Each one is moving along, with its edges partly opening and shutting. It is plain that this waving motion causes the creatures to move through the water. Also, they can rise to the surface, or fall to the depths, and do not collide with one another. So the Jelly-fish is not at all helpless.
At night Jelly-fishes sometimes look very beautiful. Each one shines in the water, with a soft yet strong light, like fairy lamps afloat in the sea.
They are of all sizes. Some you could put in a small winegla.s.s, others measure nearly two feet across. Evidently the Jelly-fish grows, and, in order to live and grow, it must eat; but what does it eat, and how does it obtain its food?
Before noticing the wonderful way in which this animal finds its dinner, let us look at its body. In any large Jelly-fish you can see marks which run from the centre of the body, and another mark round the edge of the "umbrella." These are really tubes. They all join with a hollow s.p.a.ce inside the body, which is the creature's stomach. The mouth-tube opens under the body, as can be seen by turning the Jelly-fish on its back, and moving the lobes of jelly aside. All the food goes up this tube-mouth, and so into the stomach of the animal. The whole creature is little more than so many cells of sea-water, the walls of the cells being a very thin, transparent kind of skin.
Perhaps the strangest thing about it is the way in which it catches prey. Jelly-fish feed on all kinds of tiny sea animals, such as baby fish, and the young of crabs, shrimps, and prawns. These small creatures form part of the usual dinner of many a hungry dweller in the sea, and the Jelly-fish takes a share of them.
[Ill.u.s.tration: A MEDUSOID.]
From the edge of the "umbrella" there hangs a fringe of long, delicate hairs, rather like spiders' threads. These are fishing lines, yet much more deadly. They trail through the water, stretching far from the main part of the Jelly-fish; and any small creature unlucky enough to touch them is doomed.
Down each one of these threads there are minute cells, hundreds and hundreds to every thread; and in each cell there is a dart, coiled up like the spring of a watch. The tip of the dart is barbed like a fishhook. Now the cells are so made that they fly open when touched. The dart then leaps out and buries itself in the skin of the animal which touched the thread. Not only that, but the darts are poisoned, and soon kill the small creatures which they pierce.
You see now how this innocent-looking Jelly-fish gets its food. As it swims along, the threads touch the tiny living things in the sea, the darts pierce them and poison them. Of course these stinging darts are very, very small, much too small for our eyes to see.
Sometimes there are numbers of large brownish Jelly-fish in the sea, or washed up on the sh.o.r.e. If you are paddling or swimming, keep well away from them. Their poison darts are able to pierce through thin skin, and may cause you illness and great pain. Remember that the threads are very long; after you have pa.s.sed the main body of the animal, you may still be in danger from the trailing threads.
We noticed these same poison darts when we were dealing with the flower-like animals, the Anemones. Only, in that case, they were so fine, so small, that they had no power to harm us, even though they entered our skin. You may remember that we called the Anemone a cousin of the Jelly-fish, for they both belong to the same lowly division of the Animal Kingdom.
Animals have queer ways of getting a living. Who would expect to find millions of poisoned darts in a Jelly-fish? Who would guess that these weapons are coiled up, ready to spring out at their prey? Men have made many weapons for killing, from the bow-and-arrow to the torpedo, but none of them is more wonderful than the weapon of the Jelly-fish.
1. Where is the mouth of the Jelly-fish placed?
2. How does the Jelly-fish move through the water?
3. What is the food of the Jelly-fish?
4. How does it obtain its food?
1. A FRESHWATER TURRET Sh.e.l.l.
2. EDIBLE MUSSEL.
3. CONE Sh.e.l.l.
4. SWORD-BLADE RAZOR-Sh.e.l.l.
5. EAR Sh.e.l.l, OR ORMER.
6. A TOP Sh.e.l.l.
8. SWAN MUSSEL.]
Sh.e.l.lS AND THEIR BUILDERS (1).
THE PERIWINKLE, WHELK AND LIMPET.
Most of the sh.e.l.ls which you find scattered over the sh.o.r.e 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 sh.e.l.l-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 sh.e.l.ls. But most of them build a sh.e.l.ly house for themselves; they do this to defend their soft bodies from the attacks of a host of enemies. Some build two sh.e.l.ls--the Oyster and Mussel do, as you know. These are called _bi-valves_; that is, two valves or sh.e.l.ls; and others, like the Garden Snail, the Limpet, and Periwinkle, have one sh.e.l.l 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 sh.e.l.l of a Snail, or other _mollusc_. The Snail's sh.e.l.l 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 sh.e.l.l-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.
[Ill.u.s.tration: PRECIOUS WENTLETRAP.]
The sh.e.l.l 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 sh.e.l.l must therefore be made larger. So the mantle is stretched out, and it puts a film of lime to the edge of the sh.e.l.l. Bit by bit the sh.e.l.l is thus added to by the wonderful mantle. Look at a snail's sh.e.l.l, and notice the lines which show how many times the little house has been made larger.
Each kind of sh.e.l.l-builder has its own style of building. If you go to a museum and examine the sh.e.l.ls 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 sh.e.l.ls, some of them built by uni-valve molluscs and some by bi-valve molluscs.
Wherever there are weeds along the sh.o.r.e 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 sh.e.l.l. 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 sh.e.l.l, closing the entrance with the h.o.r.n.y 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 sc.r.a.pes off his dinner. Of course the teeth wear away.
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 sh.e.l.l, 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 sh.e.l.l-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 sh.e.l.ls, 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 sh.e.l.ly 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 sh.e.l.l of the animal he is attacking. No sh.e.l.l is safe from him; and no tool could make a neater hole.
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