Part 7 (1/2)

When you next gather sh.e.l.ls on the beach, look at them closely; in some you will see where Mr. Whelk, the burglar, has been at work. He needs but a small entrance to enable him to suck out his helpless prey at his ease. Is it not strange that this creature, with a body as soft as your tongue, should earn its living by breaking into houses made of hard sh.e.l.l!

There are other molluscs which find their meals in this strange manner, and many others which, like the Periwinkle, feed more easily on seaweed.

One of these, the Limpet, you can always be sure of finding at low tide; indeed, there are so many Limpets on the rocks that it would be hard _not_ to see them. You will know, if you have tried to force a Limpet from its hold on the rock, how very tightly it clings. It is as if the sh.e.l.l were glued or cemented by its edges.

Yet there is no glue or cement used, but only a simple dodge. The Limpet has a broad ”foot,” which almost fills up the opening of its sh.e.l.l. Like the foot of the Snail, it is used when the animal wishes to take a walk; but it serves another purpose too. It can be used as a sucker; and it is this which enables the Limpet to cling so firmly to its rock.

When the tide is out, the Limpet clings to the rock, its soft body tucked safely away in the sh.e.l.l. Its feeding time comes when the water covers the rocks once more. Then the Limpet's sh.e.l.l may be seen to tilt up, and a foot, and a head with feelers and eyes, come out. The Limpet crawls to the seaweed and begins to browse, using a rasp like that of the Periwinkle. It then crawls back to its own place on the rock. In time this resting-place becomes hollowed out, and the Limpet's sh.e.l.l fits into the groove thus made.

Limpets are useful as bait for fish. The Whelk and Periwinkle are gathered in immense numbers, and are used 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 sh.o.r.e.

EXERCISES

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

2. What is the Periwinkle's sh.e.l.l made of?

3. Describe how the Periwinkle eats seaweed.

4. How does the Whelk obtain its food?

5. Give the names of three one-sh.e.l.led molluscs.

LESSON XI.

Sh.e.l.lS AND THEIR BUILDERS (2)

THE MUSSEL AND OYSTER.

As everyone knows, the Mussel and the Oyster live between two hinged sh.e.l.ls. In the last lesson we called them _bi-valve molluscs_, which is only another way of saying ”soft-bodied animals with two sh.e.l.ls.” 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 sh.e.l.ls.

Like the Periwinkle and other sh.e.l.l-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 sh.e.l.l.

The Snail, and other one-sh.e.l.led molluscs, poke their heads out of the sh.e.l.l when feeding or moving. Oysters and their two-sh.e.l.led 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 ma.s.ses 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 ma.s.s 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 cl.u.s.ters 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 sh.e.l.ls. Now the threads are fixed by the foot, just where the Mussel wishes to anchor himself. They are made from a liquid which forms in the body of the creature. This liquid hardens in the water so that it can be pulled out into long, fine threads.

Our ordinary Mussels do not make very long threads, but those of some kinds are so long that they can be woven into silky purses or stockings.

The Mussel which makes such long anchor-threads might be called ”the silkworm of the sea.”

If the Mussel is such a stay-at-home, how does he find his food? The answer is, that the food comes to him, brought by the ever-moving water.

There are countless specks floating in the sea, mostly specks of vegetable stuff. These settle on the floor of the sea, just as dust settles on our house-floors; and the waves wash this ”sea-dust” hither and thither. The Mussel or Oyster, with sh.e.l.ls gaping wide open, is bound to get some of this food with the water which enters the sh.e.l.ls.

The Oyster has no ”foot,” and is fixed in one place nearly all its life.