But it is just as well that they do not all get eaten, for sandhoppers are very useful little creatures indeed. They feed upon the masses of decaying sea-weed which are constantly flung up on the shore by the waves. For they, too, belong to the great army of “Nature’s Dustmen,” like the “zoeas” of the crabs and lobsters, and help to clear away all kinds of rubbish which would poison the air and the water if it were left to decay. Indeed, they will eat almost anything, and if you were to tie up a number of sandhoppers in your handkerchief, and leave them there for a few minutes, you would never be able to use the handkerchief again; for you would find that their sharp little jaws had nibbled it into holes. If you watch a sandhopper carefully when it is skipping about, you will find that it leaps by doubling its body up, and then straightening it out again with a sudden jerk. [Illustration: PLATE XXIX 1. SANDHOPPER (enlarged). 1A. ” (life-size). 2. SAND SCREW (enlarged). 2A. ” (life-size).] PLATE XXIX THE SAND SCREW (2 and 2 A) If you follow the tide as it goes out on a still day, you will notice that it leaves the sand quite smooth behind it. But if you come to the same spot about half-an-hour later, you will often find that it is marked by numbers of winding tracks, which look just as if they had been made by worms. These, however, are the work of the Sand Screw, a curious little creature which in many ways is very much like a sandhopper. But instead of sinking its burrows almost straight downwards into the sand, as sandhoppers do, it drives them along almost as a mole does, just below the surface. If you stand quite still for a few minutes near the water’s edge, when the tide is going out, you may sometimes see this odd little creature at work; for as it pushes its way along it raises the sand into a kind of low tunnel, which generally falls in behind it, and so forms a groove. And if you suddenly turn over the sand in front of the tunnel you will find the little animal which was making it, and will see at once why it is called the “sand screw.” For instead of skipping about like a sandhopper, it will lie on one side and wriggle its way along with a curious “screwing” movement, just as though it were trying to bore its way into the sand. PLATE XXX ACORN SHELLS (1) If you examine the rocks which are left dry when the tide goes out, you will often find that they are covered almost all over with small shells which look rather like those of tiny limpets. Only at the top of each shell there is a little hole, from the margin of which a number of ridges run down to the bottom. And these ridges are so sharp, that if you happen to slip when you are wandering about among the pools, and catch at a rock to save yourself, they will cut your fingers almost as if they were knives.