MOLLUSCA. 107 



greater part perish before achieving their develop- 

 ment, if they are abandoned to themselves in the 

 ocean. 



These animals spawn about the commencement 

 of spring, and, according to most naturalists, they 

 fecundate their own eggs ;* but instead of aban- 

 doning its- spawn, like many other shell-fish, the 

 oyster keeps it lodged between the gills, where it 

 undergoes the process of incubation. This process 

 continues for some time, and that is why oysters 

 are not generally esteemed from May to September. 



But the depth of the water in which the oyster 

 lives seems to have a considerable influence upon 

 the time of spawning. In its first state, the young 

 oyster exhibits two semi-orbicular films of trans- 

 parent shell, which are constantly opening and 

 closing at regular intervals. As they grow larger 

 they attach themselves to the rocks ; but for this 

 purpose they do not secrete long silky strings, as 

 the mussels do. When they find nothing solid to 

 adhere to, they become cemented together in large 

 quantities, each adhering to its neighbour, and con- 

 stitute solid shoals or oyster-beds, which sometimes 



* The gasteropod and bivalve mollusca are all hermaphrodite ; 

 but with the snails and slugs we have been studying, the concourse 

 of two individuals (four organs) is necessary to ensure reproduc- 

 tion ; with bivalves, such as the oyster, it appears the male organ 

 can render fertile the products of the female organ in the same 

 animal. 



