THE NOTOCHORD AND ALIMENTARY CANAL 437 



animal ; on each side of this groove in Apus are closely set the 

 gnatho-bases of the appendages, in such a manner that the groove 

 can be easily converted into a canal by the movements of these bases 

 — a canal which, owing to the great number of the appendages and 

 their closeness to each other, can be completely and efficiently closed. 

 All those who have seen Apus in the living state assert that this 

 canal so formed is actually used by the animal for feeding purposes. 

 By the movements of the gnatho-bases food is passed up from the 



Fig. 164. — Undee-Subface op Apus. 

 (After Beonn.) 



Fig. 165. — Undeb - Sueface of a 

 Teilobite (Triarthrus). (From 

 Beeches.) 



hind end of the animal along the whole length of this ventral canal 

 to the mouth, where it is taken in and swallowed. In this way Apus 

 has been seen to swallow its own eggs. 



In the trilobites there is a similar deep channel formed by the 

 mid-ventral surface, similar gnatho-bases, and closely set appendages, 

 and the membrane of this ventral groove was extremely thin. 



Here, then, in the very group of animals which were the pro- 

 genitors of the presumed palaeostracan ancestor of the vertebrate — a 

 group which is characterized by its extensive prevalence and its 



