408 
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 ( Ostreidai) 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 OF COMMON OYSTER. 
a, Position of mouth ; b, Mantle ; c, Adductor ; d, Junction of mantle-lohes ; 
e s Gills. (Nat. size.) 
