A FRESH-WATER CLAM 73 



the animal is dead and the muscles are relaxed. It is the elasticity 

 of the hinge ligament which causes it to open.^ 



Examine the animal as it lies in the shell. It will be seen that 

 the inner surface of each valve is covered with a soft, slimy mem- 

 brane, the lower edge of which is parallel to the edge of the shell. 

 This is the mantle ; it is a double fold of the dorsal integument of 

 the body, one side of the body being covered by either fold. The 

 mantle is the matrix of the shell ; that is, it secretes the shell. 

 Its lower edge is provided with muscle fibers and can be extended 

 beyond the edge of the shell ; it also possesses sensory functions. 



Observe the large, soft visceral mass hanging between the two 

 lobes of the mantle ; it contains the viscera of the animal. On the 

 lower side of the visceral mass, that is, toward the gape of the 

 shell, is the muscular foot, which can be extended below the edge 

 of the shell and is the organ of locomotion. Observe the two leaf- 

 like gills on each side of the visceral mass and foot ; notice also 

 the two large adductor muscles, one in front of and the other 

 behind the visceral mass, which pass from one valve of the shell 

 to the other and serve to close them. 



Pass a knife between the mantle and the left shell and separate 

 them from each other. Cut the two muscles close to the shell ; 

 cut the elastic hinge ligament and remove the left shell. 



Study the inner surface of the shell. Note the two large scars 

 marking the surfaces of attachment of the adductor muscles; 

 just above each is the scar of a much smaller muscle, the re- 

 tractor of the foot, and just posterior to the anterior adductor 

 muscle is the scar of the protractor muscle of the foot. Note the 

 broad line which joins the scars, running parallel with the edge 

 of the shell. This is the pallial line ; it is formed by the insertion 

 in the shell of the delicate muscle fibers at the edge of the mantle. 

 Do you find hinge teeth in the shell just beneath the hinge liga- 

 ment ? Unio has such teeth ; Anodonta is without them and is 

 also characterized by the thinness of its shell. 



1 The shell may also be opened by inserting some sharp, wedge-shaped instru- 

 ment between its valves. The valves are thus pressed apart far enough to admit 

 the blade of a scalpel, by means of which the adductor muscles should be cut close 

 to the left valve of the shell. The hinge ligament should then be cut and the left 

 valve be removed. 



