 139, 205, 206, 302, 329, 346, 359, 440, 443
  Cilia, 206
  Circulation, branchial, 174
  Cirri, 357
  Clarke's column, 467
  Clepsine, nephridial glands, 423
  Cochlea, 378
  Coelenterata, 465, 472
  Coelolepidæ, 344
  Coelom, 167, 251, 400, 472, 481
  Coelomata, 472
  Coelomocoela, 472, 475
  Coelomostomes, 477, 481
  Colleneytes, 474
  Commissure, anterior, 49
      "       oesophageal, 14
      "       posterior, 48, 280
  Comparison of brains of Ammocoetes and Arthropod, 61
      "           "       invertebrate from Branchipus to Ammocoetes, 54
      "           "       vertebrate, 40
      "         branchial circulation in Ammocoetes and Limulus, 174
      "             "     lamellæ of Scorpion and Ammocoetes, 175
      "             "     segments of Ammocoetes and Petromyzon, 169
      "         Cephalaspidian and Palæostracan fish, 31
      "         Coelom of Peripatus and Vertebrate, 400
      "         dermal covering of Pteraspis with chitin of Limulus or
                  dentine of fish scales, 346
      "         entosternite or plastron of Limulus with trabeculæ of
                  Ammocoetes, 145
      "         excretory organs of vertebrates and invertebrates, 389
      "         gut of Arthropod and tube of central nervous system of
                  Vertebrate, 43, 244, 433, 440, 455, 457
      "         head-shield of Cephalaspis and Ammocoetes, 291, 329, 338
      "         hypophysial tube with olfactory tube of Arthropod ancestor,
                                   229
      "              "       "   with position of palæostoma, 317
      "         mesosomatic region of Ammocoetes and Eurypterus, 192
      "         muscles, branchial, of Ammocoetes and appendage muscles of
                           Scorpion, 171, 447
      "           "      eye, of Vertebrate with dorso-ventral muscles of
                           Scorpion, 267, 272, 459
      "           "      of oral chamber of Ammocoetes and prosomatic
                           musculature of Limulus, 247, 447
      "           "      longitudinal body-muscles of Vertebrate and dorsal
                           longitudinal muscles of Arthropod, 411, 447
      "         nerves, appendage of Limulus and Branchipus to lateral root
                          system of Vertebrate, 157
      "           "     cranial and spinal segmental, 152
      "         nervous systems of Vertebrate and Arthropod, 36
      "         pineal gland of vertebrates and median eyes of Arthropod,
                  63, 456
      "         pituitary body and coxal glands, 246, 319, 321
      "         prosoma and mesosoma of Limulus and Ammocoetes, 140, 141
      "         prosomatic region of Ammocoetes and Eurypterus, 244, 333
      "         retina in Ammocoetes and Musca, 97
      "           "    compound in Arthropod and Vertebrate, 87
      "         skeleton of Limulus and Ammocoetes, 126, 136
      "         sense-organs of Arthropod appendages with auditory organs
                  of Vertebrate, 375
      "         thyroid with endostyle, 198
      "            "     "   uterus of Scorpion, 205
  Corneagen, 69
  Corpora quadrigemina, 47
  Corpuscles, Pacinian, Herbst, Grandry, etc., 470
  Coxal glands, 242, 246, 319, 321, 389, 398, 403, 429
  Cranium, 121, 145, 339
  Crayfish, 442, 489
  Crest, neural, 281
  Cromatophores of frog, 470
  Crura cerebri, 14
  Crustacea, first appearance, 27
       "     eyes, 76, 87
       "     retina, 100
       "     segmental glands, 422
  Ctenophora, 474
  Cyathaspis, 29, 326, 340, 343
  Cyclostomata, 165, 229, 343, 353, 424
  Cysts, 50

  Daphnia, 112
  Degeneration, 17, 19, 59, 74, 78, 94, 107, 212, 309, 333, 336, 343
  Deiters' nucleus, 489
  Dendrites, 72
  Development, parallel, 497
       "       of two types of eye, 73
       "       vertebrate retina, 101
  Diaphragms, 161, 167
  Didymaspis, 327, 338
  Digestion, 441
  Dinosaurs, 17
  Dipnoans, 23, 45, 148
  Diptera, 89, 369
  Diverticula, optic, 102
  Dogfish, skull, 121, 123
  Drepanaspis, 344, 345, 450
  Drepanopterus Bembycoides, 238

