142 MANUAL OF ANIMAL BIOLOGY 



by carefully removing the left valve of the shell, together with the 

 left fold of the mantle. This discloses the various organs of the 

 body, which consist of the visceral mass, gills, and foot. All 

 these lie in the space between the folds of the mantle, termed the 

 mantle cavity. At the posterior end of the animal, the edges of 

 the mantle form two definite openings, the inhalent siphon and 

 the exhalent siphon, through which a current of water enters 

 and leaves the mantle cavity. (W. f. 49.) 



The visceral mass, composed of a number of organs which will 

 be considered in detail later, lies between and somewhat dorsal to a 

 line connecting the prominent anterior and posterior adductor mus- 

 cles which were cut through in removing the left valve. Ventrally 

 the visceral mass merges into the heavy muscular foot. The 

 latter is an organ of locomotion which may be protruded between 

 and beyond the ventral edges of the valves. A pair of gills is 

 present on each side of the visceral mass. They are attached 

 dorsally and hang down into the mantle cavity, reaching almost 

 to the edge of the mantle. 



1. Nutritive System 



The mouth opening is at the anterior end of the body and lies 

 just posterior to the anterior adductor muscle. Surrounding the 

 mouth are two pairs of elongated, ciliated structures, the labial 

 palps, which are in close connection with the anterior end of the 

 gills. Leading dorsally from the mouth there is a short esopha- 

 gus which opens into the anterior end of the stomach. The 

 anterior wall of the stomach lies close to the posterior wall of the 

 anterior adductor muscle. The stomach is a rather large, undif- 

 ferentiated sac. Leading from near the middle of the ventral wall 

 of the stomach is a long, greatly coiled intestine which, after 

 proceeding ventrally and posteriorly down into the region of the 

 foot, makes a number of coils and then turns dorsally and con- 

 tinues this course until it is above the dorsal wall of the stomach. 

 At that point it makes a right angle turn and runs directly pos- 

 teriorly until it ends at the anal opening which lies just a little 

 posterior to the posterior adductor muscle and near the exhalent 

 siphon. 



The heart of the Clam lies in a pericardial cavity situated in 

 the median line, and just below the dorsal body wall. Strangely 

 enough, the intestine passes into the pericardial cavity and directly 



