24 
little plates are the two valves of the shell, and in the oyster 
they are separated from each other from the first, and make 
their appearance independently. 
Soon after they make their appearance, the embryos cease 
to crowd to the surface of the water, and sink to various 
depths, although they continue to swim actively in all direc- 
tions, and may still be found occasionally, close to the surface. 
The region of the body which carries the cilia now becomes 
sharply defined, as a circular projecting pad, the velum , and 
this is present and is the the organ of locomotion at a much 
later stage of development. It is shown at the right side of 
the figure in Figure 37, and in Figure 45 it is seen in 
surface view, drawn in between the shells, and with its cilia 
folded down and at rest, as they are seen when the little 
oyster lies upon the bottom. 
The two shells grow rapidly, and soon become quite regu- 
lar in outline, as shown in Figures 37 and 44, but for 
some time they are much smaller than the body, which 
projects from between their edges around their whole circum- 
ference, except along a short area, the area of the hinge, upon 
the dorsal surface, where the two valves are in contact. 
The two shells continue to grow at their edges, and soon 
become large enough to cover up and project a little be- 
yond the surface of the body, as shown in Figure 44, and 
at the same time muscular fibres make their appearance 
and are so arranged that they can draw the edge of the 
body and the velum in between the edges of the shell, in the 
manner shown in Figure 4 5. In this way that surface of the 
body which lines the shell becomes converted into the two 
lobes of the mantle, and between them a mantle cavity is 
formed, into which the velum can be drawn when the animal 
is at rest. While these changes have been going on over the 
outer surface of the body, other important internal modifi- 
cations have taken place. We left the digestive tract at the 
stage shown in Figure 36, without any communication with 
the exterior. 
Soon the outer w T all “of the body becomes pushed inwards, 
to form the true mouth, at a point (Figure 37), which ig 