  Ectognath, 238, 242, 271, 304, 342, 381
  Eel, 488
  Elasmobranchs, 23, 343, 423
  Elastin, 435
  Embryo, head of dogfish, 121, 123
    "     skull of pig, 121
  Embryology, principles of, 455
  Encepalomeres, 262
  Endognath, 238, 271, 304, 381
  Endostoma, 241, 306
  Endostyle, 198, 212
  Entapophysis of Limulus, 139
  Enterocoela, 472
  Enteropneusta, 438, 494
  Entochondrites, 377
  Entosclerite, 222, 271
  Entosternite, 143
  Epiblast, 444, 445, 459
  Epithelium cells of Ammocoetes, 347
       "     of central nervous system of vertebrates, 38, 457
       "        coelomic spaces in annelids, 421
       "        optic diverticula, 103
       "        peritoneal, pleural, and pericardial cavities, 477
       "        velum of Ammocoetes, 301, 302
  Equilibration, 358
  Eukeraspis, 326
  Eurypterus, 26, 150, 191, 237
      "       appendages, 150, 236, 237
      "       classification, 249
      "       comparison with Ammocoetes, 170, 323
      "       diagram of sagittal median section, 240, 245
      "       endostoma, 241, 306
      "       eyes, 275
      "       mesosomatic segments, 192
      "       muscles of carapace, 269
      "       operculum, 150, 190, 212
  Evidence of alimentary canal, innervation, 446
       "      auditory apparatus and lateral line organs, 355
       "      coelomic cavities in Limulus, 251
       "      degeneracy in Ammocoetes, 59, 94, 343
       "      embryology, cartilage, 20, 129
       "           "      eye-muscles, 263
       "           "      excretory organs, 390
       "           "      heart, 179, 451
       "           "      nervous system, central, cerebral vesicles, 48,
                                                     458
       "           "         "      "       "      epithelial tube, 37, 42,
                                                     102, 244, 433, 455
       "           "         "      "       "      neurenteric canal, 37
       "           "         "      "       "      neuropore, 220, 457
       "           "         "      "       "      optic diverticula, 102
       "           "         "      "       "      spinal cord, 46
       "           "      oral chamber, 228, 242, 243, 290
       "           "      olfactory organ, 220, 227
       "           "      palæostoma or old mouth, 317
       "           "      pineal or median eyes, 15, 63, 74, 456
       "           "      pituitary body and coxal glands, 246, 319
       "           "      thyroid, 192, 194
       "           "      segmentation, double, of head, 157, 234, 258
       "           "      skeleton, cranial, 120, 153
       "      nervous system, central, 8
       "      notochord, origin from segmented region, 443
       "      olfactory apparatus, 218
       "      organs of vision, 68
       "      palæontology, 20, 497
       "      pineal or median eyes, 74
       "      prosomatic musculature, 247
       "      respiratory apparatus, 148
       "      segmentation in head-shield, 339
       "      skeleton, 119
  Evolution, 8, 15, 20, 149, 482, 497
      "      of brain in brain-case, 210
      "         cranium of Vertebrate, 342
      "         excretory organs, 389
      "         eye of Vertebrate, 114
      "         nervous system, central, 34
      "         tissues, 19
      "         Vertebrate from Balanoglossus and Amphioxus, 33
  Eyes, 68
    "   lateral, 87, 105, 108
    "   median or pineal, 74, 77, 78, 79

  Fat-cells in muco-cartilage, 332
  Fat-column of Ammocoetes, 181, 182
  Fibres, Mauthnerian, 488
     "    Müllerian, of Ammocoetes central nervous system, 489
     "        "         retina, 96, 107
  Fishes, classification, 218
     "    ancient, classification, 326, 343
     "       "     cloacal region, 450
     "       "     dominance, 23
     "       "     eyes, 75
     "       "     head-shields. _See_ Head-shields
     "       "     pleural folds, 414
  Fissure, posterior, 43
  Fittest, survival of, 16, 34
  Flabellum, 359, 360, 362, 363, 366
  Folds, pleural, 410, 414
  Function of auditory organ, double, 358
      "       lateral line sense-organs, 357
      "       nerves, 448
      "       thyroid, 212, 215
  Fusion of ganglia, 52

