SOFT-BODIED ANIMALS— THE MOLLUSKS 



277 



to see just what this projection is, the clam may suddenly jerk the neck 

 down and send a squirt of water into the eye of the curious person. 



Oysters furnish us with a very important source of food and are 

 cultivated in many places in order to supply the great demand for them. 

 Oysters are not able to move about like the clams ; the left side of the 

 shell becomes attached to a solid object when the oyster is in the larval 

 stage and there it remains for the remainder of its life. Since the foot is 



heart pericardial cavity 



artery to mantle 



vein from 



blood sinus of foot 



Animals Without Backbones, Buchsbaum, The University of Chicago Press 



Fig. 19.6. Diagram of the circulatory system of the clam. Note the sinuses which 



make this an open system. 



a locomotor organ, it is not needed by the oysters and hence it is not 

 present. Otherwise the body structure is very similar to that of the 

 clams. 



Whenever a foreign body, such as a grain of sand or a small parasite, 

 gets between the mantle and the shell of one of the bivalves, a layer of 

 mother-of-pearl will be secreted around it to prevent injury to the soft 

 body parts. Additional layers of mother-of-pearl will be formed around 

 this as time goes on and at the end of several years a pearl will be formed. 

 Pearls are commonly spherical in shape, but many odd shapes may be 

 formed such as the pear-shaped or "tear drop" pearls that may be made 



