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 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 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 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