Fig. 19.—Some British Marine Bivalve Molluscs.
a, The smaller Piddock, Pholas parva, which bores into chalk, clay, and hard rock. Natural size.
b, The Razor-shell, Solen siliqua. The muscular foot is seen protruding from the shell. One-third the natural size, linear.
c, Venus verrucosa. Natural size.
d, Cardium echinatum. Two-thirds the natural size, linear.
e, Pinna pectinata, the "cappy longy." One-fifth of the natural size, linear.
Though "shells" are often numerous on parts of the seashore, some beaches (as, for instance, at Falmouth, at the mouth of the Eden of St. Andrews, and at Herm in the Channel Islands) being so placed in regard to the currents and waves of the sea that great quantities of shells of dozens of species are thrown up, and even "make up" the beach, yet there are not so very many Molluscs which live commonly on the shore between tide-marks. The shells which are accumulated as shell-beaches have come from animals which lived in quantity at depths of ten or twenty fathoms, whence they can be brought up alive by the dredge. There are, however, certain bivalves and certain univalves which are commonly to be found in the living state between tide-marks. You will not find the oyster there on our own coast, but in Australia they have picnic parties where every guest provides himself with a hammer and a bottle of vinegar and a pepper-pot, and at low tide proceeds to chip the oysters off the rocks on which they grow tightly fixed, and to eat them "right away" before they have time to lose their good temper and sweetness! In Jamaica they show you oysters apparently growing on trees high up in the air, but they are dead, having attached themselves to the branches of a young tree which dipped into the water. Once fixed there, they were unable to move as the tree grew and carried them up with its branches above the sea-level.
The only bivalve at all common and visible to the eye between tide-marks is the common or edible sea-mussel, which is attached in purple clusters to the rocks (as in North Cornwall), or forms a wide-spreading pavement, called a "scalp," of as much as an acre in extent, on which thousands of mussels lie side by side. But by digging in the sand and mud between tides there are other living bivalves to be found, which burrow more or less deeply. The razor-shell ([Fig. 19, b]) is one of these [(see p. 80)]. Often (as at Teignmouth and Barmouth) we find "cockles" buried in the sand, and those delicate, smooth bivalves not an inch long, white outside and purple within, which are made into soup at Naples and are called "vongoli," but have no English name. Other "clams" (Tapes, which is eaten in France, even in Paris, and Mya, and Scrobicularia which lives in black mud) may be dug up, but they are devoid of English names because we do not eat them; hence I have to speak of them by their Latin scientific names. As to univalves, there are three which are found almost everywhere on our coasts where there are rocks, namely, the periwinkles (one species of which actually lives above high tide-mark), the limpet, and the dog-whelk. A small species of top-shell or trochus is also very common, and so is the chiton, or armadillo-shell, which, though really the most primitive and nearest representative of the ancestors of all univalve molluscs, yet has its own shell of a very peculiar character (sometimes with very minute eyes—true eyes—dotted about on it), and always divided transversely to its length (not right and left) into eight separate pieces, which, indeed, seem to be really separate, independent little shells, corresponding to eight segments like the segments of a shrimp or an earth-worm.
Let us now compare the soft animal of one of the bivalves—say the common cockle—with the soft animal to which a univalve shell belongs—say the limpet. They can be kept alive and watched in a finger-glass of sea-water, and can be removed from their shells and examined more closely—by killing them by dipping them for half a minute into very hot (not boiling) water. Both these molluscs—like all others—adhere tightly at one place to the shell. They cannot be removed from it alive, and make a new shell or creep back into the old one, as can some worms (e.g. the serpula) and other creatures which form a hard shell to live in. Certain muscles of the soft mollusc are so closely fixed to the shell that they must be torn in order to separate it. These muscles draw the two valves of the bivalve together, and shut it tight. You can verify this whenever the oyster-man "opens" an oyster for you. When at rest the shells gape, being kept open by the horny, elastic hinge-piece. Some bivalves (for instance, the common scallop, or pilgrim's shell, which can often be dredged in shallow water, and of which a large kind is sold in the London fish shops) actually swim in the sea-water by aid of this mechanism, the shells opening by elasticity and being closed by the muscle joining one to the other, at rapid intervals, flapping like the wings of a butterfly.
In the univalves the attachment of the muscle to the shell gives a fixed point for all the movements of the animal. The limpet has a well-marked head and neck—a pair of sensitive tentacles, and a small pair of dark-coloured eyes. The mouth is at the end of a sort of short snout. Just within the mouth, and capable of being pushed forwards to the level of the lips, is a most extraordinary rasp. It consists of a long ribbon, beset with fine horny teeth—very sharp and complicated in pattern. The ribbon extends far back into the body, and is worn away by constant use at the orifice of the mouth. It grows forward, like one of our finger-nails, as it wears out, and a new, unworn portion takes the place of that worn away. It is constantly in use to rasp and bring into the mouth the particles of the seaweed on which the limpet feeds. It is easy to remove this rasping ribbon with a needle or pen-knife, and examine it with a microscope. Every one of the hundreds of kinds of univalve molluscs has this ribbon-rasp, and its teeth are of different patterns in the various kinds. It is worked by very powerful little muscles, backwards and forwards, and is strong enough in the whelks to bore a round hole into other shells (for instance, that of the oyster), when the whelk proceeds to eat the soft animal, whose protecting shell has been thus penetrated. Some of the large marine snails produce a poisonous secretion from the mouth, which renders their attack with the ribbon-rasp all the more deadly to other marine creatures. The cuttle-fishes and octopods, which are molluscs too, possess, like the univalve limpets, snails, and whelks, this terrible ribbon-rasp in the mouth. It is an indication of a common parentage or ancestral relationship in the forms which possess it.
