B. Lindsay

The shell-fish are called Mollusca, the soft-bodied animals. It will easily be seen that this name was intended to point out the distinction between them and the Arthropoda, as regards the way in which the skin is protected. In the latter, as we have seen, the skin itself is hardened. In the shell-fish, the skin secretes a covering which lies outside it. Just as our skins pass out superfluous moisture to the outside, in the form of perspiration, so the skin of the mollusc continually passes to the outside the solid substances which the body has taken in from the sea-water; and by the continual accumulation of these, the shell is formed. This, at least, is the view taken by modern authorities of the formation of the shell in most instances.

The juvenile shell-collector usually begins his knowledge of the classification of the Mollusca, by learning that shells are classified as Univalves and Bivalves. This distinction is useful as a beginning. Univalves, that is to say shells which consist of one piece, are those of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to spell it. Bivalves, or shells which consist of two flaps, are those of the Lamellibranchiata or animals with plate-like gills, such as the mussel or oyster.

Let us begin with the former. Everybody knows the snail. The snail proper bears a typical univalve shell: though in its relatives (the slugs), the shell is more or less suppressed. The name, Gasteropoda (stomach-footed animals), is supposed to be descriptive of the way in which a snail crawls. Half getting out of its shell, so to speak, it does its best to lay its body to the ground, and its so-called "foot" is an extensive muscular expansion underlying its body, not just a muscular organ thrust out of the shell, as in some other groups. The shell, the mode of crawling, and the "horns," tipped with eye-specks, and directed, intelligently and inquisitively, towards things of interest—these make up, for most people, the idea of Snail. But the most distinctive feature of the class is a less obvious feature, namely, the structure of the tongue. We may see, on any damp day or dewy evening, the snail working away with its tongue at some tender leaf. Its tongue is practically a file with which it files away the substance of the leaf, the resulting green mash being thus made ready in minute quantities for the snail to swallow. Thus are made the too familiar holes which disfigure the leaves of plants in our garden. When seen under the microscope, the file-like structure of the tongue is visible; indeed, in large tongues, it may, to some extent, be made out with the naked eye. Across the tongue, which is a flat ribbon-like structure, there runs a pattern of small teeth, bilaterally symmetrical, and this pattern is repeated over and over again throughout the whole length of the tongue. It might be thought that snails' tongues, being so much alike in their mode of use, would not need to be very various in pattern: but far from this, they vary in appearance as much as the shell. Not only is there a different pattern for every different order of the class, but a different pattern for every genus; nay, there are even distinctions between the tongues of different species in the same genus. Consequently some authorities on shell-fish prefer to classify them by their tongues, a classification which for the most part holds good. So characteristic is the tongue of the Gasteropod, that when new animals have turned up which were difficult to classify by means of the structure of the body, they have been finally recognised as Molluscs, somewhat related to the snails, by the tongue. This file-like tongue-ribbon of the snails is often called the Odontophore or Tooth-Carrier; sometimes the part which actually bears the teeth receives the name of the radula.

The snail and its relative, the slugs, belong to the Pulmonate (i.e. air-breathing) division of the Gasteropoda. The sea-slugs, in which, like the land slugs, the shell is absent or reduced, are relatives of the land snails. Some of those found on our own shores are handsome creatures, brilliantly coloured. Both groups fall under the division Euthyneura, while the majority of the marine univalves belong to the division Streptoneura (i.e. Gasteropods with twisted nerves). The Gasteropods, in the course of the evolution of their shell, have had the body thrown crooked by the burden of carrying it; the Streptoneura are the forms in which this crookedness is most pronounced; in the Euthyneura it is less so. There are degrees of crookedness even among the Streptoneura; and the limpet is less crooked than the periwinkle (see Table, p. 30).

The older classifications of the Gasteropoda were largely founded on the characters of the shell; but these, though in the main they hold good, have required some modifications in recent times. Conchology, the study of shells, was at one time the hobby of many collectors whose knowledge of the animals possessing the shells was not of a very extensive kind; and consequently the very name of conchology is often enough to ruffle the feelings of the zoologist of the present day. Yet many interesting problems of variation may be studied from shells alone, by those whose circumstances forbid them to study the living animal. Nor is there any branch of zoology which is more useful to the teacher who wishes to catch the eye and the attention of the beginner in the study of natural history, especially if the beginner is young, as beginners ought to be. Therefore we must by no means undervalue the past labours of conchologists, or the valuable collections which their industry has brought together and set in order for the benefit of the world.

