MOLLUSCA 



143 



The fresh water clams, Anodonta and Unio, often called 

 mussels, are present in large numbers in some fresh water 

 streams and lakes. The shell of the Unio is smaller and 

 thinner than that of Anodonta. The sexes are separate, 

 but the male re- 

 sembles the female 

 so closely that a 

 microscopic exam- 

 ination of the re- 

 productive glands 

 is necessary to dis- 

 tinguish one from 



the Other. FIG. 16o. Sof t-shelled clam with a piece cut out of 



The ejrprs of clams the end of the sipllon to snow botl1 tubes. Photo- 



graph natural size. 



are hatched within 



the shell of the mother, and in a few months the young 

 pass out into the water and attach themselves to fishes 

 by a pair of sharp projections 

 on the ventral margins of the 

 shell. Here they lead a para- 

 sitic life, absorbing juices from 

 the body of the fish for some 

 weeks and pass through a 

 metamorphosis, after which 

 they drop to the bottom and 

 feed on small crustaceans and 

 other aquatic organisms. Most 

 clams move about from time 

 to time by means of a single ventral projection, the foot, 

 which is protruded between the open valves. 

 The oyster (Ostrea virginiana), so highly prized for food, 



FIG. 167. Hard-shelled clam. 

 Photograph half natural size. 



