CHAPTER IX 
THE PROSOMATIC SEGMENTS OF AMMOCGETES 
The prosomatic region in Ammocotes.—The suctorial apparatus of the adult 
Petromyzon.—Its origin in Ammoccetes.—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 paleostoma.— 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 
chelicere, 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 Ammoccetes. 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. 
