CHAPTER IV
CRABS
HOW CRABS GROW
IF you hunt about in the pools among the rocks when the tide goes out, and look behind the masses of sea-weeds which cover them, you are quite sure to find a good many crabs of several different kinds. Before I tell you about these, however, I think you would like to know something about the way in which these curious creatures grow.
Remember, then, in the first place, that what we always call the “shell” of a crab is not really a shell at all. That is, it is not in the least like the shell of an oyster, or a periwinkle, or a cowry, or a whelk. In these creatures the shell grows together with the animal inside it, and is never thrown off all through their lives. But the “shell” of a crab never grows at all. It is really a kind of crust of lime on the outside of the skin, which will not even stretch in the very least degree. So the only way in which crabs can grow is by throwing off their “shells,” in order that the soft bodies underneath may increase in size.
So once in every year, until it reaches its full size, every crab has to cast off its shelly covering and get a new one in its place. A few days before the change takes place it always goes and hides away in some dark crevice among the rocks, or behind an overhanging mass of sea-weed, where none of its many enemies are likely to find it. It knows perfectly well, you see, that while it is without its coat of mail it will be quite helpless; for its claws will be so soft that it will not be able to use them, while its body will be quite unprotected. Then a very strange thing indeed takes place. Something like a third part of its flesh turns into water! If you were to catch the animal at this time and to shake it, you would be able to hear the water swishing about inside its shell! Then it gets very restless indeed, and begins to wriggle about a good deal, turning and twisting from side to side, and rubbing its legs against one another, till it is quite tired out. It then rests for a little while, and begins to wriggle and twist about again. The fact is that it is trying to get loose, as it were, inside its “shell.” After a time it succeeds in doing this, so that the “shell” is no longer fastened to its body at all. Then, quite suddenly, a rent opens right across its back, and the crab gathers itself together and leaps, with a mighty effort, right out of its old coat! And as soon as it has done so the rent closes up again, so that unless you look very carefully indeed you cannot see it. You might really think that two crabs were lying side by side together.
For about a couple of hours the crab now lies perfectly still; and if you were to feel it you would find that its body was hard and knotted all over. That is because its muscles are cramped after the violent efforts which it has been making. After a time, however, the cramp passes off. Then the animal begins to grow. It grows very fast indeed. In fact it grows so fast that you can almost see it growing, and in less than twenty-four hours it is sometimes nearly half as big again as it was before. A new “shell” then begins to form upon the skin, and in about a couple of days more the animal is able to leave its retreat, clothed once more in a suit of good stout armour.
That is the way in which crabs, and lobsters, and shrimps, and prawns all grow. Once in every year at least they get new “shells”; and every time that they do so they increase in size. But after they reach a certain age they grow no more; and the coats of mail which they are wearing then are kept to the end of their lives.
HOW CRABS SEE
Perhaps, too, you would like to know something about the eyes of crabs; for these creatures see in a very odd way. On each side of the head is a kind of stalk, something like those which you may see on the heads of slugs and snails, only very much smaller. And at the tip of each stalk is a small black spot. Now if you were to put one of these little stalks under the microscope, and to look at the black spot, you would find that it was made up of hundreds and hundreds of very tiny eyes, very much like those of insects, except that instead of being six-sided they are square. So that altogether, perhaps, a crab may have three or four thousand eyes, or even more!
That sounds a very large number, doesn’t it? But then, you see, a crab cannot move its eyes up and down, and from side to side, as we can. They are fixed, and cannot be moved at all. Each eye, however, looks in rather a different direction from all the rest. Some eyes look upwards, some look downwards, some look forwards, some look backwards, and some look out on either side. So without moving its head at all the crab is able to see all round it.
Think of it in this way.
Suppose that you take a telescope and look through it. You can only see the objects at which the telescope is pointed, not the objects above it, or below it, or on each side. But if you had four thousand telescopes, fastened together in two bundles of a couple of thousand telescopes each, all pointing in different directions, and if your eyes were made in such a way that you could look through all the telescopes at once: then you would be able to see all round you, though you would only be able to look in any special direction through just one or two of the telescopes.
Now that is very much like the way in which the eyes of crabs are made. Each of these four thousand eyes is really a kind of telescope. And as they all point in different directions, the crab is able to see above it and below it and on all sides, though it only looks at any special object through one or two eyes.
HOW CRABS HEAR AND SMELL
The way in which crabs hear and smell is almost as curious as the way in which they see, for they have very odd little ears and noses in very odd places.
On its head, as perhaps you know, a crab has two pairs of feelers. We call them the “lesser feelers” and the “greater feelers.” Now if you were to look at the first joint of the lesser feelers through a good microscope, you would find on each a little gland, or bag, containing a very tiny drop of salt and water. These are the crab’s ears. Of course they are not nearly so good as our ears are. Indeed, I do not think that a crab can hear sounds in the air at all. But water carries sounds much more readily than air does, so that if you were to dive into a lake, or into the sea, on a calm, still day you could easily hear the beat of the oars in a boat half a mile away. And the ears of the crab are made in such a way that they can hear sounds in the water quite well, even though they may be deaf to sounds in the air.
Then if you look at the first joint of the greater feelers through the microscope, you will see two other tiny glands. These are the crab’s noses, by which it can smell odours in the water just as we can smell odours in the air. It always seems to find its food by scent, and if one of those basket-like traps which we call crab-pots is baited with a few pieces of decaying fish and lowered into the sea, crabs will smell the bait from quite a long distance away, and come hurrying up to obtain a share in the banquet. And they seem to do so by means of those odd little noses on the lower joints of their greater feelers.
PLATE XXI
THE EDIBLE CRAB
Now let me tell you something about the different kinds of crabs which you may find on the shore.
First of all, of course, there is the Edible Crab. This is the crab which is so largely used for food, and which you may see in any fishmonger’s shop. Sometimes it grows to a very great size, and has claws so big and strong that if it were to seize a man by the wrist he would find it very difficult indeed to set himself free. You will not find crabs as big as this among the rocks, for these giant creatures always live in rather deep water. But one often discovers a crab four or five inches across hiding in a rock-pool, and even he is quite big and strong enough to give one a very sharp nip.