90 INVERTEBRATE ZOOLOGY 



The digestive system. Find the mouth between the two pairs 

 of palps and place a bristle in it ; note the upper and the lower 

 lips, which connect the upper and the lower pairs of palps, 

 respectively. The mouth is seen to the greatest advantage in a 

 specimen which has been deprived of both valves of the shell. 

 Trace the rectum from the anus to the place of its entrance into 

 the visceral mass. Carefully remove with forceps and knife 

 the tough, white integument which covers the left side of the 

 visceral mass, taking care not to disturb the organs beneath. 

 The soft cream-colored mass just above the foot is the reproduc- 

 tive gland ; the light greenish mass lying just above this is the 

 liver. Imbedded in these masses lies the alimentary tract, a 

 narrow delicate tube, which will be injured in the dissection 

 unless the greatest care is taken. Beginning with the mouth, 

 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 

 must be taken not to break the tract. The mouth opens into 

 the short oesophagus, after which the canal dilates to form the 

 stomach. The liver communicates with the stomach by several 

 ducts. Back of the stomach is the intestine, a narrow tube 

 which runs backward and downward to the hinder end of the 

 visceral mass; it then turns upward and runs forward to a 

 point above the stomach, where it turns downward to the lower 

 side of the visceral mass; it then bends dorsally again and 

 runs to the point where it leaves the visceral mass. Here 

 the rectum begins and passes through the heart and above 

 the posterior adductor muscle to the anus in the cloacal 

 chamber. 



Mussels feed upon minute organisms and organic particles 

 contained in the water. Some of the water in the mantle 

 cavity is drawn by the ciliated oral palps into the mouth and 

 passes through the alimentary tract, where organic substances 

 contained in it are digested and absorbed. The mussel usually 



