Do you see how long his great claws are, and how his back is covered all over with tiny hooked spines? It is quite easy to understand why the name of thornback was given to him. But how is it that all those tufts of seaweed are growing on the upper part of the shell?
Well, the answer is a very odd one. The crab planted them there himself! The fact is that when he is lying down at the bottom of a pool he does not want to be seen, for fear that the animals upon which he preys should take alarm, and escape before he can catch them. So he actually pulls up a number of little sprigs of seaweed, and plants them on his back one after the other, pressing the roots down with his claws till at last they are held quite firmly by the little hooked spines with which his shell is covered! Then as long as he keeps quite still he is perfectly invisible, and his victims may even crawl over him without suspecting that they are in any danger.
Stranger still, if a thornback crab which has covered his back with seaweeds should be placed in a tank in which sponges are growing, he will soon find out that he is not nearly so well hidden as he would like to be, and will get very uneasy. Before long he will discover what the reason is, and will actually pull all the sea weed off his shell, and plant sponges on it instead.
Here, too, scampering and rattling about among the pebbles, are lots of hermit-crabs, dragging after them the shells in which they have ensconced their soft hind bodies, as is described on page [402]. And under the stones—turn them over and you will see—are dozens of strange little half-transparent creatures which you might easily believe were insects, but which really are diminutive cousins of the crabs and crayfish named amphipods and isopods, and so forth. You may find under some stone one of the tubes made by a certain species, composed of grains of sand glued together by sticky threads much like spiders' silk. These minute crustaceans exist in vast multitudes near the surface of the ocean at certain seasons, and form the principal food of the whalebone-whales, which gulp them down wholesale. Some of them, also, are parasitic on fishes.
But what is the curious little creature clinging flat upon this rock among the weeds? It looks like some sort of pill-bug half an inch long, doesn't it?
Ah! that is a chiton. It is really a kind of shell-bearing mollusk, like the whelk and the periwinkle; only instead of having its shell made all in one piece, it has eight shelly plates on its back, which overlap one another just like the slates on the roof of a house. Just touch it with your finger. There! Do you see? It has rolled itself up into a ball, just like those pill-millepedes which you may find in the garden. It always does this if it is frightened. And its shell is so stout and hard that as long as it is rolled up it is quite safe from nearly all its enemies.
If you were to hunt about among the rocks quite close to the water's edge when the tide is at its lowest, you would most likely meet with a number of chitons, and you would be surprised to find how much they vary in color. Some are ashy gray all over; but a great many are streaked and spotted with brown, and pink, and orange, and lilac, and white. But the strangest thing of all about chitons—there are far larger ones in the warmer parts of the world—is that some of them have nearly twelve thousand eyes scattered about all over their shells!
But we are lingering too long by the way, for our real destination to-day is that fine pool over there. It is a basin among the ledges, filled with quiet sea-water left by the retreat of the tide, half-floored with sandy mud, and its edges fringed with feathery seaweeds, corallines, and hydroids. Here is a capital home for the little folk of the sea, where there is always fresh clear water, but where only a part of the time do the surges pound, and then never with full force; furthermore, a wall of rocks protects the nook, and enemies can rarely enter to destroy the peaceful society.
In warmer parts of the coast, as in the Gulf of Mexico, or upon the Pacific coast, or most of all in some of the tropical islands which now belong to the United States, such a pool would be brilliantly carpeted with sponges, sea-anemones, coral-polyps and corallines, of which you may read on pages [431 to 435]. The water of the North Atlantic, and the winters of its American coast, are too cold, however, to allow any but a very few hardy species of these lowly sea-flowers to grow in our pool; but there are quite enough to keep us busy during the hour or two left before the returning tide creeps over the jagged rim of the basin and drives us away.
Here, for instance, is half an oyster-shell looking as if it had been bored full of holes with bird-shot. It could hardly have been any boy's target though; for, see, we can find many such fragments. There is one under water. Take it out and you will find every one of the hundreds of little pits filled with a yellow spongy material. It is real sponge, called the boring-sponge, because it riddles all sorts of old shells until they fall to pieces. This is a good thing, for then they are gradually ground to powder and dissolved in the water, and so help to keep it supplied with the lime needed by living animals for their shells.