For example of the most crooked, or Azygobranchiate division of the Streptoneura, turn now to Fig. 33, in which we see a typical Gasteropod shell, Murex ramosus, the Branchy Murex, aptly enough named from the many prickly branches which beset it. These rough points are probably assumed for protective purposes; any animal that might wish to dine upon the Murex ramosus would think twice before trying to swallow it—the morsel of shell-fish is so small, its shelly case so large and so prickly. If we look for its nearest English relative, that is Murex erinaceus, the Hedgehog Murex, or Sting-winkle. This, though a comparatively plain shell, has still enough rough ridges upon it to have secured it a comparison to the prickly hedgehog. Perhaps the most prickly member of the genus, however, is Murex tenuispina, sometimes called Venus' Comb, because the crowded parallel spines which decorate the elongated front of the shell somewhat resemble the parallel teeth of a comb.

Fig. 33.—The Branchy Murex, M. ramosus, a typical specimen of the shell of the Carnivorous Gasteropods. Sp., spire or posterior end of the shell; S, siphon or anterior end of the shell. Fig. A, shows the mouth of the shell; Fig. B, the exterior only. Less than one-half the natural size.

How does the Murex get its living? Let us notice the shape of the shell, drawn out to a point, at the end opposite to the spire. According to the older classification of the Mollusca, now somewhat fallen out of use, this point marks the shell as belonging to one of the Siphonostomata (shell-fish with a siphon at the mouth of the shell, i.e.). These shell-fish are, with few exceptions, carnivorous; not that the siphon shape of the shell has any direct connection with the animal's way of feeding. Just as the snail files among soft vegetable substances, so the Murex and many of its relations file away much harder things. A Sting-winkle, or a Dog-whelk, can sit down over a helpless bivalve shell-fish, and patiently file away, until it has worked a neat round hole in the protecting shell of the latter. You may find, among the dead shells on any sandy part of the English coast, any number of bivalve half-shells with a neat little round hole in them, indicating unmistakeably how the tenant came to its death. There is some controversy as to the spot chosen by the assailant for its attack. Some authorities have stated that the predatory mollusc is so wise that it knows where to find a weak spot, and makes a hole just over some vital organ of the bivalve, or else above its adductor muscles, so that, when these are cut, the half-shells cannot be drawn tightly together and kept shut. Recently this has been denied, and statistics of the attacks of Purpura, the common small whelk, a relation of the Murex, on Mytilus edulis, the Common Mussel, have shown that the perforation occurs in every part of the shell. It is possible, however, that the Mussel, from the peculiar shape of its shell, offers an exceptional case; and I am inclined to think that in the case of bivalves of a more flattened shape, the earlier statement holds true. At South Shields, England, perforated half-shells of the Common Venus (Fig. 34) are so abundant that the children string them for necklaces; yet I have never been able, by the most industrious search, to find more than one or two specimens in which the hole is at all near the lip of the shell. It is possible that these exceptional instances were the work of a young and inexperienced univalve mollusc, or a stupid one. It is possible, also, that the mode of attack differs somewhat according to the species of the assailant. (It should perhaps be explained, for the benefit of those who have no experience in the ways of children or of shell necklaces, that the hole must be moderately near the beak of the shell, to enable the shell to "sit" properly on a string. Every unit in the necklace may therefore be counted as one in favour of the older theory.) Many of the Siphonostomatous molluscs are surprisingly active and strong, so that they are well fitted for a predatory existence. In fact, they not only eat bivalves, but occasionally attack the vegetable-feeding univalves when nothing better is to be got, so that occasionally the shells of these also may be found displaying the deadly little round hole we have described.

Fig. 34.—Half Shells of the Common Venus, several of them perforated by carnivorous molluscs. From South Shields, England.

Let us contrast with the Murex one of the shells which are "holostomatous," i.e. possessing an unindented shell-mouth—that is to say, one without a "siphon." The common edible periwinkle, Littorina littorea, may be taken as an example. No shell is more familiar; even the town-dweller, who has never found it on the sea-shore, has seen it often on stalls in the slums. The mouth of the shell is quite round and unindented, and in this case the character holds good as the mark of a vegetable-feeder—a non-predatory sea-snail. It is hardly necessary to remind the reader that its name (the shore-shell) is given it because it lives where the tide leaves the rocks exposed during part of the day. Another common species of Littorina, which frequently lives a little lower down, where the large sea-weeds grow, has been described in 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