MUSEUM OF COMPARATIVE ZOOLOGY. bo 



These capsules contain the eggs, and the young are developed in 

 them ; each one contains but one egg or young one. As soon as the 

 young has reached a certain size, the capsule bursts and the young is 

 ejected into the gill-sac (PI. III. Fig. ix. i). 



It is in the gill-sac that the young grow, and when they have attained 

 half a line in length they are ejected through the crescent-shaped lateral 

 opening of the gill-sac and introduced into the cavity of the mantle ; 

 they then leave the parent and become free agents. 



This genus brings forth young all summer. 



We find in the gill-sac, at one and the same time, capsules and young, 

 both large and small. 



All these changes, I am led to believe, occupy but a very limited time. 



Young taken out of the capsules measure about one quarter of a line ; 

 they are -white, spherical, nearly flat, and the foot protrudes between the 

 gaping valves, which are soft and somewhat membi'anaceous ; they are 

 very similar to the adult, but are somewhat flatter (PI. III. Fig. xi.). 



The young either lie free in the gill-sac or are suspended to its mem- 

 branes, but they are not attached to it. No special motion is observed 

 in the young while in the gill-sac ; they grow rapidly, the shell becomes 

 gradually of -a pale yellowish color, the foot gradually withdraws within 

 the valves, and when the shell attains one half a line in size the valves 

 are closed ; the shell then becomes harder, the color turns to a bright 

 sulphurous, and its shape becomes more convex ; when the young attain 

 a size of one and one half lines, they leave the parent. 



The young resemble the parent during all stages of their development; 

 they undergo no metamorphosis. 



• The genus Gyclas {Sphoerium) belongs to the class Vivipara. The eggs 

 or the capsules containing them are not in a sufficient state of develop- 

 ment to enable them to be hatched out if detached from the parent. 



