ON THE DIPLOCHORDA. 488 
ingestion, i.e. the collection of food-particles towards the 
mouth. The complication of the band and its attendant 
increase in size of the larva, is probably traceable to a pro- 
gress towards more efficient ingestion of food by an increase 
of ingestive surface. ‘I'he same distinction into motor and 
trophic bands can be made out, as has been indicated, in 
echinoderm larve.t The action of the circum-oral band in 
collection and transmission of food is complex, and several 
points remain to be determined, but in the immediate neigh- 
bourhood of the mouth the food-particles can be watched 
with ease as they are waved towards the mouth opening and 
pass rapidly into the alimentary canal. 
The mouth opens into the circum-oral area at a point on 
the median ventral line. A modified portion of the area in 
the immediate neighbourhood of the mouth may be distin- 
guished as the vestibule, or buccal cavity. Fig. 11 is a 
view of the living larva, showing the anterior and posterior 
parts of the circum-oral band bending forwards in a loop, at 
the apex of which the mouth is seen. 
he limits of the vestibule are clearly distinguishable. It 
forms a wide chamber, open laterally to the remainder of the 
circum-oral area. Its floor, or ventral wall, has a patch of 
cilia: forming a more or less complete connection between the 
circum-oral band and the floor of the pharynx. This patch 
(or buccal pad) (Fig. 12, 6.p.) has already been noticed in 
American Tornariae by Morgan (loc. cit.) Apparently it 
tends to disappear or to become diminished in size in the 
fully-grown larve. It is probably the action of the buccal 
cilia which is chiefly instrumental in bringing the food 
particles into the mouth. In the Bahama larva of Morgan 
the mouth does not, in the later stages, open directly to the 
exterior, but into a long ectodermal tube, up which the 
ciliated bands can be traced. No such tube appears to be 
developed in the Heligoland larva. The first portion of the 
alimentary canal is usually known as the esophagus. It 
seems desirable to distinguish this part as the “ pharynx,” 
1 «Quart. Journ. Mier. Sci.,’ vol. xl. 
