42 



MOLLUSC A. 



depth in the mud. The shell is held with the beak 

 uppermost, and the mouth or larger end is always di- 

 rected forwards, and ploughs more or less through the 

 mud, being dragged onward by the long, extensible 

 foot. Thus, although it is headless, like the clarn and 

 the scallop, it nevertheless, in all its motions, carries 

 the mouth end forwards as they do ; and also, being an 

 upriglit, free-moving animal, is, like all such, perfecdy 

 symmetrical in form and the distribution of the appen- 

 dages. The manner in which these animals contract 

 and swell the body at will, and the great increase of 

 bulk which they can effect when they wish to protrude 

 the foot or extend the mantle edges, will occasion great 

 astonishment to those who see it for the first time. 



This is accomplished by the taking in of sea-water 

 by means of pores, which occur in the skin of the foot, 

 and which communicate with vessels which permeate 

 the interior of this organ, and also the external parts 

 of the body. These are peculiarly large and numerous 

 in the Beach Clam, and can be seen if the foot is in- 

 flated with air, or distended by injecting fluid into it in 

 a specimen which has been carefully killed in fresh 

 water, and allowed to lie until all the stiffness of the 

 muscles, occasioned by death, has passed off. 



It is not yet settled whether this water system con- 

 nects with the blood vessels or not ; but it is notice- 

 able, that when one of these animals is taken out of 

 water, the fluid expelled from the body is sea-water, 

 more or less mixed with blood. 



The number and variety of the kinds of Fresh-^Vater 

 Clams, Unio and Anodonta, in the Western rivers, u 

 so great that a class may be taught most effectually by 



