MUSEUM OF COMPARATIVE ZOOLOGY. 53 
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 (Pl. ITI. Fig. xx. 2). 
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 lino ; 
they are white, spherical, nearly flat, and the foot protrudes between the 
gaping valves, which are soft and somewhat membranaceous ; they are 
very similar to the adult, but are somewhat flatter (Pl. III. Fig. xr.). 
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 Cyclas (Sphærium) 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. 
