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.
These creatures are generally known as “Acorn Shells,” and I dare say that you might think that they must be very closely related to the limpets. But in reality they are much more closely related to the shrimps and sandhoppers, though they look so very unlike them, and lead such different lives. For while shrimps and sandhoppers are always swimming or skipping about, the little animals which live inside these acorn shells never move at all after they are a few days old, but spend their whole lives fastened down to the surface of the rocks. But there is this great difference between the two. When the eggs of a limpet hatch, out come a number of very tiny limpets, just like their parent in everything except size. But when the eggs of an acorn shell hatch, the little creatures which come out from them are not like their parents at all. They are “zoeas,” in fact, or acorn shell caterpillars; and they do not reach their perfect form for some little time.
When these little “zoeas” first make their appearance in the world they are able to swim about by means of three pairs of tiny feathery legs, with which they paddle their way along through the water. And they also have a round black eye in the middle of the body, with which they can see quite well. Every two or three days they throw off their skins, just as caterpillars do, and appear in new ones, which have been gradually forming beneath. And each time that they do this their shape changes. At last they are ready to take their perfect form. Then each of the little creatures clings to the surface of a rock by means of its feelers, and pours out a kind of cement, which hardens round them, and anchors it firmly down. It then throws off its skin once more, and appears in the form of an acorn shell just like its parent. And, strange to say, it throws off its eye at the same time, and is perfectly blind for the rest of its life!
If you look down into a shallow pool, the rocky sides of which are covered with these acorn shells, you may often see a very pretty sight. You may see the little animals fishing. Out from the hole at the top of each shell comes a kind of little net, which sweeps through the water, and is then drawn back into the shell. This net is really formed by the limbs, which are fringed with long hairs, and as it passes through the water it collects the little tiny scraps of decaying matter on which the animal feeds.
You may find these acorn shells in great numbers, not only on the rocks which are left dry when the tide goes out, but also on the wooden beams which support piers and jetties. Indeed, these beams are often so closely covered with the odd little shells that you cannot see the surface of the wood at all. And very often they fasten themselves to the shells of limpets and oysters, and even on the backs of crabs.
