THE MUSSEL OR CLAM AND OTHER BIVALVES 153 



sal of the species through the migrations of the fish. Only in 

 this way can we account for the rapid colonization of certain 

 streams by mussels, since the adult plows its way through the 

 muddy bottom very slowly. 



The Oyster. - - The oyster is the best-known relative of the 

 mussel principally because of its use as food. Oysters are 

 widely spread, being found on all seacoasts. Those occurring 

 in different localities often belong to different species; those on 

 the Atlantic coast are known by the name Ostrea virginiana, 

 and the principal species in Europe as Ostrea edulis. In Japan 

 lives a species that sometimes grows to be three feet long. 



The adult oyster is unable to move from place to place. It 

 lies on the bottom of the sea, near the coast, attached by its left 

 valve, which is the larger. This attached condition probably 

 explains the absence of the foot in the oyster, since this loco- 

 motor organ could be of no use to a stationary animal. The 

 lack of the foot renders the oyster soft and is really responsible 

 for the oyster's edible quality. The mussel, on the other hand, is 

 not relished as a food because of the toughness of its muscular 

 foot. In general structure the oyster differs very little from the 

 mussel. 



" Few realize what an enormous business the oyster trade has 

 become in the United States. The value of it is stated to be 

 over thirteen million dollars annually, twenty-five million bush- 

 els of oysters being taken from the Chesapeake alone. The 

 edibility of the oyster has been known from early times, for vast 

 heaps of empty oyster-shells, known as kitchen middens, occur 

 in various parts of the world. Some of them are of such size 

 and extent as to warrant the belief that their formation must 

 have required centuries. Shell mounds are found along the 

 coasts of Florida and are of some archaeological value. The 

 cultivation of oysters, as recorded by Pliny, dates from the first 

 century B.C. 



' The poet Gay's opinion of the first man who ever ate an 

 oyster is expressed thus: 



