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 
