The skin of the cuttle-fish presents in one vast hollow, occupying all the extent of the back, a great calcareous part, the form and structure of which is quite characteristic of this genus. It is known as the cuttle-bone (Fig. 316). This bone is used for many purposes; among others, it is used in a powdered state as a dentifrice. It is sometimes suspended in the cage with captive birds, that they may whet their beaks on it, and collect phosphate of lime for the formation and repair of their bones. The osselet is oval or oblong, some provided with a slightly salient point. The upper part is surrounded with a horny or cretaceous margin, and presents in the centre a combination of spongy cells.
Most of the Cephalopods secrete a blackish, inky fluid, to which some allusion has been made, but the uses of which, in the economy of the animals, is imperfectly known. The cuttles have considerable quantities of this liquor, which is contained in a sort of sac or ink-purse, placed low down in the abdomen. When the animal is pursued or threatened with danger it discharges a jet of the fluid, which renders the water thick and muddy, and permits it to escape in the obscurity from its pursuers. It appears that the cuttle-fish avails itself of this stratagem when left accidentally ashore. It is related of an English officer, that, having dressed for dinner, and having some time to spare, he proceeded along the shore on his favourite search for objects of natural history. He reached a hollow rock in which a cuttle-fish had established its quarters; he soon detected the animal, which looked at him for some time with its great prominent eyes; in short, they watched each other with fixed attention. This mute contemplation came to a sudden and unexpected termination by the discharge of a voluminous jet of inky fluid, which covered the officer, which was the more unfortunate, since he was in his summer dress of white trousers.
Fig. 316. Internal bone of Sepia officinalis.
Fig. 317. Sepia tuberculosa (Lamarck).
The ink of the cuttle-fish is a favourite pigment, used in water-colour painting under the name of sepia. It is truly indestructible; and the hard and black substance found in the sac of fossil species of cuttle-fish when diluted with water produces a brilliant sepia. This property of the inky fluid was well known to the Romans, who used it in making ink. It was long supposed to be the chief ingredient in China ink; but a recent traveller, Mr. Seebold, who has visited the manufactory, and investigated the subject, has revealed the true process by which it is prepared.
The cuttle-fish affects the sea-shore; they are along-shore molluscs. The flattened form of their bodies is favourable to a coasting life, by permitting them to rest easily on the bottom. Still they do not remain all the year round upon the coast. The cold in temperate regions, and the opposite reason in warm regions, leads them to withdraw from the shore, to which they only return in the spring. They are rarely seen in the Channel in winter, but with the vernal sun they appear in large shoals. What is the mechanism by which these animals are moved? When the cuttle-fish wishes to swim rapidly and backwards, they advance in the water by means of the locomotive tube, sending back the ambient liquid. When they wish to approach a prey slowly in order to seize it, they swim by the aid of their fins and arms. In order to swim backwards, they contract the arms provided with tentacles, and spread out horizontally the arms without tentacles.
The cuttles are flesh-eaters, and tolerably voracious. They feed themselves upon fishes, molluscs, and crustaceans. They are true aquatic brigands, who kill not to feed themselves, but for the sake of killing; and Nature, by a just equilibrium, applies to them the lex talionis. They fall victims, in their turn, to the vengeful jaws of the porpoises and dolphins. Such is the terrible law of Nature: some must die that others may live. Michelet gives us a glimpse of the manner in which the dolphins dispose of the cuttle-fish in his "Livre de la Mer." "These lords of the ocean," he says, "are so delicate in their tastes that they eat only the head and arms, which are both tender and of easy digestion. They reject the hard parts, and especially the after-part of the body. The coast at Royan, for example, is covered with thousands of these mutilated cuttle-fish. The porpoises take most incredible bounds, at first to frighten them, and afterwards to run them down; in short, after their feast, they give themselves up to gymnastics."
In the spring the cuttle-fishes deposit their eggs, but without abandoning them. On the contrary, they exhibit a truly maternal care, taking much trouble to attach them to some submarine body, in which position the temperature of the water serves to hatch the eggs. Sepia officinalis, for example, chooses, at the moment of laying, a stem of Fucus, a foot of Gorgonia, or some other solid submarine body not less in dimensions than the little finger, and there it firmly attaches its eggs, which are pear-shaped, that is, pointed at one extremity, while a long lanière of a gelatinous nature, flat and black in appearance, with which they are provided, surrounds the solid body like a ring. Each female lays and attaches in this manner from twenty to thirty eggs, which are clustered together somewhat like a bunch of fine black grapes (Fig. 318). About a month after this the eggs are hatched.
The colours of Sepia officinalis vary considerably; but in general it may be remarked that the males are ornamented with deeper colours than the females. Transverse bands of a blackish brown furrow their backs, and diminish their breadth. Outside of these bands are small spots of a vivid white: very near the edge there is a white border, accompanied inside with a second edging of a beautiful violet. The median and anterior parts of the body are spotted here and there; beneath, a whitish tint with reddish speckles prevails.