THE CLAM AND OTHER EIVALVED SHELL-FISH. 71 



through the oyster's shell, and feeds upon the animal until 



it kills it (Fig. 70). The starfish also destroys hundreds 



of thousands of dollars' worth of the oysters. 



The starfish protrudes its capacious stomach, 



turning it inside out between the open valves of 



the oyster, and sucks in the soft parts. They 



are so numerous and destructive that they kill 



great numbers of oysters. Mussels also suffer 



from their attacks. When the oyster is dead, 



its shells are bored in all directions by a sponge, 



which causes the shell to break up and thus aid FIG. 70. The 



in forming the sand or mud of the sea-bottom. 



We thus see how varied is the life of the oyster, and, in 

 fact, of all shell-fish. Early in life they have a different ap- 

 pearance, different habits, different enemies, and are, so to 

 speak, different animals from what they are when grown 

 up. Were it not for their power of adapting themselves 

 to their different surroundings they would probably all 

 die out. We also see that nothing alout them is wasted. 

 The surplus oyster population serves as food for other ani- 

 mals, and even the fragments of their dead shells are used 

 to build up the sea-bottom, to aid in forming layers of sand 

 and mud ; and thus through past ages the shells of shell- 

 fish, whether whole or broken into pieces, have been used 

 to build up the rocks which form a part of our earth. 



Indeed, a large part of the earth's population consists of 

 the bivalves. There are about 14,000 different species of 

 these shell-fish ; of these, however, from 8000 to 9000 are 

 extinct or fossil. The species differ from one another in 

 various ways, but nearly all ayrce in having two valve*, 

 while the animal is headless, with a small foot, though this 

 may be occasionally, as in the oyster, wanting, and there 

 arc usually t/i'o pairs of leaf-like ntlls. By examining the 

 kinds we have described as examples or types, the student 

 can get a good idea of the appearance of all the members of 

 the class Lamcllibranchiata, which means " shell-fish with 

 leaf -like gills," 



