432 



NATURE STUDY AND LIFE 



bivalves of most importance for the school aquaria and for 

 the work of purifying surface waters are the common 

 fresh-water mussels, or clams. They may be found in any 

 pond or stream. Ask the children to bring in a few and 

 keep three or four large ones in each of the larger aquaria 

 and one in each of the small aquaria. Now ask a group of 



the class to test the puri- 

 fication of water by clams. 

 They may do this by ar- 

 ranging two small aqua- 

 ria, exactly alike, with 

 sand and plants, filling 

 them with water that is 

 turbid from growth of 

 algae or bacteria. Put a 

 clam in one but not in 

 the other and watch the 

 result. Generally the 

 water with the clam in it 

 will become perfectly clear in a few hours or days, accord- 

 ing to the size of the aquarium, while the other grows 

 more and more turbid. 



Next we will try to discover how the clam works. If 

 we look at the clam from above, we notice two openings 

 between the posterior ends of the valves. These are sur- 

 rounded with a fringe of dark papillae which are sensitive to 

 light. This may be demonstrated by suddenly cutting off 

 the sunlight, when the valves will close. With a fine straw, 

 or a glass tube drawn to a fine opening, let fall close to 

 the upper siphon a little colored liquid, — carmine or dilute 

 India ink. It is sent whirling away from the clam. Now, 



Fig. 175. Fresh-water Clam 



a, inhalant siphon ; b, exhalant siphon ; 



c, gills ; dj mouth ; e^ foot. 