  Galeodes, 230
      "     brain, and camerostome, 222, 223
      "     primordial cranium, 341
      "     racquet-organs, 369, 375
  Ganglia, infraoesophageal, 4, 12, 14, 51, 221
      "    supraoesophageal, 4, 12, 14, 49, 52, 221, 225
      "    origin of, of cranial and spinal nerves, 281
  Ganglion, epibranchial, 164, 282
      "     habenulæ, 48, 78
      "     optic of retina, 72, 89, 97
      "     of posterior root, 466
      "     cells of sympathetic system, 424, 428, 448
  Ganoids, 23, 345
  Gastrula theory, 165, 459
  Genital corpuscles, 470
  Geological record, 20
      "      strata, 22
  Geotria australis, 80
  Germ-band, 482
  Germ-cells, 471
  Giant-fibres, 489
  Gigantostraca, 25, 234
  Gills, 148, 161, 185, 214, 494
  Glabellum, 339
  Glands, carotid, 427
     "    coxal, 242, 246, 319, 321, 425, 429
     "    ductless, 418
     "    generative, of Limulus, 209
     "    internal secretion of, 214
     "    lymphatic, 418
     "    pineal, 15, 63, 75, 456
     "    pituitary, 244, 246, 319, 425
     "    segmental, of Crustacea, 422
     "    submaxillary, 466
     "    sweat, 448
     "    thymus, 425
     "    thyroid, of Ammocoetes, 193, 194, 196, 201, 205, 429
     "    tissue round brain of Ammocoetes, 209, 379
     "    uterine, of Scorpion, 202, 203, 204, 205
  Gnathostomata, 60, 343
  Goblet, 359, 360, 373
  Goitre, 215
  Gonad, 475, 479
  Gonocoele, 475, 481
  Grooves, ciliated, 188, 197, 212
     "     hyper-pharyngeal of Amphioxus, 410
     "     ventral, of apus and trilobites, 436
  Gymnophiona, 393

  Hæmocytes, 472
  Head of embryo dogfish, 121, 123
  Head-shield, dorsal, of Ammocoetes, 330, 331, 338
        "         "       Auchenaspis, 29, 31, 338
        "         "       Cephalaspis, 327, 328, 330, 338, 348
        "         "       Cyathaspis, 340
        "         "       Didymaspis, 338
        "         "       evidence of segmentation, 339
        "         "       Keraspis, 328
        "         "       Ostreostraci, 327, 348
        "         "       Palæostracan, 348
        "         "       Pteraspis, 29
        "         "       Thyestes, 29, 31, 327, 332, 338, 340, 341, 348
        "      ventral, Scaphaspis, 349
  Heart, nerves, 2, 447
     "   origin of vertebrate, 179, 451, 459
     "   relative position in vertebrate and invertebrate, 175
     "   veins forming vertebrate, 180
  Hemiaspis, 24, 25, 249, 250, 351, 414
  Hemispheres, cerebral, 47
  Hepatopancreas of Ammocoetes, 452
         "          Limulus, 211
  Heterostraci, 29, 275, 326, 343
  Hirudinea, 478
  Histolysis in transformation of the lamprey, 59
  Homology of branchial region of vertebrate and invertebrate, 149
       "      ductless glands and nephridial organs, 418
       "      external genital ducts of arthropods and nephridia of
                annelids, 429
       "      germinal layers in all Metazoa, 459
       "      pituitary body of Ammocoetes and coxal glands of Limulus, 319
       "      tubular muscles of Ammocoetes and veno-pericardial muscles of
                Limulus, 309
       "      ventral aorta of vertebrate and longitudinal venous sinuses
                of Limulus, 178
  Hydra, 441, 465, 472, 476
  Hydrophilus larva, eye, 84
  Hyoid segment in Ammocoetes, 186, 267
  Hypoblast, 434, 438, 444, 445, 459
  Hypoctonus, cartilage cells in entosternite, 133
      "       operculum, 189, 207
  Hypogastric plexus, 3
  Hypogeophis, 393
  Hypophysis, 229, 244, 317, 318, 340

  Infundibulum, position, 122,132
       "        tube, the ancestral oesophagus, 4, 37, 244, 318
       "          "   relation to neural canal, 14, 36, 318, 440, 457
       "          "         "     notochord, 318, 435,440
       "          "         "     olfactory tube, 220, 228, 318, 340
  Insects, chordotonal organs, 364, 370
  Invertebrate, heart, 175, 179
        "       excretory organs, 418
        "       nervous system, 13, 54
        "       segmental nerves, 152

  Keraspis, 75, 328, 338
  Kidney, 420, 459, 476
     "    nerves, 477
  King-crab, _v._ Limulus

