Now this mantle is one of
Nature's cleverest inventions. It is able to take the substance called
_lime_ from the food of the animal, and to use it as building stuff.
[Illustration: PRECIOUS WENTLETRAP.]
The shell is built to fit the soft body. When a Periwinkle is hatched
from the egg, it is as big as a pin's head. It eats and grows, and the
shell must therefore be made larger. So the mantle is stretched out, and
it puts a film of lime to the edge of the shell. Bit by bit the shell is
thus added to by the wonderful mantle. Look at a snail's shell, and
notice the lines which show how many times the little house has been
made larger.
Each kind of shell-builder has its own style of building. If you go to a
museum and examine the shells gathered from all over the world, you are
surprised at their wonderful shapes, markings and colours. Another
surprising thing is their size. Some are enormous, so large that they
make good washing-basins. Others are so small that you can hardly see
them. Each one was made by the folds of the mantle of the animal that
lived in it.
In our coloured pictures you see many different kinds of shells, some of
them built by uni-valve molluscs and some by bi-valve molluscs.
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