XV 

 MATERNAL CARE AND MOLLUSCS 



THE American and Portuguese species of oysters, 

 which are called respectively Ostrea virginiana 

 and Ostrea angulata, as opposed to the common oyster, 

 which is known as Ostrea edulis, are not hermaphrodite 

 like the latter, but have distinct males and females. 

 Moreover, the young are not fertilised within the parent's 

 body, nor do they pass their earliest stages of growth 

 within the parent's shell adhering to the " beard," or gills, 

 as in the common oyster. The eggs (Fig. 31) are, on the 

 contrary, discharged by the females into the sea, and at 

 the same time the males discharge a cloud of microscopic 

 sperm filaments, or spermatozoa (Fig. 32), which dart 

 about in the water and fertilise the eggs. That is a more 

 prodigal and less certain process than that pursued by 

 the common oyster. The American and Portuguese 

 oyster have to pay for it. The female produces in one 

 season not a million eggs, as does the common oyster, 

 but nine millions. And out of every fifty million so pro- 

 duced (in some five or six years) only a single male and 

 a single female individual, taking the whole oyster 

 population of these species into consideration, survive 

 to maturity. 



This enormous excess of egg-production in order to 

 ensure the survival of a single pair to replace their 



