94 INVERTEBRATE ZOOLOGY 



the two muscles which pass between the valves, it will gape open 

 as soon as the animal is dead and the muscles are relaxed. It is 

 the elasticity of the hinge ligament which causes it to open. 1 



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 membrane, whose lower edge is parallel with the 

 edge of the shell. This is the mantle ; it is a double fold of the 

 dorsal integument of the body, one side of which is covered by 

 either fold. The mantle is the matrix of the shell, i.e., it secretes 

 it. The lower edge of the mantle is provided with muscle 

 fibers and can be extended beyond the edge of the shell; it 

 also possesses sensory functions ; in some lamellibranchs eyes 

 are situated in the mantle's edge. 



Observe the large, soft visceral mass hanging between the two 

 lobes of the mantle; it contains most of the viscera of the 

 animal. On the lower side of the visceral mass, i.e., towards 

 the gape of the shell, is the muscular wedge-shaped foot, which 

 can be extended beneath the edge of the shell and is the organ 

 of locomotion. Do you see the two leaf-like gills on each side 

 of the visceral mass and foot ? Observe the two large adductor 

 muscles, one in front of and the other behind the visceral mass, 

 which pass from one valve to the other and serve to close 

 them. 



Pass a knife between the mantle and the left shell and sepa- 

 rate them from each other. Cut the two muscles close to the 

 shell ; cut the 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 mus- 

 cles; just above the anterior scar is that of a much smaller 



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

 ment between the 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. 



