SOFT-BODIED ANIMALS— THE MOLLUSKS 273 



of the water filters through limestone deposits and is rich in this impor- 

 tant mineral. In this region the clams are collected and their shells used 

 to make "pearl" buttons, such as those found on most men's shirts, "pearl 

 handled" knives and revolvers, and similar ornaments. 



The outer surface of the shell is made up of numerous concentric 

 rings. These are rings of growth since the rings are deposited by the 

 mantle around the outer rim of the shell to increase its size. A number 

 of rings will be deposited each year, but the rings representing the dor- 

 mant periods of the winter months are usually more pronounced. Thus, 

 the age of a clam can be estimated. Slightly anterior to the hinge is 

 a little hump called the umbo which is encircled on three sides by growth 

 rings, showing that this is the oldest portion of the shell. 



Photo by Winchester 



Fig. 19.2. Internal and external views of a clam shell. Note the concentric lines of 

 growth, with heavier lines at the termination of each year's growth. H, hinge ; U. 

 point of muscle attachment; P.L., palial line; U. umbo; 1, 2, 3, 4, yearly growth rings. 



The outer surface of the shell is covered by a tough and somewhat 

 eroded horny layer, but the inside is a very beautiful pearly layer. This 

 inner layer has an opalescent quality, reflecting the light waves in their 

 various colors due to the presence of thousands of tiny prismatic bodies 

 which are imbedded in the mother-of-pearl. 



The two parts of the shell are closed by a pair of adductor muscles 

 which are so strong that a considerable force is required to pry the shell 

 open. The clam can open it easily, however, by relaxing these muscles 

 and the elastic hinge causes the halves of the shell to swing apart. 

 When the shell is open the clam may extend its foot and pull itself 

 along very slowly in the mud at the bottom of the river or lake where 

 it lives. 



There are some unusual developments in the organ systems of the 

 clam which allow it to carry on its life processes in its sluggish state. At 



