Stories of the Universe: Animal Life

by B. Lindsay

We have referred above to the ridges on the outside of the shell. Now that we have learnt that the Pecten is a very active animal, and moves in the manner described, we see that these ridges run parallel to the direction in which it moves as it darts away ears foremost. Let us try to realise what is the effect of this.

Take a mat with parallel stripes and move it along the floor or table in the direction of the stripes; then try moving it in an opposite direction across the stripes. It is easy to perceive that in the former case one's eye does not detect the movement nearly so soon as in the latter case. To explain this would necessitate a lengthy digression on the subject of optical illusions: that the fact is so everyone may easily ascertain by experiment. The ridges, therefore, converging in the direction towards which the shell is going, are a protective decoration, enabling it to slip away more easily from under the eyes of its foes. The reader will readily recall a parallel instance in the common Cockle. This also is a very active creature; it takes leaps by means of a strong muscular foot; and the ridges on the shell, like those of the Scallop, converge towards the hinges, that is to say, in the direction in which the shell moves. Another instance of a very active shell-fish with similar markings is afforded by certain kinds of Lima, a near relative of the Scallops. It may be added that all Scallops are not equally active, nor all Limas; and various modifications of their form and colour might be pointed out which lead us to suspect that in the less active kinds the pattern of ridges is often somewhat obscured by means of these differences.

Now, take up a comb and draw it over your fingers, firstly along the teeth, and secondly across them, and you will be able to estimate the gain in speed and comfort to the comb-shell, Pecten, and to the common Cockle, from having its ridges set in the direction in which it is going. Were the ridges concentric, as is so often the case in bivalve shell-fish of a more sluggish disposition, the friction caused by the ridges would seriously delay the progress of the shell.

Something must be added regarding the colouring of the shell, which is vivid, corresponding with that of the animal within. It is capable of great variety, though perhaps not so great as in some of the smaller Pectens. The predominant shades are pink, crimson and yellow, either separately or mixed; that is to say, some shells are pure pink, some almost pure yellow, some almost pure crimson, while others present every imaginable shade of pinkish yellow, reddish brown and brownish crimson. Local variation of colour is so marked that we may suspect the variations in tint to be in some degree protective. The shell also varies considerably in size and strength according to the neighbourhood in which it has grown.

This scallop-shell is but one of many: a number of other species are found on our own shores, and many others again in foreign seas.

One shell of the English coast is very annoying to the juvenile shell-collector who gathers specimens on the shore. This is Pecten pusio, a very small and delicate kind, with a raised pattern of fine markings upon the ridges, which are very narrow. A good specimen of the deeper valve is common enough, but the shallow valve, if of any size, is distorted into all manner of shapes, as if it had been squeezed and crumpled. The disappointing character of these specimens, from an æsthetic point of view, is explained when we learn that it not only lies on its shallow valve, but becomes fixed in this position, instead of hopping about freely like the P. opercularis. It therefore has frequently to adapt its shape to the nature of the ground where it has happened to fix itself. Thus arises the disfigurement of the shell.

So far we have only considered two great groups of the Mollusca, two which are represented by common shells, familiar to everybody. We must not leave the subject of the Mollusca without referring to their most aristocratic group, the Cephalopoda. These are represented in museums by the shells of the Pearly Nautilus, and of its not very near relative, the Paper Nautilus; and they are represented on English shores by the cuttle-fishes. All these agree with the Gasteropoda in the possession of a tongue-ribbon, and in classification are therefore treated with them under the name Glossophora.

With the Pteropods, transparent forms found swimming over the surface of the deep sea, the reader is not likely to have much to do. In classification they are now placed near the Sea-Slugs.

The Placophora, or Polyplacophora, wholly different from our usual idea of a shell-fish, may be named as creatures which the reader is quite likely to meet with. Though not very common, they are widely distributed over our coasts, and may be found near low-tide mark clinging to stones. Imagine a wood-louse without any apparent head which has taken to clinging to the rock like a limpet, so that it cannot be removed without injury, and you have a rough idea of their general appearance. Chiton is the name of these animals, which have received the group name of Polyplacophora, carriers of many plates, because their external covering consists of an armour of successive shelly plates. These also belong to the Glossophora or Tongue-ribbon Carriers, of which they present a comparatively primitive form.

Reference has already been made to the labours of the earthworm and of the insects, and to their important effects upon the vegetable world. Although the Mollusca include but one terrestrial group, the Snails, they, too, have played an appreciable part in modifying plant life. If we owe our flowers to the insects, we have probably to thank the snail for our medicines. For the snail dislikes bitter-tasting leaves, and lets them alone, thus exercising an artificial selection in favour of the survival of medicinal plants. In the same way the snail has favoured the survival of hairy and thorny plants, upon which it cannot easily crawl.