THE PINNA.
Well, the reason is a very odd one. It is setting a trap for fishes! For fishes, as perhaps you know, are very inquisitive creatures. They always want to know all about everything, and whenever they see a hole they think that they must find out what is inside it. So when a little fish comes swimming past a pinna, and catches sight of its gaping shells, it is almost sure to venture in between them. Then the shells close tightly, and it finds itself in a prison from which there is no escape; and very soon it is killed and devoured.
In colour, the shells of the pinna are very pale brown, and a number of ridges run down it from the smaller end to the larger. When the animal is full-grown it is sometimes not at all easy to see its shells, for they are covered almost all over with barnacles and the tubes of sea-worms.
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.