96 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 esophagus, 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 
