THE CLAM. 



UNIO SP. 



THE common fresh-water mussels or clams, of almost any 

 genus, are excellent for study. Those of some size are best. 

 The marine clam (Venus) while not so large nor so satis- 

 factory, is quite similar in most of its structural features. 

 Clams live partly buried in the sand or mud, w r ith the pos- 

 terior end of the body protruding. They may be collected 

 by digging. It is possible to keep them alive for some time 

 if placed in tanks of running water. 



I. External Anatomy. 



1. Shell. Note the general form and relations of the shell. 

 Are the valves (the two parts of the shell) similar in size and 

 form? The dense horny hinge is the dorsal part, and the 

 knob or elevation on the shell (umbo) is nearer the anterior 

 end. Has the animal a right and a left side? Observe the 

 parallel, concentric lines extending from the umboes to the 

 margins of the valves; they indicate lines of growth, though 

 their number gives no evidence of the age of the clam. 

 Each line w r as at one time the edge of the shell. Study 

 the action of the ligament or hinge in a recently opened shell ; 

 what function does it serve? The ligament is an uncal- 

 cified portion of the shell. 



Make a drawing of the shell from the side, and one from 

 the anterior aspect. 



