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 (_g^1_ to _g^4_ 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. [Illustration: 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; _g^1_ to _g^4_, 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.]