188 
THE MOLLUSKS 
174. Food. — The food of the clam consists of microscopic 
plants and animals that are caught in a sticky fluid (mucus) 
on the gills, as the water passes through them. The food, to¬ 
gether with the mucus, is moved into the mouth by means of 
cilia. The mouth is simply an opening into the body and the 
cilia are on triangular flaps or lips (palps) on either side of 
the mouth. From the mouth food passes into the digestive 
canal, where the nutritious parts are absorbed (Figure 172). 
175. Respiration. — The clam, like other aquatic animals, 
gains its oxygen from the water and gives off carbon dioxide. 
A close inspection of the mantle shows the presence of blood 
vessels which are more numerous than in the gills. For 
this reason, the mantle is regarded 
as the main organ of respiration, 
although the gills also assist. 
176. Excretion. — The wastes 
of the body are absorbed by the 
kidneys and passed out into the 
water through the upper siphon. 
177. The Nervous System is 
not so well developed as in the 
crayfish. There are three groups 
of ganglia (nerve cells). One lo¬ 
cated far back in the body near 
the posterior adductor is called 
the visceral ganglion because it 
largely regulates the activities of the viscera (vis'se-ra), the 
internal organs of the body. Another in the foot region is 
called the pedal (pe'dal) ganglion, and regulates the move¬ 
ments of the foot. A third located in the region of the gullet 
(esophagus) is the cerebral ganglion, which regulates the 
activities of the part near the mouth. All these are con¬ 
nected by nerves. 
178. Life History. — In clams the sexes are distinct. 
Eggs formed in the ovaries of the female fuse with sperm 
Figure 173 . — Embryo of Clam. 
At this stage it becomes at¬ 
tached to the gills or fins of a 
fish. Here it remains for some 
weeks, gradually transforming 
into a clam. 
