56 COMPARATIVE ZOOLOGY. 



in those wliicli freely move about, as tlie Jelly-fisli, it is 

 generally underneath, tiie position of the animal being re- 

 versed (Fig. 193). In some, the margin, or lip, is protruded 

 like a proboscis; and in all it is exceedingly dilatable. 



The mouth of the Star-fish and Sea-urchin is a simple 

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

 Star-fish, it is enclosed by a ring of hard tubercles and a 

 membrane. In the Sea-urchin, it is surrounded by a mus- 

 cular membrane and minute tentacles, and is armed with 

 five sharji teeth, set in as many jaws, resembling little 

 conical wedges (Fig. 28). 



Among the headless MolUisks, the oral apparatus is very 

 simple, being inferior to that of some of the radiated ani- 

 mals. In the Oyster and Bivalves generally, the mouth 

 is an unarmed slit — a mere inlet to the oesophagus, situ- 

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

 at 'their origin, and between two pairs of delicate lips. 

 These lips make a furrow, along which pass the particles 

 of food drawn in by the cilia, borne by cells which cover 

 the surface of the lips. 



Of the higher Mollusks, the little Clio (one of the Ptero- 

 pods) has a triangular mouth, with two jaws armed with 

 sharp horny teeth, and a tongue covered with spiny hook- 

 lets all directed backward. Some Univalves have a sim- 

 ple fleshy tube. Others, as the Whelk, have an extensible 

 proboscis, which unfolds itself, like the finger of a glove, 

 and carries within it a rasp-like tongue, which can bore 



into the hardest shells. Such 

 as feed on vegetable matter, 

 as the Snail, have no probos- 

 cis, but on the roof of the 

 Flu. 20 — jnw of the Common Suaii mouth a curved liomy plate 



{Helix albolahriii). £i.i. j x , i ... 



titted to cut leaves, etc., which 

 are pressed agaitist 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. 



