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].)

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.

Probably every reader will be able to appreciate the above instances of creatures adapted to their surroundings. For there are few people who are not familiar with the common Limpet of the shore between tide-marks, and with the great seaweed called Tangle, which has its habitat a little lower down, and forms great sea-meadows, whose upper limits alone are ever laid bare by the tide. The Patella pellucida, too, is fairly common, and the dead shell may be found on most rocky parts of our coast all the year round. As for the blue-striped young shell, floating on the blades of the tangle, those who have leisure to visit the seaside during the months of spring and early summer, may have seen it as I have described it; and the mention of it will recall pleasant memories of clear spring skies, and fresh sea-winds, and fields of heavy tangle swaying gently on the swell that comes in from the open sea. It is interesting to know something of the habits of the creatures whose forms we study, and we have already spoken of the snug little hiding-place that the Semi-transparent Limpet finds for itself in the tangle-root. It is of interest to remember that the Common Limpet, too, is a home-loving creature, which knows and prefers the spot of rock on which it habitually rests; and can find its way back to it, aided by its two eyes and two smelling patches. This has been proved by Professor Lloyd Morgan, who has recorded the result of his observations, made on the coast of Dorsetshire. It is not easy to detach a Limpet from the rock without injuring or exhausting it, but these specimens were caught when moving of their own accord, and were therefore uninjured and brisk. They were removed to short distances, and the following table shows the result of the experiment, clearly proving that the Limpet prefers home, but regards a distance of two feet as a very long journey.

Number
Removed.		Distance
in Inches.		Number
Returned in
Two Tides.		In Four
Tides.		Later.
2562100
21121350
21181062
3624113

Similar observations were made at an earlier date, by Mr. George Roberts, at Lyme Regis.

Let us now take an instance of adaptation in form. And this time we will take a shell so common that everybody will know it.