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? PLATE XXV CRAB CHRYSALIDS (3 and 3 A) When the caterpillar of an insect has reached its full size it throws off its skin and appears as a chrysalis, or pupa. And the caterpillar, or zoea, of a crab does exactly the same thing. It casts its skin, and appears in quite a different form. Only we do not call it a chrysalis, as a rule. We call it a “Megalopa.” [Illustration: PLATE XXV 1. PEA CRAB (life-size). 2. CRAB CATERPILLAR (enlarged). 2A. ” ” (life-size).