CHAPTER IX 



THE PROSOMATIC SEGMENTS OF AMMOCCETES 



The prosomatic region in Ammocoetes. — The suctorial apparatus of the adult 

 Petroniyzon. — Its origin in Ammocoetes. — Its derivation from appendages. 

 — The segment of the lower lip or 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 palreostoma. — The 

 pituitary gland. — Its comparison with the coxal gland of Limulus. — 

 Summary. 



In the last chapter it was seen not to be incompatible with both the 

 anatomical and morphological evidence to look upon the trigeminal 

 nerves as having originally supplied the seven prosomatic pairs of 

 appendages of the invertebrate ancestor, the foremost of which, the 

 chelicerse, and the four pairs of endognaths dwindled away and became 

 insignificant, leaving as trace of their former presence the descending 

 root of the Vth nerve ; while the two hindmost pairs, the ectognaths 

 and the chilaria, or metastoma, remained vigorous and developed, 

 leaving as proof of their presence the nucleus masticatorius. Evi- 

 dence in favour of this suggestion and of the nature of the dwindling 

 process is afforded when we examine what the trigeminus does supply 

 in Ammocoetes. In all vertebrates this nerve supplies the great 

 muscles of mastication which, in all gnathostomatous fishes, move 

 the jaws. The lowest fishes, the cyclostomes, possess no jaws ; they 

 take in their food by attaching themselves to their prey and by 

 means of rasping teeth situated in serried rows within the circular 

 mouth, combined with a powerful suctorial apparatus, they suck the 

 juices of the fish they feed upon. Not possessing jaws, they feed 

 by suction on the living animal, a method of feeding which gives 

 them no more claim to be classed as parasitic animals than the 

 whole group of spiders which feed in a similar manner on living 

 flies. 



