140 INTRODUCTION TO ZOOLOGY. 



where its presence is perceived by the palpi, and it is 

 taken into the mouth. 



The valves of the fresh-water clam seldom fit so 

 closely to each other as do the valves of the salt-water 

 clam. The starfish is the principal enemy of salt- 

 water clams. It kills them by inserting its stomach, 

 which is a very thin membrane, into the space between 

 the valves, and then digesting the clam within its 

 own shell. The salt-water clam, then, must have a 

 shell that fits very tight, or it will be killed by the 

 starfish ; bvit there are no starfish in fresh water, so 

 the clam that shuts its shell tight has no advantage 

 in that respect over the one that does not shut its shell 

 tight. 



The principal danger to the fresh-water clam is 

 the possibility of having its valves torn apart when a 

 strong current pounds it over the stones. The 

 mechanical device of the interlocking teeth, near the 

 umbo, is employed to guard against this danger. 

 Another cause of the death of many clams is their 

 stranding in shallow pools which afterward dry up 

 completely. This occurs especially in river bottoms, 

 when the water recedes after a freshet to its usual level. 



The eggs of the fresh-water clam are carried for 

 some time in the outer gill and hatched there. A 

 gill that is carrying eggs may be known by its dark 

 color and swollen appearance. 



The shell is closed by the two adductor muscles. 

 The shell is opened by the hinge ligament, which acts 

 somewhat like the case-spring of a watch, opening 

 the valves whenever the adductor muscles are relaxed. 

 Fresh-water clams are too tough to be edible. 



THE OYSTER. 



Oysters in the shell can readily be secured from 

 most markets. 



