56 
COMPARATIVE ZOOLOGY. 
in those which freely move about, as the Jelly-fish, it is 
generally underneath, the 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 sharp teeth, set in as many jaws, resembling little 
conical wedges (Fig. 28). 
Among 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, 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 jnouth, 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 
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. 
Fig. 20.—Jaw of the Common Snail 
