The Animal World, A Book of Natural History Part 56

A good many different kinds of slugs are found in Great Britain. The largest of all is the great gray slug, which often grows to a length of more than six inches. Then the black slug is very common in many parts of the country. It is not always black, however, for one may often find examples which are brown, or yellow, or gray, or even white. The milky slug, which has a thick creamy slime, is plentiful everywhere. And sometimes one may dig up a very curious slug--testacella--which feeds on earthworms, and follows them down to the very bottom of their burrows.

When the weather is cold, this slug makes a kind of coc.o.o.n of earth and slime, and lies fast asleep inside it, often for many months at a time.

SNAILS

In many ways snails are very much like slugs, but they have a sh.e.l.l large enough to contain the entire body when the animal withdraws inside it. Several hundred different kinds of snails are found in North America, and many more in other parts of the world, varying in size from that of a small pinhead to that of a big walnut. Some are exceedingly numerous, others so rare and singular in their living-places that they are highly prized by conchologists. All snails lay eggs, usually in damp soil; and if you will turn over an old log in the woods in summer, you will be almost certain to find some of the minute s.h.i.+ning globules. When winter draws near all the snails go into hiding, and they have a most curious way of closing the entrances to their sh.e.l.ls by making little doors across them, composed partly of slime and partly of very small fragments of earth. This is in order to prevent the frosty air from getting in and killing them. But it would never do, of course, to keep all the air out, for in that case they would be unable to breathe. So they always leave a tiny hole in the middle of each door, through which just enough air can pa.s.s to prevent them from being suffocated.

Among the largest of all is the edible snail, which is largely used for food in many parts of Europe and is imported into the United States and pickled, to be eaten by those who like this delicacy.

Most of the gastropod mollusks, however, live in the water, some inhabiting ponds and streams, while others dwell in the sea.

In almost every brook and every ditch, for example, you may find water-snails of different kinds. Some are quite flat, and some are conical and pointed. Some are as large as land-snails, and some are so tiny that they are almost always overlooked. Most of them feed upon decaying leaves, and they have an odd way of traveling by floating upside down at the surface of the water, each with its broad fleshy "foot" expanded, so as to convert themselves into tiny boats. You may sometimes see quite a fleet of these little creatures being carried along by the stream. But if you throw a stone into the water they all sink down to the bottom at once, and do not resume their journey until many hours or even days afterward.

The eggs of this snail are laid in long jelly-like ribbons, which are generally fastened either to the stems and leaves of water-plants, or under the edges of large stones lying at the bottom of the stream. A very large number of gastropods live in the sea. One of the best known of these is the whelk, of which one reads in all books of English natural history. On almost every sandy and s.h.i.+ngly beach, in Western Europe, one may find it lying about in hundreds; and even in large inland towns one often sees whelks for sale, both in fishmongers'

shops and on barrows at the corners of the streets. Its eggs are one of the curiosities of the sea-beach--small, yellowish-white objects about the size of peas, made of tough, parchment-like skin, and fastened together in bundles about as big as cricket-b.a.l.l.s. You may often find these bundles on the sh.o.r.e in dozens; and most likely you will wonder how the whelk ever managed to lay a batch of eggs a good deal bigger than itself.

But the fact is that the eggs of the whelk are just like those of the frog. When they are first laid they are very tiny; but the tough skin of which they are made is very elastic, so that it will stretch almost like a piece of india-rubber. Besides this, it has the curious property of allowing water to soak in from the outside, but not to pa.s.s out again.

So as soon as the eggs are dropped into the sea they begin to swell, and before very long they are quite twenty or thirty times as large as they were when they were first laid.

We do not have these whelks in North America, but we have a variety of small gastropods, whose sh.e.l.ls are sometimes rough and coiled in a spiral form, sometimes round like land-snails, and of various sizes. One of them is the purpura, which has many ribs, and broad dark and light stripes running spirally. The purpura of the Mediterranean is famous for the purple dye obtained from its body; but our species yields such a dye also in small quant.i.ty. This was the dye anciently known as Tyrian purple. It is contained in a little bag behind the throat, which holds just one small drop of liquid, and no more. And if you were to see it you would never think that it was dye at all, for it looks only like rather yellowish water. But if it is squeezed out on a sheet of white paper, and laid in the suns.h.i.+ne, it very soon begins to change color.

First it becomes green, then blue, and then purple. And it is really the dye which the ancient Romans valued so highly that no one who did not belong to the royal family was allowed to dress in purple raiment.

