104 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. Itis 
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 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, z.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 pelecypods 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, 2.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 instru- 
meut 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. 
