Chapter II.; and another, _L. rudis_, lives a little higher up, so that

it spends most of its time in a dry state, and is fast on its way to become a land-shell. At most of the familiar English seaside resorts one may see dozens of it baking in a hot July sun on rocks where only the highest tides can reach them: and yet under these conditions they continue to live and flourish. The periwinkles are remarkable for the great length of the tooth-ribbon, in comparison with the size of the animal. The number of separate teeth upon it has been estimated at 3500.

A familiar feature of the common periwinkle is the lid or stopper (Operculum), with which the animal can close the mouth of the shell. This is developed and carried by the outside of the animal's foot. In the periwinkle and other English molluscs it is comparatively soft and semi-transparent, and reminds one of a thin slice of horn. In many tropical molluscs, however, it is hard and shelly. The large tropical shells named _Turbo_ have massive lids of considerable weight. These shells, which are nearly allied to the pearly Top-shells (_Trochus_) of the English shores, are sold as ornaments, the outer coat of the shell being partly scraped off to show the inner coat of pearl: it is rarely, however, that the purchaser obtains a lid, or even knows that the creature had one. The reverse is the case with some of the smaller kinds, the lids of which, being brightly coloured, are imported without the shell, and sometimes set as articles of jewellery. Some of these are of a bright green hue.

While the lids of the Holostomata are rounded in shape, those that belong to the Siphonostomatous shells are necessarily more or less modified so as to fit the mouth of the shell, and are consequently oval or even claw-like in shape. The Sting-winkle already spoken of, the common small whelk, _Purpura lapillus_, and the large whelk, _Buccinum undatum_, are common shell-fish in which the elongated lid may be studied. The lid is not, however, like the tongue-ribbon, an essential feature of the structure of every univalve mollusc.[D] Not only are there special instances in which it is greatly modified, but also there are whole groups of univalve molluscs in which it is absent.

[D] There are one or two exceptional cases of gasteropod molluscs that have no tongue-ribbon. The majority of these are parasitic forms, which can get their food without the trouble of filing it down.

A curious suggestion has been made with regard to the lids of univalve shell-fish; namely, that the snapping to of the lid is capable of producing a sound, which may perhaps be audible at a distance under the water. Various molluscs have been credited with producing sounds, either by muscular movements or by the grating of the shell as the animal walks. The common Tortoise-shell Snail, _Helix aspersa_, sometimes makes a most alarming noise when crawling over a window. It has been disputed whether the sounds thus made are produced by the grating of the creature's tongue-ribbon on the glass, as it files off small particles of algae and vegetable moulds, which are invisible to our eyes: or whether they are sounds due to suction of the muscular surfaces, such as may be produced by drawing a wet finger across glass. The noise, however produced, is, as I can testify from experience, sufficiently loud and weird to be very startling, if heard in the dead of night.

Turn now to the Bivalves or Lamellibranchiate molluscs, which include the familiar oyster, cockle, and mussel. These are also known as the Pelecypoda, and as the Aglossa, or molluscs without a tongue-ribbon. The name Lamellibranchiate refers to the shape of the gills--"plate-like," or flat; the name Pelecypoda to the shape of the foot, "hatchet-foot."

The animal usually chosen as a type of these, the fresh-water mussel, is rather a dull sort of creature, so we have chosen a prettier and more lively specimen as a representative of the class; namely, one of the Scallops, _Pecten opercularis_, sometimes called the Quin, the shell of which is shown in the frontispiece of the book. This is one of the most beautiful, perhaps the most beautiful, of the English shells. The generic name, _Pecten_, the Comb-shell, probably refers, not to the shape of the gills, which is somewhat peculiar, but to the marking of the shell, which presents raised ridges, side by side. Anyone familiar with shells will see at once that this is an unusual pattern. There are plenty of bivalve shells with concentric ridge markings, comparatively few with radiating ridges. We shall see presently that there is a good reason for this. The specific name "opercularis," lid-like, refers to the neat round shape of the shell. Each half of the shell has a pair of "ears," so-called. The person who first gave this name to these flaps of shell, three of which are three-cornered and the fourth nondescript, must have been familiar in his youth with books afflicted with the "dog's-ear" disfigurement; for certainly there is no other kind of ear which greatly resembles these. The notch beneath the irregularly shaped ear is called the "byssal notch": many Pectens spin a byssus or thread, like that spun by the common Sea-Mussel, and thus anchor themselves to fixed objects for a time; this notch is the place where threads of this kind leave the shell.