BORERS

In many parts of our eastern coast occur in great numbers two or three kinds of small, rough, spiral gastropods, called borers by the fishermen, who hate them because of the great number of oysters they kill. Each of these spends its whole life in seeking and devouring other sh.e.l.l-bearing mollusks. It kills and eats these in a very curious way.

Like all the gastropods, it possesses what we call a tooth-ribbon--that is, a narrow strip of very tough gristle in its mouth; set with row upon row of sharp, notched, flinty teeth. There are some times more than six thousand of these teeth, and although they are so small that they cannot be seen without the aid of a powerful microscope, they are nevertheless very formidable. For every tooth is hooked, with the points of the hook directed toward the throat.

The tooth-ribbon is used in this way: When a borer meets with a victim, it fastens itself to it by means of its fleshy, muscular "foot." Then it bores a round hole through its sh.e.l.l, as neatly as if it had been pierced by a drill. And then it pokes the tooth-ribbon down into the body of the creature inside, and draws it back again. As it does so, of course the hooked teeth tear away little bits of the victim's flesh. The borer swallows these, and then pokes down its tooth-ribbon once more.

And so it goes on, over and over again, until the sh.e.l.l of its victim has been completely emptied, when it goes off to look for another.

PERIWINKLES

These are common on rocky parts of the coast, and you may find them crawling about on the weed-covered rocks in thousands when the tide is out. They have tooth-ribbons just like that of the borer, but they do not use them in the same way, for they feed only upon seaweeds. And they are remarkable for having the foot divided by a kind of groove, which runs right down the middle. When a periwinkle crawls, it moves first one side of this foot forward, and then the other side, so that although it has no legs it may really almost be said to walk.

THE COWRY

One of the prettiest of the gastropod sh.e.l.ls, is that of the cowry, in some parts of Africa used as money. It would seem strange to earn one's living just by picking up money on the sea-sh.o.r.e, wouldn't it? And perhaps you might think that every one who lived near those parts of the coast where cowries are found must be very well off. But then sixteen hundred of these sh.e.l.ls are only worth about a quarter of a dollar, so that you would have to hunt for a very long while and stoop a great many times in order to obtain sufficient even to buy food. And it must be very awkward to have to carry several sacks of money when one goes out marketing! Many of them, however, are extremely beautiful.

LIMPETS

Commoner still are the limpets, which you may find in thousands clinging to the rocks that are left bare when the tide goes out. They fasten themselves down by means of the broad, fleshy foot, which acts as a big sucker. And so firmly do they hold that it is almost impossible to pull them away.

After a time, the edges of a limpet's sh.e.l.l cut a circular groove in the rock to which it clings, so that even the sea-birds cannot drive their beaks underneath and force it from its hold. And though, when the tide is up, the mollusk will wander to a distance of two or even three feet in search of food, it always seems to return to its resting-place before the retreating waves again leave the rock uncovered.

AMPHINEURANS

This order of mollusks contains the curious creatures which are known as chitons. These may be described as sea-armadillos, for they are covered with a kind of sh.e.l.ly armor, consisting of a series of plates, and can roll themselves up into b.a.l.l.s, in order to protect themselves from the attacks of their enemies.

One of these mollusks is called the p.r.i.c.kly chiton, because it is covered all over with sharp spines, like a hedgehog. It grows to a length of nearly six inches. But long before it reaches its full size the spines are rubbed off, so that a large example of this creature is nearly always perfectly bare. The chitons live among muddy rocks at low-water mark, and are not common outside the tropics or in shallow water.

The order of the amphineurans is quite a small one, and so is that of the scaphopods, which consists only of the tooth-sh.e.l.ls, which are very common on the sandy coasts of the Northern Pacific, and look rather like very tiny elephants' tusks. The Indians of the Puget Sound region used to string them as ornaments, and valued them highly.

BIVALVES

The order of the bivalves is a very large and important one. All these creatures have their sh.e.l.ls made of two parts, or valves, which are fastened together by means of a hinge. They have no heads, and the mantle forms a kind of flap on either side of the body. They are found both in fresh and salt water. Every one knows the "fresh-water clams,"

or mussels, which abound in our lakes and rivers. In the central and southern parts of the United States they are exceedingly numerous and of many kinds, some rough, others smooth. All are lined with mother-of-pearl, and pretty b.u.t.tons and other ornaments are made from them. Moreover, pearls are very frequently discovered inside their sh.e.l.ls, and sometimes they are of great value.

