THE KIVER MUSSEL 35 



B. EXTERNAL FEATURES OF THE SHELL. Of how many 

 pieces is the shell composed ? Are they of the same shape 

 and size? These pieces are called the valves. Note that 

 they are joined or hinged together along the broad edges. 

 These edges are held uppermost by a living mussel when it 

 is moving, and this side of the shell is therefore known as 

 the dorsal margin. The thin edge of the shell is called 

 the ventral side or margin. Note the concentric lines run- 

 ning parallel with the ventral edges of the shell. These are 

 the lines of growth. They all begin and end around a raised 

 point near the anterior end of each valve. This point is 

 called the umbo, or beak. 



Knowing the dorsal side of the shell a"nd its anterior 

 end determine which is the right valve and which the left. 



Draw a side view of the left valve showing all the fore- 

 going points possible. 



C. THE MUSSEL IN THE SHELL. 



(For dissection, the valves must be opened. To do this, place the 

 animal in warm water, about as hot as the hand can bear. The 

 valves will gape, and a block may be slipped between them to hold 

 them dpen.) 



Note the mantle lining the inside of the valves. With 

 the handle of a scalpel, separate the edge of the mantle 

 from the upper valve. Observe that it is loose to a line 

 some little distance from the edge of the valve. Beyond 

 this line the mantle is grown tight to the valve. This line 

 is known as the pallial line. With the scalpel, carefully 

 tear the mantle loose from the valve that is uppermost, 

 and then cut away the strong muscles that hold the valves 

 together. Note that the valves spring apart as this is 

 done. Find the strong ligament, near the umbo, that acts 

 as a hinge. Note the hinge teeth on the dorsal edges of 



