THE RIVER MUSSEL. 141 



eral inches of sand in the bottom and several inches of 

 water above the sand. Here something of their habits 

 may be seen. 



Study of a Live Specimen. Take a live specimen in 

 hand, and note : 



1. That the animal has entirely withdrawn itself 

 within its shell. 



2. That its shell is composed of two equal pieces 

 called valves. 



3. That the valves are hinged together at one side. 



4. That they are firmly held together at their free 

 margins. Try pulling them apart. 



5. That there is a central prominence in each valve 

 (the beak or umbo) near the hinge. 



6. That each umbo is a center of growth, with lines of 

 growth arranged concentrically around it. 



Now observe one of the mussels that have been placed 

 on the sand in the aquarium, one which, having been 

 left for awhile to itself, has gotten up on edge and started 

 to travel. Observe : 



1. That the hinge is up. It is on the back or dorsal 

 margin. 



2. That the free margins of the valve are down. They 

 form the ventral margin. 



3. That they are slightly separated, and that the animal 

 is partly extended between them. 



4. The direction in which the mussel is traveling. Take 

 some measured observations and compute its rate of speed. 



5. That the umbones are nearer the forward or anterior 

 end. Looking at the moving mussel from behind, the 

 valve on the right hand is the right valve ; the other, the 

 left valve. 



6. That the animal moves by a succession of pulls. 

 Find out what does the pulling. 



