  SHELLS AND THEIR BUILDERS (1). THE PERIWINKLE, WHELK AND LIMPET. Most of the shells which you find scattered over the shore are empty. The little animals which built them are gone; and their empty houses, of wonderful shapes and colours, are all that you find. Let us look at the builders of these pretty homes. The shell-builders have soft, juicy bodies, and they are put in one big division of the animal kingdom--the mollusca, which only means soft-bodied. Some of these molluscs do not build shells. But most of them build a shelly house for themselves; they do this to defend their soft bodies from the attacks of a host of enemies. Some build two shells--the Oyster and Mussel do, as you know. These are called bi-valves; that is, two valves or shells; and others, like the Garden Snail, the Limpet, and Periwinkle, have one shell only, and so are called uni-valves. The crab, and other crustaceans, also have a hard covering to their soft bodies; but it is not at all like the shell of a Snail, or other mollusc. The Snail's shell is like the little boy's suit which is altered and made bigger as the boy grows. The crab's covering is a suit which cannot be altered. It must be thrown away, and replaced by a larger one. The body of the shell-builder is wrapped in a soft covering, a kind of outer coat, which is called the mantle. Now this mantle is one of Nature's cleverest inventions. It is able to take the substance called lime from the food of the animal, and to use it as building stuff. The shell is built to fit the soft body. When a Periwinkle is hatched from the egg, it is as big as a pin's head. It eats and grows, and the shell must therefore be made larger. So the mantle is stretched out, and it puts a film of lime to the edge of the shell. Bit by bit the shell is thus added to by the wonderful mantle. Look at a snail's shell, and notice the lines which show how many times the little house has been made larger. Each kind of shell-builder has its own style of building. If you go to a museum and examine the shells gathered from all over the world, you are surprised at their wonderful shapes, markings and colours. Another surprising thing is their size. Some are enormous, so large that they make good washing-basins. Others are so small that you can hardly see them. Each one was made by the folds of the mantle of the animal that lived in it. In our coloured pictures you see many different kinds of shells, some of them built by uni-valve molluscs and some by bi-valve molluscs. Wherever there are weeds along the shore you can find whole armies of the Periwinkle--the "Winkle" we all know so well. It browses there, among the weeds, just as its cousin, the land Snail, browses on your cabbages. You must have seen the little door with which the Periwinkle closes the entrance to his house. The land Snail does not own a door, but he makes one when he goes to sleep for the winter. The Periwinkle crawls on a broad, slimy foot, which is put out from the shell. It is stretched on this side or that, and so draws him and his home in any direction. There are two sensitive feelers in front of his head; and behind these are two short stalks, on each of which is a tiny eye. If alarmed, the Periwinkle can shorten his body, and pull it back into its shell, closing the entrance with the horny door. But the strangest part of him is the tongue. It is not for tasting, but for rasping. It is like a long, narrow ribbon, on which are hundreds of tiny points, all sloping backwards. They are arranged three in a row. The Periwinkle rasps the seaweed with his tongue, and so scrapes off his dinner. Of course the teeth wear away. COWRIES.But only part of the toothed ribbon is used at a time, so there are plenty of teeth behind the worn ones, ready to take their place. The shell, as we have seen, is made of limestone. But the teeth are made of flint. This is a hard substance, so hard that it is used for striking sparks. Now we will look at a shell-builder, the Whelk, who uses his flinty tongue in quite another fashion. The Whelk does not care for a vegetable dinner. He prefers to eat other molluscs--he is carnivorous, a flesh-eater; but these other molluscs do not wait to be eaten. As the enemy draws near they retire into their shells, and shut themselves up as tight as they can. The Whelk, however, is a clever burglar; he knows how to make a way into the hardest of shelly houses. His front part--we might call it a nose--will stretch out to a fine point; and it contains a rasping tongue even harder than that of the Periwinkle. He sets to work. Moving the rasp up and down, he drills a neat round hole in the shell of the animal he is attacking. No shell is safe from him; and no tool could make a neater hole. When you next gather shells 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 shell! 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 shell 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 shell. 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 shell. Its feeding time comes when the water covers the rocks once more. Then the Limpet's shell 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 shell 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 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.
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