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 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 shells gaping wide open, is bound to get some of this food with the water which enters the shells.

The Oyster has no "foot," and is fixed in one place nearly all its life. It is an interesting animal; and one of such value as food, that hundreds of thousands of Oysters are reared in special "beds," and sent to the market at the proper season. Our British Oysters were famous even in the time of the Romans; they were carefully packed and sent to Rome, and, at the Roman feasts, surprising quantities of them were eaten.

Many sea-animals have wonderfully large families, but the Oyster, with its millions and millions of eggs, beats most of them. Strangely enough, its eggs are not sent into the sea at once, but are kept between the Oyster's shells until they hatch. Needless to say, these babies are very small indeed, else their nursery could not contain them all Though so small that thousands of them together look more like a pinch of dust than anything else, yet each one has two thin shells; so that, if you eat the parent Oyster, they grate on your teeth like sand. Oysters, at this time, are "out of season"--that is, unfit for food.

At the right moment, the Oyster gets rid of its numerous family. It opens its shells, then shuts them rapidly; and, each time this happens, a cloud of young Oysters is puffed out like smoke. Now these mites must fend for themselves in a sea full of foes.

They have no defence, and countless numbers of them are gobbled up by crabs, anemones, and others. If this did not happen, the sea would soon be paved with Oysters.

For a time, the baby Oysters--which are known as "spat"--are able to swim here and there. In rough weather they are driven far into the deeps of the ocean, and lost. The rest of them, before they have been free for two days, settle on the bed of the sea--sometimes on their own parents; and there they remain for life. Only a very few out of each million become "grown-ups"--the rest are eaten by enemies, or smothered in mud or sand. In a year or so they are as big as half-a-crown. In five years they are fine, fat grown-up Oysters--that is to say, if they have not been dredged up from their bed and sent to market.

Their shells open and shut like a trap. You may have seen a picture of an inquisitive mouse trapped by an Oyster. Thinking to have a nice taste of Oyster, the mouse had poked its head into the open shells, but they were snapped together, and the mouse was firmly held in the trap.

Between the hinge of the two shells there is a pad, which acts like an elastic spring, and forces the shells open. The Oyster can close them by means of a strong muscle. They are its only defence, so it closes them at the least hint of danger.

Even these thick walls are sometimes of no avail, as we saw in our talk on "Five-fingered Jack." We saw how the starfish forces the shells open with the help of its strong tube-feet. The whelk and his cousins know how to bore a hole in the shell, and suck out the helpless Oyster. Then there are certain sponges, with the strange habit of making holes in shells, and living in and on them. Sometimes the Oysters are stifled in their "beds" by other Oysters settling and growing over them. Thick masses of Mussels may cling to them and suffocate them. And grains of sand sometimes get in the hinges of their shells, so that they cannot close up the house when they wish.

Like the other animals which are useful as food, Oysters have been carefully studied and cultivated by man for many, many years. The story of the Oyster-beds is a long and interesting one.

Oysters feed in rather a strange way. You may have looked inside the shells and seen two delicate dark-edged fringes, known as the "beard." This fringe is the Oyster's gills or breathing arrangement. Trace the "beard" as far as the hinge of the shells, and you see the mouth with its white lips. If you could watch the creature having its dinner, you would see a constant stream of water flowing over the gills and towards the mouth.

What makes the water move in that way? The gills are covered with very tiny lashes, like little hairs. There are so many of them that, as they keep moving, they 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 Mussel?

4. Of what use is the "beard" of the Oyster?

5. Why is the Oyster called a bi-valve?

6. Why is the Oyster sometimes unfit for use as food?