86 PHYLUM MOLLUSCA 



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, 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 ; that is, 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, that is, toward the gape of 

 the shell, is the muscular, wedge-shaped foot, which can be ex- 

 tended beneath the edge of the shell and is the organ of locomo- 

 tion. Notice the two leaflike 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 separate 

 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 muscles; 

 just above the anterior scar is that of a much smaller muscle, the 

 anterior retractor of the foot. Note the broad line which joins 

 the scars and runs parallel with the edge of the shell except near 

 the posterior muscle scar, where it bends forward, forming a tri- 

 angular indentation. This is the pallial line ; it is formed by the 

 insertion in the shell of the delicate muscle fibers near the edge of 

 the mantle. The indentation is the pallial sinus. Note the hinge 

 teeth just beneath the umbo. 



Exercise 3, Draw a view of the inner surface of the shell. 



Break the shell and examine the broken edge with a hand lens. 

 Study the structure of the shell. It is composed of three layers : 

 the inner mother-of-pearl layer, which is secreted by the entire 

 surface of the mantle, the prismatic layer, and the organic layer, or 



