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.