In the sea the Jelly-fish looks like an umbrella of bluish-white jelly, from which hang tassels and threads. Look over the side of a boat, or from the pier, and you often see them drifting by, hundreds of them, like so many ghosts.
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 wineglass, 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 space 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.
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 shore. 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 passed 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.
EXERCISES
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?
SHELLS1. A FRESHWATER TURRET SHELL.5. EAR SHELL, OR ORMER.2. EDIBLE MUSSEL.6. A TOP SHELL.3. CONE SHELL.7. SCALLOP.4. SWORD-BLADE RAZOR-SHELL.8. SWAN MUSSEL.
SHELLS1. A FRESHWATER TURRET SHELL.5. EAR SHELL, OR ORMER.2. EDIBLE MUSSEL.6. A TOP SHELL.3. CONE SHELL.7. SCALLOP.4. SWORD-BLADE RAZOR-SHELL.8. SWAN MUSSEL.
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--themollusca, which only meanssoft-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 calledbi-valves; that is, two valves or shells; and others, like the Garden Snail, the Limpet, and Periwinkle, have one shell only, and so are calleduni-valves.
The crab, and othercrustaceans, also have a hard covering to their soft bodies; but it is not at all like the shell of a Snail, or othermollusc. 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 themantle. Now this mantle is one of Nature's cleverest inventions. It is able to take the substance calledlimefrom 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.
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 oflimestone. But the teeth are madeof 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 hardnotto 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.
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 thembi-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 wonderfulmantle; 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?