  Labyrinthodont, 21, 28
  Lamina terminalis, 49
  Lamprey, _v._ Ammocoetes and Petromyzon
  Larva, _v._ Transformation of the Lamprey
  Lateral line system, 261, 355, 411, 470
  Law of Progress, 19
    "    Recapitulation, 434, 456, 498
  Layer, germinal, 459
     "   laminated, 347, 348
  Leech, 421
  Lens, formation, 83, 115
  Lepidosiren, 148, 461, 466
  Limulus or king-crab, 25, 140, 236, 240
      "   appendages, branchial, 138, 164, 175
      "   appendages, prosomatic, 381
      "   brain, 54
      "   circulation, 174, 176
      "   classification, 26, 249
      "   coelomic cavities, 252, 328
      "   coxal glands, 321, 389, 397, 403, 429
      "   eyes, median, 62, 74, 81
      "   entosternite or plastron, 142, 143
      "   flabellum, 360, 362, 363, 380, 381
      "   generative organs and ducts, 189, 202, 208, 209, 380
      "   heart, 180
      "   musculature, branchial, 170
      "        "       prosomatic, 247
      "        "       veno-pericardial, 177, 297, 309, 313
      "   nerves, appendage, 140, 157
      "      "    cardiac, 314
      "      "    segmental, tripartite division of, 157, 235, 267, 355
      "   segments, branchial, 152
      "       "     first mesosomatic, 188
      "       "     prosomatic, 233
      "   operculum, 189, 202, 235, 295
      "   sense-organs, poriferous, of appendages, 359
  Lip, lower, of Ammocoetes, 246, 289, 297, 458
    "  upper,      "         228, 243, 303, 336
  Liver, Ammocoetes, 452
     "   Limulus, 209, 211
  Lizard, pineal eye, 80
     "    suprarenals, 424
     "    tail, 50
  Lobes, optic, 101
  Lobster, 489
  Lungs, 148
  Lung-books of scorpions, 150
  Lymph, 474
  Lymph-corpuscles, 463, 490
  Lymphocytes, 472

  Malapterurus, 470
  Mammal, dominance of, 21
  Man, dominance of, 17
  Marsipobranchs, 23, 35
  Medullation of nerve-fibres, 20, 267, 467, 477
  Membranes, basement, 436
  Meroblastic egg, 485
  Merostomata, 25, 249, 321
  Mesencepalon, 48
  Mesoblast, 444, 455, 459
  Mesogloea, 474
  Mesonephros, 389, 400, 424, 429
  Mesosoma, 52
  Mesothelium, 472, 477
  Metanephros, 389
  Metasoma, 52, 387, 411
  Metastoma, 239, 246, 272, 289, 342, 458
  Metazoa, 444, 459, 471, 472
  Meynert's bundle, 48, 77
  Mollusca, dominance of, 23
  Mouth, old, or palæostoma, 14, 317, 322, 440, 458
    "    vertebrate, 317
  Muco-cartilage, _v._ Cartilage
  Muscles, antagonistic, 447
     "     branchial, 170
     "     connection of, with central nervous system, 464
     "     eye, and their nerves, 263
     "     prosomatic, 243, 247
     "     phylogeny of origin of skeletal, 478
     "     rudimentary, in Ammocoetes, 289
     "     somatic trunk, origin of, 406
     "     striated, 20, 155
     "     tubular, of Ammocoetes, 309
     "     unstriped, 20, 447, 491
     "     visceral and parietal, 155, 172
     "     veno-pericardial of Limulus and Scorpion, 177, 297, 309
  Muscle-spindles, 267
  Mygalidæ, stomach, 109
      "     segmentation, 249, 306
  Myomeres, 262, 337, 414, 479
  Myotomes, 332, 337, 338, 391, 407, 408
  Mysis, eyes, 100
    "    ductless glands, 422
  Myxine, 220, 392, 402, 419

