



Centimeters 



Figure 276. — Actively feeding C. virginica. The valves are wide open, the pallium and the tentacles are extended out- 

 ward and the cloaca, the adductor muscle, and rectum are clearly visible. Drawn from life. The opening between 

 the valves is slightly e.xaggerated. 







Centimeters 



Figure 277. — Position of the mantle and tentacles of C. virginica shortly before the discharge of eggs through a small 

 window left open (right side of figure). Drawn from life. 



bj' the lateral cilia. Laboratory observations 

 show that when the valves open the gill lamellae 

 also expand and the water tubes dilate. xVt the 

 same time the closing of the cloaca and of the 

 mantle cavity cuts off the access of water from the 

 outside. As a result, a suction is produced inside 

 the water tubes by the expansion of gill lamellae, 

 forcing the eggs into the water tubes and through 

 the ostia to the surface of the gills. 



Eggs of larviparous oysters {0. lurida and 0. 

 ediilis) also pass through the gills but are retained 

 in the pallial cavity until fully developed larvae 



are formed and escape from the mother's body. 

 Stafford (1913) thought that the eggs of 0. 

 lurida are too heavy to be carried by the respira- 

 tory current and so fall into the water tubes and 

 are forced tlu-ough the ostia by the pressure of 

 their own mass. The correctness of such an ex- 

 planation seems doubtful. The act of spanning 

 in the larviparous species has not been studied 

 adequately, probably because the ovulation and 

 passage of eggs into the pallial chamber proceed 

 without any outward indication (Yonge, 1960). 

 Eggs f)f these species are fertilized inside the body 



ORGANS OF REPRODUCTION 



305 



