Branch Mollusca 79 



INTERNAL ANATOMY 



VIII. — The shell is a true exoskeleton, and its removal 

 does not expose the interior or body cavity of the clam. 

 On account of the size of the animal and its lateral 

 compression, we can do no more than indicate the rela- 

 tions of the body cavity and alimentary canal. 



IX. — The membrane above mentioned as found closely 

 adhering to the inner side of the shell is the mantle. 

 This has two lobes, corresponding in shape to the valves 

 of the shell. Demonstrate this fact. Lift the upper- 

 most lobe carefully to determine its place of attachment. 

 It is simply an outgrowth of the body wall. At the pos- 

 terior end find two well-defined oval openings, formed 

 by the two mantle lobes coming together. These two 

 openings together form the siphon. In the salt-water 

 clam the two lobes of the mantle have developed into 

 a siphon that extends outward, sometimes an inch or 

 more beyond the shell : a black, hard, elongated pro- 

 jection, through which these openings pass as wholly 

 closed tubes. Water containing food material passes 

 in at the ventral or lower opening, and out from 

 the dorsal opening. It may be necessary to remind 

 the student that we are still talking about the posterior 

 end of the animal, and that we have not yet penetrated 

 its body wall. 



X. — By raising the mantle lobe examine the exposed, 



hard, white, muscular foot. This is also an extension 

 of the body wall, and within its base (the visceral mass) 

 are embedded the long coils of the stomach region of 

 the alimentary canal. 

 XL — On either side of and posterior to the abdomen find 

 the gills, a pair of ribbed membranous organs. Just 



