HOW ANIMALS EAT 257 



The mouth of the starfish and sea urchin is a simple 

 round aperture, followed by a very short throat. In the 

 starfish, it is inclosed by a ring of hard spines and a 

 membrane. In the sea urchin it is surrounded by a 

 muscular membrane and minute tentacles, and is armed 

 with five sharp teeth, set in as many jaws, resembling 

 little conical wedges (Fig. 226). 



Among the headless mollusks, the oral apparatus is 

 very simple, being inferior to that of some of the radiate 

 animals. In the oyster and bivalves generally, the mouth 

 is an unarmed slit a mere inlet to the esophagus, situ T 

 ated in a kind of hood formed by the union of the gills 

 at their origin, and between two pairs of delicate flaps, 

 or palpi. These palpi make a furrow, along which pass 

 the particles of food drawn in by the cilia, borne by 

 cells which cover the surface of the flaps. 



Of the higher mollusks, the little clio (one of the 

 pteropods) has a triangular mouth, with two jaws armed 

 with sharp horny teeth, and a tongue covered with spiny 

 booklets all directed backward. Some univalves have a 

 simple fleshy tube or siphon. 

 Others, as the whelk, have 

 an extensible proboscis, 

 which unfolds itself, like the V y \ 

 finder of a glove, and carries FIG. 218.- j aw of the 

 within it a rasplike tongue, (Helix albolabr ^ P~ti y magnified. 

 which can bore into the hardest shells. Such as feed 

 on vegetable matter, as the snail, have no proboscis, but 

 on the roof of the mouth a curved horny plate fitted to 

 cut leaves, etc., which are pressed against it by the lips, 

 and on the floor of the mouth a small tongue covered 

 with delicate teeth. As fast as the tongue is worn off 

 by use, it grows out from the root. 



The mouth of the cuttlefish is the most elevated type 

 below that of the fishes. A broad circular lip nearly 

 DODGE'S GEN. ZOOL. 17 



