PBLECYPODA 



75 



By its contraction the shell is closed. Its location is changed from year to 

 year as the animal grows. A brown scar in the shell indicates where the 

 attachment has been. The oyster can open its shell but little. 



The oyster, since it is fixed, needs no organ of locomotion, and so has no 

 foot. Neither has it any siphon, but the food-bearing water (Fig. 55) 

 enters along the curved border of the shell and passes out near the larger 



Fig. 54. — Shell of typical American oyster: 1, Inner face; 2, outer face. 

 (Report U. S. Geol. Survey.) 



end on the straight side. A fresh supply of sea-water is necessary to fur- 

 nish it with food and oxygen. If the oysters settle too deep in the mud 

 or if they are covered by silt and sand in time of storms they smother. 



Our species of oysters (Ostrea virginiana) is bisexual, while the European 

 species are hermaphroditic' The reproductive organ is attached to the 



1 "Hertwig's Manual of Zoology," Kinglsey, p. 367. 



