186 
THE MOLLUSKS 
enters, carrying food and oxygen. Through the other the 
water passes out, carrying the waste from the body. 
Between the mantle and the body proper are four gills, 
which hang free in the shell cavity. The gills are filled with 
holes through which water passes. 
The foot, which is attached directly to the body proper, is 
that part of the clam hard to chew when it is eaten. v The foot 
and body form a solid mass that nearly fills the space be¬ 
tween the shells. 
The two valves of the clam shell are held together by 
means of strong muscles, attached to each shell. One of 
these, located in front of 
the body, is known as the 
anterior (front) adductor 
(ad-duk'ter) muscle; the 
second is just back of the 
body and is the posterior 
(back) adductor muscle. 
When these two muscles 
contract, the two valves 
are held tightly together. 
Before the live clam can be 
examined these two muscles have to be cut, as it closes its 
valves when handled. When the clam is dead, these muscles 
relax and the hinge forces the valves apart. It is not safe to 
eat clams and oysters that have died in their shells. 
When the two adductor muscles are cut free from the 
valves, a round or oval surface is seen which is marked off 
from the rest of the interior of the shell. These* areas 
are called muscle scars (Figure 171). 
When the empty clam shell is examined, it is found 
that the hinge, sometimes called the hinge ligament, is 
elastic. This is shown by compressing the two valves and 
seeing how promptly they open when the pressure is taken 
off. Where the two valves come in contact just beneath 
Figure 171. — Left Shell of Clam. 
Showing mantle and muscles. a.a., 
anterior adductor muscle; p.a., posterior 
adductor muscle. 