THE PEARL-OYSTER

Pearls are obtained chiefly, however, from the pearl-oyster, which is found in warm seas in many parts of the world, the princ.i.p.al fisheries being in Ceylon, the Persian Gulf, the South Sea Islands, and off the northeast coast of Australia. They are deposited by the mantle, and it is most likely that they are really due to a grain of sand, which has lodged inside the sh.e.l.l and set up irritation. Indeed, it has been found that if small objects, such as tiny stones, are forced between the valves of one of these oysters, they become covered with layers of pearl in a very short time. The best mother-of-pearl is also obtained from the sh.e.l.ls of the pearl-oyster.

OYSTERS

The ordinary oyster belongs to another family of bivalves, in which one part of the sh.e.l.l is a good deal larger than the other.

The early life of this mollusk is very curious. The sp.a.w.n is known as spat, and is produced in enormous quant.i.ties. This spat looks at first like very fine gray dust, and remains for some little time within the sh.e.l.ls of the parent. But one day in early summer the oyster opens its valves a little way, and squirts it out like a cloud into the water. For a few weeks the little oysters are able to swim, and they generally travel backward and forward with the tide. But after a while they attach themselves to some object at the bottom of the water, and there they remain without moving any more for the rest of their lives.

One would think that, since a family of oysters is so enormously large, these creatures must be the most plentiful mollusks in the sea. But by far the larger number are destroyed by other creatures before they are able to settle down; while even after that they have a great many enemies. We have already told you how mischievous starfishes are in the oyster-beds. Then borers and dog-whelks are almost equally troublesome, and besides these there is a curious kind of sponge, called the cliona, which burrows into the sh.e.l.ls of the mollusk and gradually destroys them, sometimes actually causing them to fall to pieces.

BLACK MUSSELS

Two or three kinds of black mussels live in vast numbers on almost all coasts, clinging to rocks and submerged timber. The way in which a mussel fastens itself to its hold is very curious, for instead of turning the whole of the foot into a big sucker, as the limpet does, it spins a number of very strong threads from that part which lies nearest to the hinge; and every one of these threads is separately fastened to the support, so that the creature is moored down, as it were, by a kind of cable. These threads are known as the byssus, and hold so firmly that it is not at all easy to pull them away. Some of these mussels are good to eat, but are not as much used in the United States as in Europe.

THE c.o.c.kLE

This is another very well-known bivalve. Its heart-shaped sh.e.l.ls, covered with low ridges, you must know by sight. It is one of the burrowing mollusks, spending its life buried in sandy mud. It is especially common at the mouths of large rivers, where enormous quant.i.ties are collected to serve as human food. And its large muscular foot is not only used in digging, but also enables it to leap to a considerable height. It is to this family that the quahog or hard clam of our markets belongs.

RAZOR-Sh.e.l.lS

These, too, are inhabitants of the mud, and if you want to find their burrows all that you have to do is to visit a patch of sandy mud when the tide is out, and stand quietly watching it. Before long you are sure to see a little jet of water spurt out of the mud to a height of three or four inches. Now this water has been squirted out of the siphon-tubes of a razor-sh.e.l.l, and if you walk to the spot, treading very carefully, you will find a tiny hole in the mud. This is the entrance to the burrow, and if you want to get the animal out, the best way to do so is to drop a little salt down the hole. For it is a very strange fact that although the razor cannot live in mud at the bottom of fresh water, it does not like pure salt at all, and is sure to come up to the surface and try to get rid of it. But if you fail to seize it at once it will retreat to the very bottom of its burrow, and no amount of salt will persuade it to come up again. The soft clam, which is sold in our markets in such enormous quant.i.ties, is a near relative of the razor.

THE PIDDOCK

One of the most wonderful of all the bivalves is the piddock, as it is a boring mollusk, living buried in the solid chalk or limestone.

If you should examine the rocks which are left bare at low water along the sh.o.r.e of the Mediterranean, or some other warm sea, you would often find that they are pierced by numbers of rather large round holes. These are the entrances to the burrows of piddocks; and if you could split the rock open you would find several of these creatures lying in their tunnels.

Sometimes, when they are boring, their burrows become choked up behind them with the material which they have sc.r.a.ped away. Then they just squirt out a jet of water from their siphon-tubes, and so wash the pa.s.sage clear.