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 
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Fic. 164,— UNDER-SURFACE OF APUS. Fic. 165.—UNDER-SURFACE OF A 
(After Bronn.) TRILOBITE (Triarthrus), (From 
BEECHER.) 
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 paleostracan ancestor of the vertebrate—a 
group which is characterized by its extensive prevalence and its 
