force the water along, over the gills and towards the
mouth. In this way the Oyster breathes the air which is in the water;
but not only that. As we have already noticed, there is a kind of
"vegetable dust" in the sea. This is driven to the Oyster's mouth and
swallowed. The Oyster, fixed in its "bed," unable to hunt for food, thus
makes its dinner come to it. What a strange use for a "beard"! It not
only serves as lungs, but also helps the animal to catch its "daily
bread"!

Another mollusc used as food is the Cockle, and its shell is one of the
commonest found along the shore, especially near sandy places. It lives
in sand, and can bury itself so quickly that you would have to use your
spade with all your might in order to keep pace with this little
shell-fish. Where Cockles have buried themselves you will see spurts of
water and sand, showing where they are busy down below in the wet sand.

Besides being so skilful at digging, the Cockle is a first-rate jumper.
If left on the beach, it jumps over the sand, towards the sea, in the
funniest way. It is strange to see a quiet-looking shell suddenly take
to hopping and jumping like an acrobat.

To perform this astonishing feat the Cockle makes use of its foot, which
is worked by very strong muscles. It is large and pointed, and bent: if
the Cockle wishes to move quickly, it stretches out its foot from
between the shells, as far as it will go. Then, by using all its power,
it leaps backwards or forwards in a surprising manner.

There are many other interesting molluscs, besides those we have looked
at. The Piddock, or Pholas, is a smallish, rather delicate one, with a
soft foot. But this foot is a most wonderful boring tool, fitted with a
hard file. Hard rocks and wood are perforated by these little molluscs.
Indeed, they are a positive danger, for they pierce the wooden piles of
piers, and weaken them. They cannot pierce through iron, however, and so
iron plates or nails are used to protect the piles from their
onslaughts. You will often see stones and rocks riddled by the Piddock
as if they were as soft as cheese. Chalk, sandstone, or oak, it is all
the same to the Piddock, which rasps them away with its file. When the
points of this strange instrument are worn out with all this hard wear,
a new set takes their place.

EXERCISES

1. How does the Mussel anchor itself?

2. Describe how the shells of the Oyster are opened and closed.

3. What is the food of the M