THE CLAM AND OTHER BIVALVES 



197 



animal lies attached to the bottom, frequently to another 

 oyster shell. Although many oysters may live thus fastened 

 to each other, there is no organic connection between them. 

 They sometimes form clumps so large and heavy that the 

 basal ones sink into the mud and die. The valves of the 

 living ones extend outward at any angle. When oysters 

 are not crowded in the "bed," the usual method of liv- 

 ing is the one shown in the illustration. There the valves 



Fig. 104. Group of living oysters. (Reduced) 



extend horizontally, and we can distinguish an upper and a 

 lower valve. The lower valve is always much larger and 

 deeper than the upper one. The lower one is the left valve. 

 Comparison with the Clam. The internal organs of Ostrea 

 virginica and Mya arenaria are very much alike. The 

 large, dark-brown digestive gland, the coiled intestine, the 

 three-chambered heart, and the reproductive glands have a 

 common plan in the two species. A noticeable difference is 

 the entire absence of a foot in the oyster. Applying the 

 principle of adaptation, we can readily explain the absence 

 of that organ. There are two adductor muscles in the clam, 

 in the oyster only one. 



