OYSTERS, CLAMS, MUSSELS 227 



boats. It has been estimated that more than twenty-five 

 million bushels of oysters are gathered from the beds in the 

 United States each year. 



The west-coast oyster, Ostrea lurida, is much smaller than 

 the eastern oyster and has a much thinner shell. It differs also 

 in being hermaphroditic and viviparous; that is, both ova and 

 spermatozoa are produced in the same individual and the eggs 

 are fertilized in the gill and mantle cavities and here also they 

 pass through the early stages of development. At spawning 

 season, when these young embryos are set free, they have 

 already reached the swimming stage and are soon ready to 

 attach themselves to some convenient shell or other collector, 

 where they remain fixed through life. 



The area available for oyster cultivation is much less on the 

 Pacific coast than on the Atlantic, but the total output of 

 oysters from the state of Washington amounts to about 

 $300,000 annually. Many years ago shipments of eastern 

 oyster spat or seed were made to the Pacific coast and planted 

 in San Francisco Bay where they were allowed to remain until 

 they reached a marketable size. Now many carloads are 

 shipped from the east each season and planted on the tide flats 

 in California and Washington, the introduced oysters attaining 

 a good size and a flavor hardly excelled in their native waters. 

 On account of the low temperature of the water during the 

 spawning season most of the young of the eastern oyster are 

 killed while they are swimming at the surface, and so the beds 

 of eastern oysters have to be replanted when the marketable 

 oysters are removed. 



There are two common species of oysters native to Europe. 

 The smaller flat oyster, Ostrea edulis, occurs along the northern 

 shores, and in many respects resembles our Pacific coast oyster. 

 Like the latter it is hermaphroditic. The Portuguese oyster, 

 0. angulata, is found on the southern shores and resembles 

 more our east-coast oyster in size and methods of breeding, but 

 is not so highly esteemed for food as the smaller northern 

 oyster. 



The European oysters have been cultivated since the earliest 

 times, and in many places the collecting of the spat on especi- 



