THE OYSTER 8i 



Class : Pelecypoda 

 THE OYSTER 



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

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

 hydroids, sponges, tube-forming annelids, and other marine ani- 

 mals. 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, and the flat right shell acts as 

 a cover which can be raised to allow the animal to draw in water 

 containing food and respiratory 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 accommodates 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 

 of the shell. Note the parallel lines of growth which extend from 

 the umbo to the ventral and posterior sides of the shell. When the 

 anterior, the right, and the left sides of the shell are known, the 

 ventral and posterior sides can be easily determined. 



Exercise 1. Make an outline drawing of the right valve, indicating 

 the anterior, posterior, dorsal, and ventral aspects and showing the 

 lines of growth. 



