CIVIC PROBLEMS RELATING TO MOLLUSKS 279 



unlike the parent. This free-swimming period enables the species to 

 be distributed widely by tides and currents, and it also offers opportu- 

 nity for the culturist to increase his stock almost beyond belief by mak- 

 ing conditions more favorable for the young. The embryo oyster, for 

 example, swims for from one to six days. By the end of this time the 

 shell begins to form and it must sink to the bottom. If it happens to 

 land on a clean, hard surface, it may survive ; if it falls in an ooze of 

 slime or silt, it is quickly smothered. So the oyster culturists scatter clean 



FIG. 126. Pearl fishing in the Mississippi River 

 Photograph by the author 



shells "cultch " over the bottom about the beginning of the spawn- 

 ing season. If too many of the young oysters succeed in attaching to 

 these, they must be dredged up and the clusters broken apart and re- 

 planted evenly over the bottom, so that all may find food and have 

 room to grow. The left valve of the oyster, which is spoon-shaped, 

 always makes contact with the support and is quickly cemented to it by 

 a secretion like that which forms the shell. The other marine bivalves 

 anchor by means of a peculiar mechanism, the byssus (byssos, " fine textile 

 fiber "), which is secreted by a gland in the foot as a viscous fluid that 

 hardens on contact with the water. The byssus is retained in Mytilus, 

 but is lost in the clams and scallops soon after they assume adult form. 

 Fresh-water mussels (" clams ") Unionid&. The fresh waters of east- 

 ern North America contain about 600 species of lamellibranchs. 

 Tough, and muddy of taste, they were considered as food only for 



