The Origin of Vertebrates

The Origin of Vertebrates

Walter Holbrook Gaskell
Walter Holbrook Gaskell

Author: Gaskell, Walter Holbrook, 1847-1914
The Origin of Vertebrates

Transcriber’s note: A few typographical errors have been corrected. They appear in the text like this, and the explanation will appear when the mouse pointer is moved over the marked passage.










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Introduction 1
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
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
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


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
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
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
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


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
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
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
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


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
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
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


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

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.




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 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 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 th

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