AN OYSTER 99 



PELECYPODA 



AN OYSTER 



Select a large live oyster in the shell, and if it is dirty wash it 

 thoroughly. The shell is sometimes covered with mud, hydroids, 

 sponges, tube-forming annelids, and other marine animals. The 

 small, round holes made by the yellow boring sponge are often 

 conspicuous. 



The two valves of the shell will be seen to be different in 

 shape, one being more or less flattened and the other much 

 deeper and more convex. These two valves cover the right and 

 left sides of the animal's body, the convex valve being on the 

 left and the flattened one on the right side. The oyster is a 

 sessile animal, after it has passed through its youthful migratory 

 period, and is fastened to a rock or shell or other stationary 

 object by its left shell. It thus lies on its left side, while the 

 flat right shell acts as a cover which can be raised to allow 

 the animal to draw in water containing food and air, and 

 closed when danger threatens. The very young oyster is a 

 symmetrical animal which swims about actively in the water. 

 While it is still very small — so small, in fact, that it is barely 

 visible to the naked eye — it settles down and fastens itself 

 to some stationary object and in its subsequent growth accommo- 

 dates itself more or less to the irregularities of this substratum. 

 This is the reason why the shell is so often rough and irregular 

 in shape. 



The smaller end of the shell is the anterior end. The hinge 

 ligament is situated here, the elasticity of which keeps the shell 

 open except when it is closed by the contraction of the large 

 adductor muscle. At this end is also the umbo, the oldest part 



