ed by us for food. Perhaps you
have seen the egg-bundle of the Whelk. It contains many eggs when first
laid in the sea. Each egg is as big as a pin's head. They swell in the
water, until the yellowish bundle is three times as large as the Whelk
that laid it. You often see the empty bundle blown by the wind along the
shore.

EXERCISES

1. Give the names of two bi-valve molluscs.

2. What is the Periwinkle's shell made of?

3. Describe how the Periwinkle eats seaweed.

4. How does the Whelk obtain its food?

5. Give the names of three one-shelled molluscs.

LESSON XI.

SHELLS AND THEIR BUILDERS (2)

THE MUSSEL AND OYSTER.

As everyone knows, the Mussel and the Oyster live between two hinged
shells. In the last lesson we called them _bi-valve molluscs_, which is
only another way of saying "soft-bodied animals with two shells." Have
you ever opened an Oyster? It is a tug-of-war, your skill and strength
against the muscles of the animal inside the tight shells.

Like the Periwinkle and other shell-builders, these creatures owe their
strong houses to a wonderful _mantle_; but in this case the mantle is in
two pieces instead of one. You can imagine the Periwinkle's mantle as a
tube enclosing the animal's body. The mantle of the Mussel or the Oyster
is in two pieces; and each half forms its own shell.

The Snail, and other one-shelled molluscs, poke their heads out of the
shell when feeding or moving. Oysters and their two-shelled cousins
cannot do this, for the simple reason that they have no heads!

In some places you see that the rocks at low tide are covered with
Mussels. In dense black masses they cling to the rocks; and, though
heavy waves bang them like so many hammers, they stick tight. Little
Mussels and big ones, they form a mass so thick that baby crabs and
other creatures use them as a hiding-place. On the piers and groynes,
and the woodwork of the harbour, you can see other clusters of Mussels;
they are placed where the high tide covers them.

Have you noticed how the Mussel anchors himself? He uses a bunch of
threads, like so many cables or tiny ropes. It is interesting to know
how these threads are made.

The Mussel is, as a rule, a stay-at-home, but he can move from place to
place if he likes. He has a long, slender foot which can be pushed out
of the shells. Now the threads are fixed by the foot, just where the
Mussel wishes to anchor himself. They are made from a liquid which forms