THE FRESH WATER MUSSEL 139 



cent, alcohol or two per cent, formalin, but at least one 

 muscle should be cut and the liquid allowed to penetrate the 

 interior. 



(6) It is desirable that the students go to the natural 

 habitat to collect the specimens if possible. 



The External Structure.^ — (i) The Living Animal. 

 Study the animal in its natural surroundings. Note the 

 covering, the shell, and the tendency of the animal to keep 

 it shut so tight that nothing can enter between the margins. 

 Does it ever open the shell ? Why does it ? What makes it 

 close it ? Note the Ueshlike fringe where the shell opens and 

 closes. Touch this fringe with a needle or scalpel. Is it 

 sensitive? Does the animal seem to have control over it? 

 What relation does it assume to the bottom of the water? 

 Watch for movment. Does it move on the sand or through 

 the sand ? How is this movement accomplished ? 



(2) The Shell. Hold the shell in your hand with the 

 narrow edge down and the beak, umbo, pointing from you. 

 The part on your right is the right side, the front is the 

 anterior end, and the top is the dorsal side. Where is the 

 left side? The ventral side? The posterior end? How 

 many parts has the shell? It is called a bivalve. Why? 

 Does it have bilateral symmetry? Select two shells that 

 fit together like the shells of the animal you have. Examine 

 limbo and the hinge on the dorsal side, the margin on the 

 ventral side. Is there an indication of an epidermal cover- 

 ing on the outside of the shell? Notice the lines on the 

 outside. What point do they seem to surround ? Are they 

 lines of growth? Examine them carefully? How many? 

 Look at the one next the margin and compare its outline 

 with the margin. If the animal should add more to its 



