56 COMPARATIVE ZOOLOGY. 



as the Jelly-fish, it is generally underneath, the position of 

 the animal being reversed. 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 sharp teeth, set in as many jaws, resembling little 

 conical wedges (Fig. 28). 



-<^j^mong the headless Mollusks, 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. 



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 flesh} 7 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 

 of the Common' siuiii mouth a curved horny plate 



(Helix albolabris). fifcted t() ^ j^^ ^ 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. 



