Anatomy of the Food Mollusks 13 



conform to the swimming habit. Certainly it is not the 

 degenerate oyster that has completely lost the organ of 

 locomotion, and the anterior adductor muscle. Probably 

 it is not the soft clam, for in it, also, the ancient foot is 

 greatly reduced. Of the short list, the hard clam, Venus 

 mercenaria, probably has a greater number of organs that 

 are most like those of the ancestral bivalve, though some, 

 like the gills, depart much farther from the primitive con- 

 dition of those organs than do those of the mussel and 

 scallop. But because Venus, not by any means one of the 

 more primitive of living bivalves, is somewhat the more 

 simple of the species here described, it may illustrate best 

 some of the anatomical characters common to them all. 



The Shell. The hard protective covering of Venus 

 consists of right and left parts known as valves. It is 

 composed of carbonate of lime, which is deposited in a 

 viscous secretion poured out by the fleshy mantle fold 

 lining its inner surfaces. On the shore, one sometimes 

 finds valves of clams or other bivalves, recently dead, 

 that are united on the upper or dorsal side by a piece of 

 stiff, elastic substance, resembling rubber. This is known 

 as the shell ligament. The position of its attachment to 

 the shell is represented in Figure i (/). 



Just within the ligament, each valve bears prominent 

 ridges or teeth that fit into corresponding depressions in 

 the opposite valve. This mechanism, serving to hold the 

 two parts of the shell in their proper relative positions, is 

 called the hinge. It may be noticed that the shells of 

 dead bivalves are always open at the under or ventral 

 margin. In the living animal, lying quite undisturbed 

 in the water, a slight gaping of the valves may also be 

 observed; but when the animal is disturbed, the valves 



