Origin of Vertebrates

by Walter Holbrook Gaskell

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 Ammocœtes 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 Ammocœtes 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 Ammocœtes 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 Ammocœtes; its topographical arrangement, its structure, its origin in muco-cartilage—The prosomatic skeleton of Ammocœtes; the trabeculæ and parachordals, their structure, their origin in white fibrous tissue—The mesosomatic skeleton of Limulus compared with that of Ammocœtes; similarity of position, of structure, of origin in muco-cartilage—The prosomatic skeleton of Limulus; the entosternite, or plastron, compared with the trabeculæ of Ammocœtes; 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 Ammocœtes—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 Ammocœtes—Its structure—Its openings—The nature of the thyroid segment—The uterus of the scorpion—Its glands—Comparison with the thyroid gland of Ammocœtes—Cephalic generative glands of Limulus—Interpretation of glandular tissue filling up the brain-case of Ammocœtes—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 Ammocœtes—Origin of the nasal tube of Ammocœtes 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 Ammocœtes—Prosomatic segmentation shown by marks on carapace—Evidence of cœlomic 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 Ammocœtes
The prosomatic region in Ammocœtes—The suctorial apparatus of the adult Petromyzon—Its origin in Ammocœtes—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 Ammocœtes to the most Ancient Fishes—the Ostracodermata
Cephalaspis—Ammocœtes 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 Ammocœtes—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 cœlom—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 cœlom of Peripatus and of vertebrate—Pronephric organs compared to coxal glands—Origin of vertebrate body-cavity (metacœle)—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 Ammocœtes—Formation of new gut in Ammocœtes 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 cœlom—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.