Ill] THE PEARL OYSTER 41 



Fertilisation depends then not upon any act of the 

 pearl ousters themselves, except of course the emis- 

 sion of the eggs or sperms into the sea-water, but 

 upon the accidental meeting in the sea of the two 

 microscopic germs. 



Where this method is adopted we always expect 

 to find myriads of eggs developed in the individual, 

 to make sure in the terrific mortality that afterwards 

 occurs that some small but adequate proportion will 

 survive. This is the case in the pearl oyster. It 

 would be a tax on the imagination to picture what 

 would happen if every egg produced by the pearl 

 oysters developed into even a young oyster. This, 

 how ever, applies to many thousands of other species 

 in the animal world. 



The description of the pearl oyster should give an 

 idea of the type of animal that is popularly known as 

 a bivalve. It would not be difficult for the reader to 

 find most of the structures described in the edible 

 oyster, the scallop, the clam, the edible mussel, cockle 

 and fresh-water mussel. The three last-named might 

 look rather different, but this is simply due to the 

 fact that instead of possessing one large central 

 muscle for closing the shell, they possess two, one at 

 each end of the body. 



