108 



MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 



slowly extended outward and downward into the mud and 

 anchored there. Then the retractor muscles contract and 

 pull the shell and body up to the foot, as it were. By a 

 repetition of these movements of the foot the mussel covers 

 considerable distances in the course of time. 



The alimentary canal and digestion. — The mouth is 

 just under the anterior adductor muscle and between two 

 pairs of soft flaps, the labial palps. It leads by a short 

 gullet to the spherical stomach, which is surrounded by a 



dark green mass, the 

 Posterior adductor muscle..._ digestive gland, or 

 Digestive gl^^^ jjgf^j^ \ u^^^^^,, The in- 



JJreter; testine passes down- 

 ward from the stom- 

 ach, coils once or 

 twice in the abdo- 

 men and foot, and 

 returning, runs pos- 

 teriorly through the 

 ventricle of the heart 

 and ends back of the 

 posterior adductor muscle (Fig. 58). The food, which con- 

 sists of minute plant and animal organisms, is brought in 

 by the inhalent cm-rent, carried to the anterior end of the 

 body (Fig. 56), and directed into the mouth by the cilia of 

 the labial palps. It is acted upon by the fluids from the 

 digestive gland and is absorbed from the stomach and 

 intestine. 



Excretory organs. — The excretory organs consist of a 

 pair of dark-colored kidneys lying just below the pericar- 

 dium and in front of the posterior adductor muscle (Fig. 

 58). Each kidney consists of a glandular portion, the 



Fig. 58. — Internal structure of a clam. 



