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.