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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