The wentletrap is sometimes known as the “staircase shell,” because the ridges which run round it are very much like those spiral staircases by which one climbs to the tops of church towers and other lofty buildings. If you want to find it, the best place to look is in the ridges of small pebbles which are washed up here and there on sandy coasts by the waves, and which are generally mixed up with broken coal which has been thrown out from passing ships. But it is not very common, and you must not be disappointed if you do not succeed in finding it.
This is a very common creature indeed, and you can find it in hundreds and thousands on any rocky part of the coast. Numbers of its empty shells are to be found lying about on the beach, and if you go down among the rocks when the tide is out you will often notice that in some places they are so covered with limpets that you can scarcely put the tip of your finger in between them.
These animals cling to the rocks in the most wonderful way. Indeed, if you take hold of a big limpet between your fingers you will not be able to move it in the least, even if you pull at it and push at it as hard as you can. But if you take the animal by surprise, and give it a sharp, sudden blow sideways with a stone, or the end of a stout stick, you can generally knock it off quite easily. And you will very often find that a deep ring-shaped mark has been worn away in the rock by the sharp edges of its shell.
However, limpets do not always remain clinging to the rocks, for they can crawl about quite as easily as snails can, by means of that soft, fleshy part of the body which we call the “foot.” And if you take them home alive, and put them into an aquarium, you may often see them creeping up and down the glass sides, through which you can examine their bodies quite easily.
There are a good many different kinds of limpets, of which one of the most curious is the Key-hole Limpet. It is generally found in rather deep water, but you may sometimes find it clinging to the rocks just above low-water mark. You must choose a season of “spring-tide,” however, for then the tide goes farther out than usual, and leaves behind it a good many creatures which at other times one hardly ever sees.
The shell of this creature is rather stouter than that of the common limpet, and has a number of ridges running down it from the peak to the margin. Even by these you can tell it at once. But if you look at it closely, you will also find that just at the top of the peak there is a hole shaped rather like a key-hole. Through this hole the animal squirts out the water which has passed over its gills; so that all the time that it is breathing, if only one could see it, a kind of little fountain is playing under water, spouting out from the top of its shell!
At first sight, perhaps, you would hardly take this creature for a limpet at all, for it is ever so much smaller than either the common or the key-hole limpets, and has a very thin and delicate shell indeed. It varies a good deal in colour, but generally the shell is pale brown, looking almost like polished horn, with eight or nine narrow streaks of bright blue running down from the peak to the margin. It is often called the “bonnet shell,” because in shape it is rather like an old-fashioned bonnet.
You may often find the empty shells of this creature lying upon the shore. But if you take them home you will find that as soon as they become dry the beautiful blue streaks begin to fade, and that after a few days you can hardly see them at all.
This is a very curious creature indeed. But if you want to see why its rather odd name was given to it, you must look inside its shell instead of outside. Then you will see that in the upper part is a curved plate which really looks very much like a tiny tea-cup, while the shell itself surrounds it just like a saucer. And if you were to examine the animal which lives inside it very carefully, and to pull out its long tooth-ribbon, you would find at the tip of it a curious little organ which looks just like a tea-spoon. So that we have cup, saucer, and spoon all in one!
Perhaps you may wonder what the odd little cup is for. Well, the fact is that the muscles by means of which the animal clings to the rock are very strong indeed. So, of course, there must be something else very strong to which they can be fastened, and this cup-shaped plate gives them a very firm hold.
The cup and saucer limpet is not a very common creature, and in many parts of the coast it is never met with at all. But if you stay by the sea-side on the south coast of England, you may sometimes find its empty shell lying upon the shore.