The two valves of the shell differ in depth, one being flatter than the other; and the "ears" of the two valves differ in shape. The inside of the shell shows muscular impressions, but these cannot be seen in a photograph. The picture, however, shows the strong hinge-ligament which joins the halves of the shell, and the difference in depth and shape of the two valves. The valve on which the animal usually lies is the lighter in colour of the two, and has one ear much longer than the other.

The creature swims by means of the "mantle," or muscular margin of the body. It contracts this suddenly, after first opening the shell and taking in as much water as possible. Thus the water is squeezed out again, and the effect of this is to propel the animal in an opposite direction.

Now we are in a position to understand a little more about the shape of the shell. These curious "ears" possessed by the two valves, together form a straight, strong edge, which cuts the water as the animal flies along. It reminds us of a ship's prow, and not without reason, for the use of each is the same. A boat's sharp prow, compared with the rounded front of a "tub," makes all the difference in the possibilities of straight steering, and favours the putting on of speed: the ears of the shell are not less useful to our Scallop. The following account of the swimming powers of this species of Scallop, quoted by Woodward, was given by the Rev. D. Landsborough, who observed young specimens, about the size of the small ones in our picture, swimming about in a pool of sea-water, left by the ebbing tide. "Their motion was rapid and zigzag; they seemed, by the sudden opening and shutting of their valves, to have the power of darting like an arrow through the water. One jerk carried them some yards, and then by another sudden jerk they were off in a moment on a different tack." To the sharp prow, the _Pecten_ owes this capability of arrow-like flight. Its eyes are situated on the fringe of its mantle, and consequently near the wide end of the shell; its peculiar mode of progression, therefore, enables it to back away instantly from any enemy it sees.

Something must be said regarding the interior of the shell. The majority of bivalve shells have a complicated system of so-called "teeth," or interlocking projections, at the hinges of the shell: these exhibit great variety in different kinds of shell, and are therefore often a ready means of distinguishing one shell from another. The Scallop, however, is very deficient in this respect, as are also some of its near relations, for instance the oyster and its family group. The Fresh-water Mussel also gains its name, _Anodon_, or _Anodonta_, the Toothless One, from the same circumstance. The name often puzzles the beginner, who asks, bewildered, "But do Bivalves ever have any teeth?" True teeth, of course, they have none--it is the shell-hinge that has teeth, not the animal inside it. Not only have the bivalve shell-fish no teeth indeed, or tongue-ribbon, but furthermore they have no head. For this reason the group has not only received the name already mentioned, of Aglossa, the Tongue-less Ones, but also that of Lipocephala, _i.e._ Molluscs in which the head is not developed. The reason of its absence is not far to seek--a head would be no use inside such a shell. The snail-shell, so differently built, allows freedom for the head; the bivalve mollusc, squeezed in between its valves, has room only for a mouth.

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.

The larval forms of the Mollusca differ considerably from the adult. That of _Anodon_, the fresh-water mussel, at first received, in consequence, a different name, that of Glochidium, by which it is still known, although it has now been long identified as a larval form. It is exceptional in the fact that it is parasitic on fish.

The usual Molluscan larva is a ciliated creature which has been compared to a modified trochosphere. It is preceded by a gastrula stage, and it develops later on into what is called a "Veliger," or "veil-carrying" larva, so called because it has in front a broad two-lobed ciliated expansion, the velum. This larva is adapted for swimming, which is accomplished by means of the velum. In terrestrial molluscs, the development is necessarily much more direct. It is worthy of note that the periwinkle mentioned above, which lives high and dry (_L. rudis_) has no larval form, while its relatives that live under water develop in the usual way.

The eggs of Mollusca are often enclosed in tough cases, calculated to resist waves and weather. Some of these are shown in miniature, in the group of eggs of various kinds, Fig. 35.

TABLE SHOWING THE CLASSIFICATION OF THE MOLLUSCA

{ AGLOSSA: the LAMELLIBRANCHIATA, also called { CONCHIFERA, and PELECYPODA. =MOLLUSCA.= { { { GLOSSOPHORA { GASTEROPODA. { CEPHALOPODA.