tle 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.

Wherever there are weeds along the shore you can find whole armies of
the Periwinkle--the "Winkle" we all know so well. It browses there,
among the weeds, just as its cousin, the land Snail, browses on your
cabbages. You must have seen the little door with which the Periwinkle
closes the entrance to his house. The land Snail does not own a door,
but he makes one when he goes to sleep for the winter.

The Periwinkle crawls on a broad, slimy foot, which is put out from the
shell. It is stretched on this side or that, and so draws him and his
home in any direction. There are two sensitive feelers in front of his
head; and behind these are two short stalks, on each of which is a tiny
eye. If alarmed, the Periwinkle can shorten his body, and pull it back
into its shell, closing the entrance with the horny door.

But the strangest part of him is the tongue. It is not for tasting, but
for rasping. It is like a long, narrow ribbon, on which are hundreds of
tiny points, all sloping backwards. They are arranged three in a row.
The Periwinkle rasps the seaweed with his tongue, and so scrapes off his
dinner. Of course the teeth wear away.

[Illustration: COWRIES.]

But only part of the toothed ribbon is used at a time, so there are
plenty of teeth behind the worn ones, ready to take their place.

The shell, as we have seen, is made of _limestone_. But the teeth are
made _of flint_. This is a hard substance, so hard that it is used for
striking sparks.

Now we will look at a shell-builder, the Whelk, who uses his flinty
tongue in quite another fashion. The Whelk does not care for a vegetable
dinner. He prefers to