  Nebalia, 144, 422
  Nemertina, 475
  Nephridia, 395, 421, 429
  Nephrocoele, 430
  Nephrotome, 393
  Nerves, abducens, 155, 263, 266
     "    auditory, 356, 376
     "    autonomic, 3
     "    facial, 155, 156, 186, 188, 192, 311, 356, 378
     "      "     ramus branchialis profundus, 311
     "    to flabellum, in Limulus, 361, 375
     "    glossopharyngeal, 155, 156, 186, 356
     "    hypoglossal, 156
     "    inhibitory, 447
     "    inedullation of, 20, 267, 467, 477
     "    occulomotor, 155, 234, 263, 274
     "    olfactory, 229
     "    optic, 101, 104
     "      "    of pineal eye, 79
     "    origin of ganglia of cranial and spinal, 281
     "    to pecten of Scorpion, 375, 376
     "    preganglionic, 2
     "    of prosoma in Limulus, 235, 355
     "    regeneration of, 469
     "    roots, of Limulus, 157
     "    sacral, 448
     "    segmental, 152, 156
     "    segmental nature of cranial, 259, 411
     "    spinal, absence of lateral roots in, 388
     "    spinal accessory, 154
     "    trigeminal, 151, 155, 156, 234, 243, 257, 279
     "         "      motor nucleus of, 280
     "         "      of Ammocoetes, 288
     "    tripartite arrangement of cranial nerves, 154, 157, 235, 267, 355
     "    trochlear, 48, 155, 234, 263, 276
     "    vagus, 151, 154, 156, 173, 186, 356, 447, 449
  Nervous system, central, comparison of Vertebrate and Arthropod, 36, 457
          "         "      connection of, with muscular and epithelial
                                          tissues, 464
          "         "           "         with retina, 71
          "         "      disease of, 50
          "         "      evidence of, 8
          "         "      evolution of, 34
          "         "      importance of, 16, 463, 482, 498
          "         "      invertebrate, 10, 13, 54
          "         "      origin of, 480
          "         "      relation of germ-band to, 483
          "         "      segmentation of vertebrate, 51
          "         "      tube of, 36-51, 102, 211, 433, 455, 457
          "         "      vertebrate, 10, 13, 40, 41, 152
          "       enteric, 447
          "       sympathetic, 2, 424, 428, 448, 491
  Neurenteric canal, 37
  Neuroblast, 465
  Neuromeres, 55, 247, 262, 312, 316
  Neurones, 72, 92, 465
  Neuropil, 71, 91
  Neuropore, 220, 457
  Nose, 219
    "   of Osteostraci, 329, 352, 458
  Notochord, 120, 122, 180, 181, 220, 244, 295, 318, 405, 417, 433, 436,
      494

  Ocelli, 70
  Oesophagus of Ammocoetes, 405
       "        Arthropod, compared to tube of infundibulum, 4, 244, 440
  Olfactory apparatus, evidence of the, 218
      "     organs of the Scorpion group, 220
      "     tube of Ammocoetes, 219, 225, 244, 317
  Oligochæta, 421, 478
  Operculum of Eurypterus, 191, 212, 291
       "       Limulus, 189, 202, 235, 295
       "       Phrynus, 191
       "       Scorpion, 189, 206, 212, 372
       "       Thelyphonus, 189, 190, 206
  Organs, arrangement of, 10
     "    auditory, of arachnids and Insects, 368
     "    branchial, innervation of vertebrate, 151
     "        "      sense-organs of embryo vertebrate, 261, 281
     "    chordotonal, of insects, 364, 369, 370
     "    electric, 470
     "    generative, of Limulus, 208, 209
     "         "      connection between Thyroid gland and, 215
     "    genital, of sea-scorpions, 206
     "    lateral line, 355, 411
     "    lyriform, of arachnids, 364, 369
     "    olfactory, of Scorpion group, 220
     "    phagocytic, 420
     "    racquet, of Galeodes, 369, 375
     "    segmental excretory, 389, 391, 408, 418, 459, 477
     "    sense, of appendages of Limulus, 358
     "    vestigial, 456
     "    of vision, evidence of, 68
     "    vital, 57
  Origin of alimentary canal, 444
     "      arthropods from annelids, 395
     "      atrial cavity, 409
     "      auditory capsules and parachordals, 377
     "      coelom, 475, 481
     "      ductless glands, 428
     "      free cells, 472
     "      heart of vertebrate, 179
     "      lateral line organs, 356
     "      muscles, 478
     "      musculature, branchial, 170
     "            "      somatic trunk, 406
     "      nervous system, central, 480
     "      notochord, 434
     "      segmental excretory organs, 389
     "      skeleton of vertebrates, 119
     "      vertebrates, 9, 36, 351, 433, 493
  Ostracodermata, 326, 343
  Osteostraci, 29, 75, 275, 326, 343
  Otoliths, 378
  Ovum, 473

