34 



LIVING CREATURES. 



rubber is elastic. Something like such a piece of 

 rubber is a ligament under the hinge of the salt-water 

 clam, as shown in Figure 2 (/). If you open a book 

 and fasten the covers open by a rubber strap running 

 over the outside of the back, you can only close it by 

 force, and then it will open again as soon as you let 

 it go. In the same way an elastic ligament on the 

 outside of the fresh-water clam (Fig. 3 /) draws the 

 valves open. In the salt-water clam the ligament pushes 

 the valves open, and in the fresh-water clam the liga- 

 ment pulls the valves open. 



Little Neck Clam. 



This is just what the clams want to make them 

 happy to be kept open without effort, so that the 

 nourishing water may always flow in. To close their 

 valves they must put forth an effort; and this is the 

 way they work. Inside the shell two strong muscles 

 pass from one valve to the other. One of these mus- 

 cles (m) is shown in each of the two figures just now 

 before us. It is called an adductor muscle, adductor 