The cockle ([Fig. 19, d]), to which we now turn, has not got a ribbon-rasp, nor anything of the kind. It has a mouth with four flapper-like lips, but no projecting head, no eyes, no biting mechanism, nor have any of the bivalves, excepting a few which like the scallop have a series of eyes on the edge of the soft mantle or flap which lines the shell. This constitutes a greater difference between bivalves and the univalves than does the shape of the shell. They are a very quiescent, peaceful lot, feeding on microscopic floating plants (diatoms and such), which are drawn to the mouth by currents of water set going by millions of vibrating hairs arranged on four soft plates hanging under the protecting arch of the shell, and called in the oyster—in which bivalve most people know them—the "beard."
The limpet adheres to rocks by a great disk-like mass of muscle, which is called "the foot." It is really the whole ventral surface, and it can loosen its hold, and, by curious ripples of contraction, cause the animal to creep or glide over the rock. At low tide the limpet is exposed to the air, and remains motionless, but when the tide is up it makes a small excursion in search of food, never going more than a foot or two from the spot which it has chosen, and returning to it, so that in the course of time it actually wears away a sort of cup or depression at this spot—if the rock is not of exceptional hardness. The word "foot" is applied to the ventral disk-like surface of the limpet, because in many univalves this region becomes drawn out, and is connected by a comparatively narrow and nipped-in stalk or pillar with the rest of the animal. This occurs in the univalves which have large spiral shells, into which the whole of the soft animal can be deeply withdrawn, which is not the case with the limpet. You may find on the shore at Torquay a sea-snail (Natica), in which the animal is quite invisible, drawn far up into the shell. Place this in sea-water and watch it. Soft semi-transparent lobes begin to issue from the mouth of the shell, part of the soft distensible foot appears swelling out and growing bigger and bigger, and soft folds spread out from the mouth of the shell, and gently creep over it, and completely envelop it; the foot begins to grip the bottom of the vessel, and the animal "crawls." At last, swelling out from the other folds of soft but tense "molluscan" substance, the head and its tentacles emerge. Touch the animal and it shrinks rapidly, disappearing into the shell.
It used to be thought (about twenty-five years ago) that the molluscs expand their bodies in this manner by taking water, through definite apertures provided with valves, into their blood, and that, having thus swelled themselves out, they could shrink and reduce themselves by pouring out again the in-taken water. The behaviour of some other marine animals, namely the sea-anemones, which really do act in this way, made this explanation of the swelling and shrinking of molluscs seem probable. It was also known that the star-fishes and sea-urchins actually do take in the sea-water into a system of vessels connected with their wonderful sucker-bearing tentacles. But it turned out on close examination that the molluscs do not take in or shed out water in this way. A hole, which was thought to let in water into the blood of sea-snails, was shown to be only the opening of a great slime-gland. In the case of some bivalves which have red-blood corpuscles, I showed that the blood is never made paler, nor are the red corpuscles shed during the great distensions and contractions of the body. Measurements were made to determine the removal of water from a glass jar by an expanding sea-snail, and it was found that none is removed or taken up; in fact, the whole of what is very often an astonishingly large and bulky distension of the foot, or of lobes of the body, and the subsequent rapid shrinking of the same parts, depend entirely on the blood being injected from the rest of the body into the swelling part, and squeezed from it into the depleted region when the swollen part shrinks again. The firm, opaque shell hides from view the change of shape of the concealed body, and we see only the distended foot or other lobes which project from the shell.
The cockle has a "foot" of a very curious scythe-like shape, usually carried bent up between the two valves of the shell. Those who rightly like to confirm statements about unfamiliar animals can do so by buying a cockle or two at the fishmonger's. Some bivalves (the Noah's-ark-shell, called "Arca," and a few others) have a great flat foot, like that of the univalves, and crawl about on it. But in most bivalves it is curiously elongated and modified, for the purpose of burrowing into sand by vigorous strokes, and in some it is suppressed altogether, as in the oyster. The cockle is remarkable for the fact that when placed on a board or a rock it will give such a vigorous kick with its bent foot as to throw itself up a yard or so into the air. A naturalist (Stutchbury) dredging in Port Jackson, Australia, many years ago was overjoyed at discovering in his net three specimens of a very peculiar kind of cockle (Trigonia), which was till then only known in the fossil state from the oolite strata of Europe. He placed the three novelties on the seat of his boat, and was looking at other things when he heard a click-like sound, then another. He turned his head and saw that two of his newly-discovered "living fossils" had jumped overboard, and had the pleasure of seeing the third perform the same feat!