34 STUDIES OF ANIMAL TYPES 



cross section of the body showing the organs. Summarize 

 the processes of digestion, circulation, excretion, and res- 

 piration as carried on in the earthworm. 



XI. THE RIVER MUSSEL 



Materials. Living mussels, carmine or India ink, dissecting in- 

 struments, dissecting pan, magnifier, hydrochloric acid, balances, 

 bristles. 



Directions. (The following outline applies especially to the 

 river mussel (Unio) that is common in the ponds and streams of the 

 United States. The marine clams (Venus and Mya) will serve quite 

 as well. These may be obtained of dealers in zoological specimens. 

 The river mussel may be carried home in water, and kept in an aqua- 

 rium containing several inches of sand in the bottom.) 



A. STUDY OF A LIVING MUSSEL. Carefully watch a liv- 

 ing specimen in the aquarium. In what position is the 

 shell held when the animal is moving? Is the movement 

 slow or rapid ? Does the mussel leave a track, or furrow, in 

 the mud? Is the shell open or closed when the mussel 

 is moving? Note a white fleshy projection between the 

 anterior edges of the shell. This is the foot. Note that the 

 mussel moves in one direction with the anterior end fore- 

 most. Is the broad or sharp edge of the shell held upper- 

 most ? From the posterior edges of the shell may be seen 

 projecting two rounded, fringed openings, the siphons. 

 By placing some carmine or India ink in the water near 

 them, the direction of the currents of water may be deter- 

 mined. Through which siphon does the water enter? 

 Through which does it emerge? In moving, the valves 

 are held partly open and the edge of the delicate mantle 

 may be seen all around the edges of the shell. 



