Oysters are delicate morsels—still appreciated by that class of the population which nevertheless shudders at the thought of eating the high-flavoured “whilk” or the gristly “periwinkle,” and neglects the admirable mussel, so rightly valued by our French friends. There are a number of interesting facts about the nature and life-history of oysters, and the different kinds of them—a knowledge of which does not diminish, but, on the contrary, rather adds to the pleasure with which one swallows the shell-fish. I remember the time when “natives” were sold in London at sixpence the score. When I was a schoolboy at St. Paul’s they were no more than sixpence a dozen at the best shops in Cheapside. That inevitable form of British enterprise which is known as “monopoly,” many years since laid hold of the oyster business, and rapidly raised the price of the best natives to eight times what it had been, while the typhoid “scare” came subsequently as a sort of poetical justice, and threatened to ruin the oyster monopolists. As a matter of fact, there is no difficulty in freeing oysters from any possible contamination by the typhoid germ. They have only to be kept for ten days or a fortnight in large tanks of sea-water of unquestionable purity—after removal from the fattening grounds (tanks or waterways), and they rid themselves of any possible infection. It is the interest of the oyster merchant to make sure that this treatment is strictly enforced. It is a noteworthy fact that the anciently established habit of drenching an oyster with vinegar before eating it is precisely the best treatment, except cooking them, which could have been adopted in order to destroy the vitality of typhoid germs—although the existence of such germs was unknown when the practice arose, and vinegar or lemon-juice was taken with uncooked oysters as a matter of taste, not as a safeguard.
The oyster is sometimes grandiloquently styled “the succulent mollusc”—and it is classed together with other bivalve shells and true “shell-bearing” shell-fish, such as whelks and snails (not lobsters and crabs, which are Crustacea), in a great division of animals known to naturalists as the Mollusca. This word is only a Latin form of the name Malakia, which was given to the cuttle-fishes by that wonderful man Aristotle, the Greek—and means “soft creatures.” A bivalve, or two-shelled mollusc, like the oyster, may be compared to an oblong notebook. The hard covers correspond to the two shells and the back to a horny piece by which the two shells are united, forming the hinge. If you place a piece of indiarubber (a thickish bit) between the covers of the notebook so that it lies near the back, and then try to shut the book, you find that it requires some pressure to do so; when you leave off pressing them the covers gape. The horny hinge-piece or ligament of the shells of the oyster and other bivalves acts in this way. The shells are only kept closed by a strong muscle, which runs across from shell to shell (Figs. [28] and [30]m). When the oyster is at rest or when it is dead the muscle does not act, and the elastic hinge-piece or ligament causes the shells to gape. The animal within the shells may be compared to the leaves of the notebook. Suppose there are twenty-six leaves, then the outermost leaf on each side corresponds to the two soft living membraneous flaps which secrete the two shells or covers of the oyster and lie closely on them (a, b, Figs. [28] and [30]); the next two on each side (rather shortened leaves, folded in from below) are the flat gills or “gill-plates” of the mollusc (g1 to g4 in [Fig. 28]); whilst we must suppose the twenty middle leaves to be “pulped” and fused together to represent the body of the shell-fish.
Fig. 28.—An oyster with the right-side shell removed; c, the pearly inner surface of the left-side shell; d, the horny outer layer projecting beyond c; a, the thick edge of the “mantle”-flap of the left side; b, the thick edge of the mantle-flap of the right side thrown back towards the centre, so as to show what underlies it; e, notch in the surface (pulled a little upwards and forwards) where the ligament is formed; h, the hinge surface, where the removed shell rested on the left-side shell; g1 to g4, the four gill-plates or flaps, two right, two left—the so-called beard; l, the four corresponding lip lobes: the mouth lies deeply between the second and third—that is between the right pair and the left pair; m, the central shell-muscle, which runs from one shell to the other.
Fig. 29.—Part of a row of the lashing hairs or “cilia” which cover the gills of the oyster. This represents part of a single row, only the 1/400th of an inch long from one end to the other. The whole surface of the gill and other parts is beset with these hairs, not in single rows, but closely, as the hairs of fur are set. The drawing is intended to show the way in which the hairs actively bend downwards (or “lash”), and then rise up again in regular waves, the movement or wave passing along in the same way as a wave of bending and returning to the upright passes over a ripe cornfield when a light breeze blows across it (see also [Fig. 40]).
The oyster’s gill-plates, commonly called “the beard,” are covered on the surface by microscopic hairs of a very remarkable kind ([Fig. 29]). They are soft, living protoplasm, and are continually “lashing,” bending forwards and straightening again at the rate of some three or four hundred strokes to the minute. They all work rhythmically together, and produce a strong current in the water, which bathes the surface of the oyster when the shells are open. Such microscopic vibrating hairs are very common in aquatic animals, and are called “cilia.” The current which they produce causes oxygen-holding water to flow from without over the gills, and so aerate the blood of the oyster, and also carries into the chamber protected by the shells excessively minute particles, chiefly microscopic plants, which are driven on to the small, open mouth of the oyster, placed far up on its body. These microscopic food-particles are wafted down the oyster’s throat by similar vibrating hairs into the stomach and intestine. An oyster has no other means of taking food, and almost without cessation, as the oyster lies on the sea bottom with its muscle relaxed and its shell “gaping,” the nourishing stream is kept going. If poisonous matter, bad water, or some violent disturbance make themselves apparent, the shell-muscle acts, and the oyster tightly closes his shell. Such things make themselves “apparent” to the oyster, for it has a nervous system, and though it has no eyes (the nearly allied “scallop” has a number of eyes) it has a delicate sense of smell and touch, and also what is usually considered to be an organ of hearing.