THE FRESH-WATER CLAM. 47 



touch the margin of one of them. What follows ? The 

 water enters one of these openings and comes out of the 

 other. Prove the action of each by stirring up a little 

 mud, to show the currents; or take a small glass tube, dip 

 the lower end into ink, or finely divided indigo in water, 

 then placing a finger over the upper end of the tube, lift 

 out a little of the ink. Keeping the finger tightly on the 

 top of the tube, thrust the other end down to a point 

 just above the siphonal openings ; then raise the finger 

 and release some of the ink. In some way prove the exist- 

 ence of these currents. Let each pupil have a clam in 

 a fruit-jar, with sand enough to support the clam. Keep 

 the clams alive for a week or more, and watch them daily. 



THE CLAM-SHELL. 



If a live clam is used, place it on a plate, or in the 

 oyster-can. 



1. Notice the two parts of the shell, — the valves. 



2. The edge along which the shell opens is the ventral 

 margin. 



3. The edge by which the two valves join each other is 

 the dorsal margin, or hinge margin. 



4. The concentric lines parallel to the ventral margin 

 are the lines of growth. 



5. The raised point around which these lines centre is 

 the beak, or umbo. 



6. The umbones are nearer the front, or anterior, end uf 

 the clam. 



7. Toward the posterior end, back of the umbones, 

 between the valves, and uniting them, is the hinge- 

 ligament. 



