4o8 



MOLLUSCS. 



examined for pearls. Pearl-fishing is also carried on in the Persian Gulf, the 

 South Sea Islands, Panama, West Indies, and a few other localities. The spherical 

 pearls, like the shell itself, are produced by the mantle, and probably other parts of 

 the animal also. They consist of layers of pearl, deposited round some foreign 

 substance which has intruded itself within the shell. This may be a grain of 

 sand, but is said to be usually an egg of the mollusc itself, which has not properly 

 developed. The hammer-oyster (Malleus), Vulsella, Crenatula, and Melina are 

 other interesting existing forms of Aviculidce, and many extinct genera have 

 been referred to this family. 



The shells of the oysters (Ostreidw) are so familiar that no description is 



necessary. The animal 

 has no foot, and the 

 mantle -lobes are free 

 nearly all round, the 

 borders being fringed 

 with short papillae. 

 The shells, excepting 

 in the very early 

 stages, are closed by 

 a single adductor. The 

 sexes are separate in 

 the American oyster 

 (Ostrea virginiana), 

 but united in the 

 British 0. edulis. In 

 a gastronomic point of 

 view the oyster stands 

 far above all other 

 molluscs, and its arti- 

 ficial cultivation was 

 practised by the 

 ancient Romans, and 

 at the present time 

 forms a most import- 

 ant industry in many 

 parts of the globe. 

 The oyster is- very 

 prolific, a single in- 

 dividual of the common species having been estimated to contain over a million 

 embryos, whilst the American form is said to discharge ten times as many. 

 0. edulis is not full-grown until about five to seven years old. Oysters are 

 incapable of motion, and attach themselves to other shells, rocks, and other 

 substances by the convex or deeper valve. During May, June, and July, the 

 eggs are discharged into the gills, where they remain until hatched ; and it is 

 during this period that oysters are out of season. Oysters are cosmopolitan; 

 wherever there is a rocky coast, excepting in Arctic climates, they are sure to 



ONE VALVE AND SOFT-PARTS OP COMMON OYSTER. 



a, Position of mouth ; b, Mantle ; c, Adductor ; d, Junction of mantle-lobes ; 



(Nat. size.) 



Gills. 