  Pacinian bodies, 470, 477
  Palæmon, 20, 422
  Palæontology, evidence of, 20, 497
  Palæostoma, 317
  Palæostraca, 27, 396
       "       median eyes, 74
       "       mesosomatic appendages, 188
       "       olfactory organs, 221
       "       segments, compared to Ammocoetes, 308
  Pantopoda, glands, 423
  Parachordals, 121, 132, 377
  Parapodia, 357
  Parapodopsis, foot glands, 422
  Parathymus, 427
  Parathyroids, 427
  Parietal organ, 76
  Pecten of scorpion, 114, 359, 366, 371, 372, 373, 374
  Pedipalpi, 190
  Periblast, 471
  Peripatus, 396, 399, 400, 411, 421, 429
  Petromyzon, alimentary canal, 405, 445
       "      auditory organ, 378
       "      branchial segments, 169
       "      life-history, 59
       "      olfactory tube, 219, 226
       "      pronephric duct, 402
       "      retina and optic nerve, 95
       "      skeleton, 125
       "      suctorial apparatus, 287, 304
       "      transformation, _v._ Transformation of the Lamprey
  Phagocytes, 420, 471
  Pharynx of Amphioxus, 410
      "      Vertebrate, 440
  Phoronis, 439
  Phrynus, brain, 53
      "    caudal brain, 450
      "    carapace and carapace removed, 250
      "    coecal diverticula, 109
      "    evidence of segmentation of carapace, 249, 250, 341
      "    operculum, 191
      "    prosomatic appendages, 306
      "    crossing of dorso-ventral muscles, 271, 277
      "    stridulating apparatus, 368
  Phyllodoce, 395
  Phyllopoda, 321
  Pigment, in Ammocoetes, in position of atrial cavity, 412
      "       epithelial lining of central nervous system, 43, 457
      "       choroid of vertebrate eye, 104, 107
      "       between glandular cells round brain of Ammocoetes, 211, 379
      "       tapetal layer of retina, 70
      "       white, of right pineal eye of Lamprey, 76, 80
  Pineal body, 14, 15
     "   eyes, 74, 233, 244
     "     "   of Ammocoetes, 80, 78, 85
     "   gland, 63, 75, 456
  Pits, epithelial, of diaphragms in Ammocoetes, 164
    "        "         skin in Ammocoetes, 173, 200
  Pituitary body, 244, 246, 319, 321, 425, 430
  Plasma-cells, 471
  Plakodes, 283
  Planarians, 475
  Plastron, formation of cranial walls from the, 86, 322, 341
      "     of Limulus, 136, 142, 143
      "     Palæostracan, compared to trabeculæ of Ammocoetes, 145, 377
      "     muscles attached to the, 270
      "     of Thelyphonus, 143
  Platyhelmia, 475
  Pleuron, 410, 415
  Plexus, of Auerbach, 447
      "      choroid, 38, 45, 49, 103
      "      hypogastric, 3
  Polychæta, 357, 395
  Pores, abdominal, 430
  Porifera, 473
  Pouch, formation of gill, 165, 166
  Prestwichia, 24, 25, 249, 351
  Principle of concentration and cephalization, 51
       "       embryology, 455
  Pristiurus, 424
  Progress, law of, 19
       "    result of, 56
  Pronephros, 389, 397, 419, 424, 449
  Prosencephalon, 48
  Prosoma, 52
  Protopterus, 148
  Protostraca, 27, 396, 417
       "       dominance of, 28
  Protozoa, 166, 479
  Pseudoniscus, 25, 249
  Pteraspis, 29, 30, 275, 326, 343, 344, 350
  Pterichthys, 29, 31, 239, 326, 351
  Pterygoid, pedicle of, 295
  Pterygotus, 25, 27, 56, 170, 191, 221, 235, 238, 249, 276
  Ptychodera, 494, 495

  Ramus branchialis profundus of facial nerve, 311
    "   communicans, 2, 3
  Raphe, 46
  Recapitulation, law of, 434, 456, 498
  Regeneration of nerves, 469
  Reptiles, dominance of, 21
  Retina, compound, 71
     "    development of, 101
     "    inversion of, in Vertebrates, 114
     "    inverted, 70
     "    layers of compound, 73
     "       "      in Crustacean eye, 100
     "    of lateral eye of Ammocoetes, 93, 95, 111
     "    Musca, 89
     "    Pecten and Spondylus, 114
     "    upright compound, 72
     "        "   simple, 69
  Rhabdites, 69, 81

