CONTENTS vii 



CHAPTER VIII 



The Segments belonging to the Trigeminal Nerve-Group 



PAGE 



The prosoruatic 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 op Ammoccetes 



The prosomatic region in Ammoccetes — The suctorial apparatus of the adult 

 Petromyzon — Its origin in Ammoccetes — 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-peri- 

 cardial muscles in Limulus — The old mouth or palseostoma — The pituitary 

 gland — Its comparison with the coxal gland of Limulus — Summary . . 286 



CHAPTER X 



The Relationship of Ammocostes to the most Ancient Fishes 



— the Ostracodermata 



The nose of the Osteostraci — Comparison of head-shield of Ammoccetes and of 

 Cephalaspis — Ammoccetes 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 

 VHth nerve passes in and out of capsule — Evidence of Ammoccetes — 

 Intrusion of glandular mass round brain into auditory capsule — Intrusion 

 of generative and hepatic mass round brain into base of flabellum — 

 Summary 355 



