FOOD 
187 
the hinge ligament, a blunt projection of one shell fits into 
a depression in the other. These are called the hinge teeth. 
LABORATORY STUDY 
Live clams can be secured in the market during the school year. The 
dissection of the clam is too difficult, but the arrangement of the organs 
in the mantle cavity can be studied. The position of the adductor 
muscles, foot, gills, palps, heart, etc., should be observed. Examine a 
small portion of a gill under the microscope for cilia. A variety of 
shells of clams should be studied in which hinge, muscle scars, and hinge 
teeth are examined. Compare clam and snail shells. 
Digestive System. — The mouth, which is located under the 
anterior adductor muscle, leads through the short esophagus 
to the stomach. The in¬ 
testine winds through the __ ' ~~ 
foot region forming a loop, 
finally ascending and pass¬ 
ing through the peri¬ 
cardium and between the 
chambers of the heart it¬ 
self and opening into the 
upper siphon (Figure 172). 
Circulation is well de¬ 
veloped. 1 From the heart colorless blood is carried through 
arteries into smaller tubes, and returns, through veins, back 
to the heart. 
173. Locomotion. — The movements of the fresh-water 
mollusks are extremely slow. In the clam the foot is 
forced out of the shell by the blood, which flows into it 
and causes the foot to be greatly enlarged. Muscles attached 
to the shell and front of the foot contract and pull the shell 
forward over the extended foot. 
Figure 172. — Digestive Tube of Clam. 
m, mouth; s, stomach: i.c., intestine. 
1 The three-chambered heart lies in the dorsal region, near the hinge, in a 
little soft-walled chamber, the pericardium (per-i-kar'di-um: Greek peri, 
around; cardia, heart). 
