The adult shell, however, retains a certain degree of translucency, which matches very well with the colouring of the tangle-root; and thus presents a great contrast to the shell of the common Limpet, which is found on rocks. The rugged surface of the latter is usually more or less irregularly speckled in harmony with the surfaces on which it lives, though this shell also presents when young occasional touches of blue, which suggests a family likeness in colour tastes on the part of the two kinds of Limpet. The blue in this case, however, is of the dullest and dingiest shade. The _Patella pellucida_ is common on the more rocky portions of our coasts; in spring the young may be seen in thousands on the seaweeds of the Isle of Man; here its habits were first observed and described in detail by the Manx naturalist Forbes, who noticed its peculiar way of finding a hiding place among the roots of the tangle. The same shell-fish, in contrast with the commoner Limpet of the rocks, affords another instance of the way in which shells adapt their forms to their surroundings. In each case the shell is a plain conical cap, and the animal within keeps the shell firmly attached to the base on which it rests. The Limpet can move about at a very creditable snail's pace when it wishes to do so, and at low-water mark, when the tide is beginning to rise, you may easily find them moving about and off their guard; but during many hours of the day, when the tide is out, the main object of the Limpet is to keep its shell as firmly fixed to the rock as possible. It will at once be seen that if the margin of the shell were smooth like that of a tea-cup, and the surface of the rock to which it clung very irregular, many chinks would be left between the margin of the shell and the surface of the rock through which unwelcome visitors might find entrance. The loss of moisture through the crevices, too, would be a serious thing to the animal during the hours when the shell is uncovered by the tide and exposed to the rays of a hot sun. On the other hand, if the margin of the shell were irregular, and the surface on which it rested smooth, unprotected crevices would in the same way be left. So the Limpets adapt the shape of their shell to their surroundings; the _Patella pellucida_, which lives on the smooth branches of the tangle-root, has a shell with a smooth regular edge; while the _Patella vulgata_, which lives upon rocks, has a shell with an irregular, indented edge, whose irregularities fit into those of the rock on which it rests. (See Fig. 2.) [Illustration: FIG. 2.--Shells mentioned in Chap. II. 1, Common Limpet, old and young; 2, Semi-transparent Limpet, old and young (the remains of the young shell may be seen crowning the adult shell); 3, Common Yellow Periwinkle; 4, Common Edible Periwinkle; and 5, High-tide-mark Periwinkle, both with a sharp spire, for comparison. One specimen of the latter stands among group 3.]