This is a very odd crab indeed. In the first place it is extremely small. Even when it reaches its full size it is scarcely ever so much as half-an-inch across, while its body is so round that it really does remind one very much of a pea. Only it is not quite the right colour for a pea, for it is creamy yellow instead of green.
And, in the second place, this crab lives in a very odd place—namely, inside the shells of living mussels, or pinnas, or even cockles! What it does there nobody seems quite to know. It does not appear to injure the animal to whom the shell belongs, although it is very fond of the flesh of mussels, and if it finds one of those creatures lying dead will certainly devour it. Perhaps it only creeps inside its shell for the sake of safety. At any rate, it is a very timid little crab, and if you open a mussel and find a pea crab lying hidden inside it, it will tuck up all its legs quite close to its little round body and lie perfectly still for several minutes in the hope that you will think that it is dead.
On some parts of the coast pea crabs are so plentiful, that three out of four mussels are found to have one of these odd little creatures inside it.
PLATE XXV
CRAB CATERPILLARS (2 and 2 A)
I dare say you did not know that crabs have caterpillars, just as insects have. We call these crab caterpillars “zoeas,” and they are not in the least like their parents. There are a great many different kinds, of course, for every crab has its own zoea, just as every butterfly and moth has its own caterpillar, and some of them are not very much like some of the others. But they are always very tiny indeed—they are scarcely as large, in fact, as the smallest grains of sand—and they always have a very long curved horn in front of the body and another one behind, and long waggly tails. And they swim in the oddest way possible—by turning somersaults in the water, over and over again!
These zoeas are very useful little creatures, because they feed upon the tiny scraps of decaying matter which are always floating about in the sea, and so help to keep the water always pure. They belong, in fact, to the great army of what I always like to call “nature’s dustmen”—those little animals whose duty it is to clear away the rubbish from the world. There are millions and millions of these busy little workers on the land, and millions and millions of others in ponds and rivers, as well as in the sea, and so well do they perform their task that both the air and the water are always kept pure.
Another very interesting fact about zoeas is that they form the chief food of no less a creature than the Greenland whale. No doubt you know that whales are of two kinds—those which have teeth, and those which have none. Those which have teeth feed upon fishes, and giant cuttles, and could easily swallow a man. But the whales which have no teeth have throats so small that they would almost certainly be choked if they tried to swallow a herring! So they have to feed on very small creatures indeed, and are very fond of zoeas, which often swim about in such vast shoals that the water of the sea is quite thick with them. And they catch them in a most curious manner.
You have heard, of course, of the very useful substance which we call “whalebone;” and no doubt you know that it has nothing to do with the bones of the whale at all. It is found in the mouths of those whales which have no teeth, and hangs down in great plates from the gums of their upper jaws. Very soon these plates split up; and then each part splits up again; and so on, over and over again, till at their lower ends they form a kind of thick fringe of close, matted hairs.
Now it is by means of this fringe that the whale catches the zoeas. When it meets with a shoal of these little creatures it opens its huge mouth wide, and swims through them. Then it nearly closes its jaws, and lets down the whalebone plates, so that the hairy fringe forms a kind of strainer all the way round. It then squirts out the water from its mouth through this fringe, which allows the water to pass through it, but keeps back the zoeas; and when it has got rid of all the water it closes its mouth completely and swallows the zoeas, a few thousand at a time, after which it opens its jaws again, and swims through the shoal once more.
Doesn’t it seem strange that the biggest animal on earth should feed on some of the very smallest?