  Saccus vasculosus, 244, 322
  Scales, 345
  Scaphaspis, 349
  Schwann, sheath of, 469
  Sclerotomes, 388
  Scorpion, brain, 54
      "     branchial lamellæ, 175
      "     development, 482
      "     entochondrites, 377
      "     excretory organs, 397
      "     eyes, 75
      "     lung-books, 150, 170
      "     lymphatic glands, 423
      "     muscles, oblique, 278
      "        "     recti, 271
      "        "     respiration, 171
      "        "     veno-pericardial, 177
      "     muscular system, 247, 268, 269
      "     nerves to Cheliceræ, 237
      "     olfactory organs, 220
      "     operculum of male, 189, 206, 212
      "     pecten, 359, 366, 371, 373, 374, 377
      "     under surface, 372
      "     uterus, 189, 202, 203, 204, 205, 212
  Sea-scorpions, 25, 26, 27, 56, 150, 170, 191, 208, 221, 232, 235, 241,
      349, 359
  Segmentation, branchiomeric, 124
        "       body-muscles in vertebrate, 388
        "       eye-muscles, 248
        "       of head, double, 155, 157, 173, 234, 258, 411, 459
        "       of head-shield, 339
        "       history of cranial, 258
  Segments, branchial of Ammocoetes, 161, 178, 186
      "     hyoid, in Ammocoetes, double, 186, 201, 267, 300
      "     innervation of branchial, 151
      "     first mesosomatic, in Limulus and its allies, 188
      "     mesosomatic, of Eurypterus, 192
      "     prosomatic of Limulus and its allies, 233, 249
      "          "        Ammocoetes, 286
      "     of spinal region of Vertebrates, 388
      "     of trigeminal nerve-group, 257, 279
      "     tubular muscles of hyoid, 299
  Sense-organs of Amphioxus, 34
         "        branchial, of Limulus, 359, 360
         "        lateral, of Annelids, 357, 367
         "        lateral-line system, 356, 411, 470
  Serum, 492
  Significance of the optic diverticula, 102
  Silurus, 488
  Sinus, longitudinal venous, of Limulus, 176, 312, 451
     "   rhomboidal of bird, 46
  Skeleton, Ammocoetes, 126, 296, 335
      "        "        branchial, 126, 126
      "        "        basi-cranial, 132
      "        "        muco-cartilaginous, 291, 296, 330, 331
      "     aponeurotic, 414
      "     Cephalaspis, 414, 415
      "     evidence of the, 119
      "     Limulus, cartilaginous, 126, 136
      "        "     mesosomatic, 137
      "        "     prosomatic, 142
      "     Petromyzon, 125
      "     Vertebrate, commencement of bony, 120, 121
  Skin, digestive power of cells of, in Ammocoetes, 58, 442
    "   of Ammocoetes, 346
    "   nerves of, 448
  Skull of dogfish, 123
     "     pig-embryo, 121
  Slimonia, 27, 56, 170, 235, 238, 249, 276, 303
  Solenocytes, 395, 477
  Solpugidæ, 109
  Sphæroma serratum, brain, 62, 90, 101, 225
  Spiders, eyes, 75
     "     stomach, 109
  Spina bifida, 50
  Spinal cord, difference between brain and, 45
     "     "   region of, 385
     "     "   termination in bird-embryo, 51
  Spondylus, retina of, 114
  Squilla, eyes, 100
     "     glands, 422
  Stomach, cephalic, 4, 43, 102, 244
  Stylonurus Lagani, 27, 235, 239, 249
  Substantia gelatinosa Rolandi, 44
  Suprarenal body, 423
  Surfaces, dorsal and ventral, 11
      "     reversal of, 15, 29, 36, 87, 175, 352, 433, 484
  Synapse, 72
  Syncytium, 464, 471, 479

  Tail of lizards, 50
  Tapetum, 69
  Teleosteans, 23, 345, 420, 424
  Tendon-organs, 470
  Tentacles of Ammocoetes, 246, 289, 303
  Tergo-coxal muscles, 247
  Test, biological, of relationship of animals, 492
  Thalainencephalon, 48
  Thelodus, 344
  Thelyphonus, 231
       "       brain, 53, 54, 56, 224
       "         "    caudal, 450
       "       coecal diverticula, 109
       "       entosternite, 143
       "       genital organs, 206
       "       lyriform organs, 368
       "       olfactory passage, 226, 306
       "       operculum, 189, 190, 206, 207
  Theory, gastræa, 444, 461
  Theories of the origin of vertebrates, 9, 411, 433, 457
  Thionin reaction, 131, 139, 213, 330, 336
  Throat, formation of, 179
  Thyestes, 30, 31, 275, 326, 328, 329, 339, 340, 341
  Thymus, 425, 430
  Thyroid gland of Ammocoetes, 61, 127, 192, 194, 196, 429, 459
      "     "   evidence of the, 185
      "     "   function of, in Ammocoetes, 213
  Tissues, connective, 471, 474, 481
      "    evolution of, 19
      "    notochordal, 435
      "    two groups of, 463
  Tongue of Ammocoetes, 246, 303
  Tonsils, 427, 430
  Torpedo, 262, 392, 470
  Trabeculæ, 121, 132, 133, 145, 277, 295, 377
  Transformation of the Lamprey, 18, 35, 59, 61, 125, 168, 193, 199, 200,
      220, 227, 228, 287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445
  Tremataspis, 32, 75, 275, 326, 351, 352
  Trilobites, 24, 25, 26, 437
       "      appendages, 351, 437
       "      diagram of section through a trilobite-like animal, 413
       "      dominance of, 26
       "      excretory organs, 396
       "      eyes, 74, 88
       "      glabellum, 339
       "      relations of, 249, 283
       "      respiratory apparatus, 170
       "      ventral surface, 437
  Tube of central nervous system, 37, 38, 42, 102, 211, 433, 455, 457
    "  from IVth ventricle to surface of brain in Ammocoetes, 209
    "  Fallopian, 431
    "  hypophysial, 229, 244, 317, 440
    "  meeting of four tubes in vertebrate, 318, 440
    "  notochord originally a, 436, 440
    "  olfactory, of Ammocoetes, 219, 225, 317, 440
    "  unsegmented, in segmented animal, 439
  Tunicata, 16
      "     budding of, 441
      "     degeneration, 12, 17, 19, 60
      "     endostyle, 198, 212
      "     hypophysis, 425
      "     notochord, 438
      "     position of, 494

