THE FRESH-WATER CLAM. 49 



in the left hand, with the hinge-margin resting in 

 the palm of the hand, and the anterior end toward 

 you. Insert' the blade of a knife between the mantle 

 and the upper (left, if held as directed) valve, and 

 gently separate them by sliding the blade of the 

 knife along the inner surface of the shell. In this 

 way proceed backward, around the posterior end of 

 the shell, then forward along the dorsal margin. 

 Back of and below the hinge is a large white muscle, 

 which extends directly across from valve to valve. 

 Cut this off close to the left valve. In like manner 

 loosen the mantle at the anterior end, and find 

 another muscle connecting the two valves near the 

 anterior dorsal margin. Sever as before, close to the 

 left valve, and loosen the mantle completely from the 

 upper valve, and turn this valve back like the lid of 

 a box. What makes the valve spring up after the 

 muscles are severed? 



2. Lay the clam in a deep plate, or in the oyster-can, 

 and cover it with water. Renew the water as often 

 as it becomes turbid. 



Observe that the left mantle lobe now covers the 

 body, and that the right lobe lines the right valve. 

 Notice the thicker margin of the mantle. Pinch this 

 thick edge ; what follows ? Observe a thin, dark- 

 colored membrane bordering the edge of the shell. 

 This is an extension of the outer covering, or epider- 

 mis, of the shell. Scrape off some of the epidermis 

 to see its relation to the limy shell. Carefully study 

 the relations of the epidermis to the mantle. Turn- 

 ing to the uninjured mantle lobe, pinch the edge of 

 the mantle, and observe the effect on this free border 



