The Origin of Vertebrates

CHAPTER I. The Evidence of the Central Nervous System

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

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 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

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 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

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

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 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 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

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

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

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 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

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?

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