166 



GENERAL ZOOLOGY 



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 

 virginiana 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. 

 The two animals are different also in the number of adduc- 

 tor muscles. In the clam there are two ; in the oyster only 

 one. The muscle-scar on the inner surface of the valves of 

 an oyster is a dark blotch. As the oyster grows from season 

 to season the location of the shell-closing muscle changes 



FIG. 85. Group of Living Oysters. Reduced 



to meet the necessity of performing its function most advan- 

 tageously. In a lot of a dozen oyster- valves the observer 

 may find several which indicate this change. In some valves 

 all trace of the change is concealed by the thick layer of 

 mother-of-pearl laid down by the mantle after the muscle 

 altered its position. 



