THE HARD-SHELL CLAM 91 



in a specimen which has been taken out of both shells. Trace the 

 rectum from the anus through the heart to the point where it 

 meets the visceral mass. With forceps and knife carefully remove 

 the tough white integument which covers the left side of the vis- 

 ceral mass. The soft, cream-colored mass filling the greater part 

 of it is the reproductive gland ; the greenish mass above is the 

 liver. Embedded in these masses lies the alimentary tract, a nar- 

 row, delicate tube, which will be injured in the dissection unless 

 the greatest care is taken.- Gently scrape away the soft mass which 

 surrounds the alimentary tract, laying it entirely bare. The water 

 in the dissecting pan must be frequently renewed to keep it clear, 

 and great care taken not to break the canal. The course of the 

 whole digestive tract is the following : The mouth opens into the 

 short cBsophagus, after which the canal dilates to form the stom- 

 ach. The liver surrounds the stomach and is connected with it by 

 several ducts. Back of the stomach is the intestine, which first 

 runs backward and downward to the posterior part of the visceral 

 mass, after several turnings in the lower part of which it bends 

 upward and runs forward parallel with the posterior margin of 

 the visceral mass to its dorsal border, where it leaves it. Here the 

 rectum begins and passes through the heart and above the pos- 

 terior adductor muscle to the anus. A small, transparent rod is 

 often present in the intestine ; its function is unknown. 



Clams feed upon minute organisms and organic particles con- 

 tained in the water. Some of the water in the mantle cavity is 

 drawn into the mouth by the ciliated oral palps and passes 

 through the alimentary tract, where the organic substances are 

 digested and absorbed. 



Exercise 9. Draw a diagrammatic view of the digestive system. 



The Reproductive System. The sexes are separate. The repro- 

 ductive glands (testes or ovaries) are very similar to each other 

 and consist of a pair of cream-colored masses which fill a large 

 part of the visceral mass. Their external openings are a pair of 

 minute pores, one on each side of the visceral mass just below 

 and in front of the opening of the ureter. They can sometimes 

 be located by pressing out from them eggs or sperm. 



