HOW ANIMALS EAT. 55 
tentacles, and leading directly to the stomach, is the ordi- 
nary mouth of the Polyps and Jelly-fishes. In those 
which are fixed, as the Actinia, Coral, and Hydra, the 
mouth looks upward: in those which freely moye about, 
as the Jelly-fish, it is generally underneath, the position of 
the animal being reversed. In some, the margin, or lip, 
‘an be protruded like a proboscis; and in all it is exceed- 
ingly 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 inclosed by a ring of hard tubercles. In the 
Sea-urchin, it is armed with five sharp teeth, resembling 
little conical wedges, set in as many jaws, and surrounded 
by a muscular membrane and minute tentacles. 
Among the headless Mollusks which do not move about, 
the oral apparatus is very simple, being inferior to that of 
the radiated animals. Thus, the immovable Ascidian has 
a mouth without tentacles or lips, and in a strange place, 
for it is in the interior of the body, at the bottom of the 
respiratory sac; the aperture at the top of the creature 
being really for the entrance of water for the double pur- 
pose of respiration and nutrition, and any alimentary par- 
ticles which enter with the water must find their way to 
the true mouth below. In the Oyster and Divalves gen- 
erally, the mouth is an unarmed slit—a mere inlet to the 
stomach, situated in a kind of hood, formed by the union 
of the gills at their origin, and between two pairs of deli- 
eate 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 tongne 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, 
