THE SHELL-ANIMALS 409 



many species indicated by minor differences in shape and 

 markings on the shells. 



The two most common marine dams sent to market in 

 eastern states are the hard shell, " little neck," or " quahog " 

 clams of the genus Venus (Fig. 141), which are the favorites 

 in New York markets ; and the soft shell, " long neck " 

 clams of the genus Mya, which are demanded by the best 

 trade in the Boston markets. The " little neck " and 

 " long neck " refers to the length of the siphons. 



The giant clams of the Indian Ocean sometimes weigh 

 500 pounds and may be two feet long. The smallest bi- 

 valve is a little fresh-water species 1 cm. long. 



Most remarkable of the allies of the clams is the ship-worm 

 (Teredo) which bores burrows in ships' timbers, piles, etc. 

 The body of the teredo is worm-like. The small shell, 

 used for cutting the burrow, is at the inner end of the burrow. 

 The siphons project above the surface of the wood whenever 

 the " worm " is not disturbed, and the ingoing current of 

 water carries in food (small organisms) and provides for 

 respiration. The remedy against these animals is copper 

 sheathing, or soaking timbers with creosote and tar. 



The oyster is peculiar in that the eggs develop within a 

 few hours after fertilization into free-swimming larvae. 

 These finally become attached and metamorphosed into 

 the oyster form. The left valve of an oyster's shell is at- 

 tached to a stone or to another oyster shell. In conse- 

 quence of this sedentary mode of life the oyster has no use for 

 a foot (organ of locomotion), and this is absent. One reason 

 why the flesh of oysters is so delicate in texture is that there 

 is no hard muscular foot; and the great development of 

 this organ makes river mussels " too tough " to be palatable. 

 The mouth of an oyster is near the ligament which hinges 

 the shells together; but as in case of clams, currents of 

 water from the opposite side of the shell carry in food 

 particles. 



