THE MANTLE-COVERED ANIMALS. 105 
opened you may see two transparent flaps, with 
thickened edges, one lying above, and the other 
below the oyster in its shell (m, Fig. 41, p. 108), 
and these two flaps are the two halves of the mantle, 
which, when they touch, enclose the animal between 
them. In the periwinkle the mantle is equally visible, 
but this time it is all in one piece, and forms a com- 
plete transparent tube, out of which the animal pokes 
its head and its crumpled foot bearing the horny lid, 
or operculwn, which closes the shell. 
When the periwinkle was very young he was not 
larger than the head of a small pin, and his shell was 
like a minute transparent bead. But as his body 
grew larger it was necessary for his home to be both 
larger and stronger. Then he stretched out his 
mantle till it reached over the edge of the tiny shell, 
and gave out from it a thin film, in which were grains 
of lime which had been passed through his body into 
the mantle. This film, clinging to the inside of the 
shell and stretching over its edge, formed a fresh 
internal layer, and a new rim to the mouth. The 
rim, however, was not white, but coloured by little 
cells of dark paint or pigment, secreted in the border 
of the mantle. The shell was now a little larger and 
a little thicker, and the mantle was drawn in till a 
still more roomy house was needed, and then the 
same thing took place again ; and so the building 
went on till the shell was completed, the lines round 
and round it marking the rims which had each in 
their turn formed its mouth. 
In this way the mantle, not only of the periwinkle 
but of all the mollusca builds up the shell for the 
animal to live in. In the oyster each half of the 
