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.
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.
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.
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 quant.i.ties 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 sh.e.l.ls 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 sh.e.l.ls; 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 sh.e.l.ls, 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 sh.e.l.ls 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 sh.e.l.ls, but they were snapped together, and the mouse was firmly held in the trap.
Between the hinge of the two sh.e.l.ls there is a pad, which acts like an elastic spring, and forces the sh.e.l.ls 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 sh.e.l.ls open with the help of its strong tube-feet. The whelk and his cousins know how to bore a hole in the sh.e.l.l, and suck out the helpless Oyster. Then there are certain sponges, with the strange habit of making holes in sh.e.l.ls, and living in and on them. Sometimes the Oysters are stifled in their "beds" by other Oysters settling and growing over them. Thick ma.s.ses of Mussels may cling to them and suffocate them. And grains of sand sometimes get in the hinges of their sh.e.l.ls, 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 sh.e.l.ls 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 sh.e.l.ls, 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 c.o.c.kle, and its sh.e.l.l is one of the commonest found along the sh.o.r.e, 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 sh.e.l.l-fish. Where c.o.c.kles 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 c.o.c.kle 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 sh.e.l.l suddenly take to hopping and jumping like an acrobat.
To perform this astonishing feat the c.o.c.kle makes use of its foot, which is worked by very strong muscles. It is large and pointed, and bent: if the c.o.c.kle wishes to move quickly, it stretches out its foot from between the sh.e.l.ls, 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.
1. How does the Mussel anchor itself?
2. Describe how the sh.e.l.ls 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?
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