  Unit, appendage, in non-branchial segments, 185
    "   branchial, 161, 165, 168, 185
  Ureters, nerves of, 448
  Uterus of Scorpion group, 189, 202, 203, 204, 205, 214
     "   vertebrate, nerves of, 448

  Valve, ileo-colic, 449
     "   of Vieussens, 48
  Variation in dominant races, 21, 88
      "     meristic, in spinal nerves, 154, 387
  Veins, forming vertebrate heart, 180
  Velum, 228, 289, 298, 302
  Vertebrates, alimentary canal, innervation of, 446
       "       atrial cavity, 410
       "       auditory apparatus and lateral-line system, 356
       "       body-cavity, 401, 430
       "       brains, 40
       "       branchial organs, 151
       "       coelomic cavities in head region, 251, 266
       "       cranium, evolution of, 342
       "       egg of, 483
       "       evolution of, 11
       "       excretory organs, 389, 391, 408
       "       glands, ductless, 418
       "          "    internal secretion of, 215
       "       heart, 175, 179, 180
       "       muscles, evidence of segmentation of eye, 248
       "          "     oblique, 278
       "          "     origin of somatic trunk, 406
       "       nervous system, central, 13
       "       nerves, segmental, 152
       "       notochord and gut, 434
       "       organs of, 10
       "       origin of, 9, 411, 433, 457
       "       segments, prosomatic, 257
       "       skeleton, commencement of bony, 120, 458
       "       spinal cord and medulla oblongata, 44
       "       spinal region, 385
       "       thyroid, connection between generative organs and, 215
       "       tubes, meeting of four, 318, 440
  Vesicles, cerebral, formation of, 48, 458
  Vitellophags, 471, 483
  Volvox, 479

  Wolffian body, 390

  Xiphosura, 24, 26, 249

  Yolk, 482


THE END



Notes.

[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.

[2] "The Origin of Vertebrates, deduced from the Study of Ammocoetes." Part
    X., "The Origin of the Auditory Organ: the Meaning of the VIIIth
    Cranial Nerve." _Journ. Anat. and Physiol._, vol. 36, 1902.

       *       *       *       *       *



Corrections made to printed text

Fig. 6: 'Dalmanites' corrected from 'Dalmatites' (which is an ammonite).

Fig. 15 caption: 'Pterichthys' corrected from 'Ptericthys'. So also on P.
239 and twice on P. 324.

P. 60: 'gnathostomatous condition' corrected from 'gnathostomotous ...'.

P. 409: 'well known' corrected from 'well know'.

P. 420: 'meso-nephros' corrected from 'neso-nephros'.

P. 432: 'had become a vertebrate' corrected from 'had became ...'.

Fig. 167 caption: 'nephrocoele' corrected from 'nephrocele'.

P. 474: 'Scyphomedusæ' corrected from 'Scyphomedusoe'.

P. 497: 'idiosyncrasy' corrected from 'idiosyncracy'.

Bibliography, Dietl: 'Gehirns' corrected from 'Gehirus'.

Bibliography, Goodrich: 'Polychæta' corrected from 'Polychoeta'.

Bibliography, Graber: 'Chordo-tonalen' corrected from 'Chordo-tonalem'.

Bibliography, Vincent: 'Phylogeny' corrected from 'Phyogeny'.

Index, Homology: 'Metazoa' corrected from 'Metozoa'.