A FRESH- WATER MUSSEL (Anodonta sp. or Unio sp.). 

 Phylum XI, MOLLUSCA; class i, PELECYPODA; order 4, EULAMEL- 



LIBRANCHIA. 



HABITAT. Fresh-water mussels are found in rivers, ponds and 

 lakes in most parts of this and other continents. Some species fre- 

 quent sandy bottoms, while others are more at home in the mud and 

 ooze. 



Technical Note. If live mussels are obtainable they should be used to 

 demonstrate water currents, movements, and protrusion beyond 

 the shell of certain organs. They can be kept alive for weeks in jars 

 or tubs of water with a layer of three or four inches of sand at the 

 bottom. The water should be changed every few days. A four 

 per cent solution of formaldehyde makes an excellent preserving 

 agent for specimens to be used in dissection. If the live animals 

 are placed for a few minutes in water about as hot as the hand can 

 bear, the shell will soon gape enough to allow a small wooden wedge 

 to be inserted. This will prevent the shell from closing again and 

 will give free entrance to the preservative fluid. Before preserving, 

 the clams should be killed by smothering them in a closed vessel 

 containing only just enough water to cover them, for if the live clams 

 are thrown directly into the preserving fluid, some of the internal 

 organs will contract and become distorted. 



EXTERNAL APPEARANCE. The mussel is bilaterally symmetrical, 

 compressed from side to side, and enclosed in a shell consisting of 

 two valves hinged together on one of the margins. Observe the 

 following : 



a) The concentric lines of growth paralleling the margin of the shell. 



Each of these represents what was at one time actually the margin. 



b) The convergence of these lines about the umbo, an elevation on 



each valve near the hinge toward the forward end of the shell. 



77 



