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. PLATE XI THE COMMON LIMPET (1) 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. PLATE XI THE KEY-HOLE LIMPET (2) 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! PLATE XI THE SMOOTH LIMPET (3)