The skeleton of the cuttle-fish (sepia officinalis) is extremely singular: its back-bone, for some purpose unknown to us, is much broader than that of any other aquatic animal of the same size, and of course would be much heavier but for a curious contrivance to prevent this effect, which consists in its being exquisitely porous and cellular, and capable, like the bones of birds, of becoming filled with air, or exhausted of it, at the option of the animal, in order to ascend or descend with the greater facility. It is an animal of this kind, or closely akin to it*, that inhabits the shell of the beautiful paper-nautilus, and still more beautiful pearl-nautilus (argonauta and nautilus tribes), and which hence obtain no inconsiderable portion of that lightness which enables them, with their extended sails, to scud so dexterously before the wind. + In the calamary (sepia loligo) we meet with an approach towards the same contrivance, in a kind of leafy plate introduced into the body of the animal; and even in

The animal has commonly been supposed to be a real sepia or cuttle-fish; but several naturalists have of late doubted this, inasmuch as there are a few marks of distinction that seem to take it out of this genus. Rafinesque has hence made another genus, for the purpose of receiving those which possess these distinctive signs; and Dr. Leach has lately distinguished it specifically, in consequence of specimens sent home from the unfortunate Congo expedition, as collected by Cranch, by the name of Ocythoe Cranchii. Even this animal, however, is regarded as a parasite in the shell, and only possessing when empty. The proper animal is not known to the present hour. See Phil. Trans. 1817, p. 293.

+ The recent discoveries of Mr. Owen with regard to the nautilus are richly worth the student's attention, though they cannot be detailed here with sufficient fulness. — Ed.

the cloak of the slug-tribe we trace something of the same sort, though proportionably smaller, and verging to the nature of horn.

Generally speaking, the bones grow cartilaginous towards their extremities, and the muscles tendinous; by which means the fleshy and osseous parts of the organs of motion become assimilated, and fitted for that insertion of the one part into the other upon which their mutual action depends. The extent and nature of the motion is determined by the nature of the articulation, which is varied with the nicest skill to answer the purpose intended. In ostraceous worms, the only articulation is that of the hinge in the cancer tribes, the tendon is articulated with the crust, whence the wonderful strength and activity of the claws; and it is articulated in a similar manner with the scaly plates of some species of the tortoise. In insects, the part received and the part receiving form each a segment of a spheroid; whence the motion may be either rotatory or lateral, at pleasure. In mammalian animals, the lower jaw only has a power of motion; but in birds, serpents, and fishes, the upper jaw in a greater or less degree possesses a similar power.

The motion of serpents is produced, according to Sir Everard Home, by their ribs, which for the most part accompany them, not only as organs of respiration, but from the hind extremity to the neck, and are possessed of a peculiar power of motion by means of peculiar muscles. "The vertebræ are articulated by ball and socket joints, (the ball being formed upon the lower, and the socket on the upper one,) and have therefore much more extensive motion than in other animals." In

the draco volans, the skeleton of the wings is formed out of ribs which " are superadded for this purpose, and make no part of the organs of respiration; the ribs in these animals appear to work in succession, like the feet of a caterpillar."

The TEETH vary in their form and position almost as much as the bones. Where jaw-bones exist, they are usually fixed immoveably in their sockets; but in some animals a few of them are left moveable, and in others the whole. The mus maritimus, or African rat, the largest species of this genus which has hitherto been discovered, and seldom less than a full-sized rabbit, has the singular property of separating at pleasure to a considerable distance the two front teeth of the lower jaw, which are not less than an inch and a quarter long. That elegant and extraordinary creature the Kangaroo, which we may soon hope to see naturalised in our own country, is possessed of a similar faculty. And the hollow tusks or poisoning fangs of the rattle-snake, and other deadly serpents, are situated in a peculiar bone on each side of the upper jaw, so articulated with the rest, that the animal can either depress or elevate them at his option. In a quiescent state, they are recumbent, with their points directed inwards; but whenever the animal is irritated, he instantly raises them; and at the moment they inflict a wound, the poison, which lies in a reservoir immediately below, is injected through their tubes by the immediate act of pressure.

In the shark and ray genera, the whole of the teeth are moveable, and lie imbedded in jaw-cartilages instead of in jaw-bones, and like the fangs of the poisonous serpents are raised or depressed at

pleasure. The teeth of the saw-fish are similarly inserted; while his long sword-like snout is armed externally, and on each side, with a taper row of sharp, strong-pointed spines or hooks, which are sometimes called his teeth, and which give rise to his popular name.

The ant-eater and manis swallow their aliment whole; and in many animals the jaws themselves perform the office of teeth, at least with the assistance of the tongue. In birds this is generally the case, sometimes in insects, whose jaws are for this purpose serrated or denticulated at the edge, and frequently in molluscous worms. The jaws of the triton genus act like the blades of a pair of scissors. The snail and slug have only a single jaw, semilunar in its form, and denticulated: but the mouth of the nereis has several bony pieces. The sea-mouse (aphrodita aculeata) has its teeth, which are four, fixed upon its proboscis, and is of course able to extend and retract them at pleasure; and the leech has three pointed cartilaginous teeth, which it is able to employ in the same way, and by means of which it draws blood freely. In like manner, though insects chiefly depend upon a serrated jaw, yet many of them are also possessed of very powerful fangs, of which we have a striking instance in the aranea avicularia, or bird-spider, an inhabitant of South America, found among trees, and a devourer of other insects, and even small birds. It is of so enormous a size that its fangs are equal to the talons of a hawk; and its eyes, which are eight in number, arranged as a smaller square in the middle of a larger, are capable of being set in the manner of lenses, and used as microscopes.

In many animals, especially the herbivorous, the tongue itself is armed with a serrated apparatus, the papillæ being pointed and recurvated, and enabling them to tear up the grass with much greater facility. In the cat-kind the tongue is covered with sharp and strong prickles, which enable the animal to take a strong hold; and similar processes are met with in the bat and the opossum. In the lamprey and myxine families, the tongue itself is covered with teeth. In that grotesque and monstrous bird the toucan, whose bill is nearly as large as its whole body, the tongue is lined with a bundle of feathers, of the use of which, however, we are totally ignorant, though it is probably an organ of taste.

In the crab and lobster tribes the teeth are placed in the stomach, the whole of which is a very singular organ. It is formed on a bony apparatus, and hence does not collapse when empty. The teeth are inserted into it round its lower aperture or pylorus their surface is extremely hard, and their margin serrated or denticulated, so that nothing can pass through the opening without being perfectly comminuted. The bones and teeth are moved by peculiar muscles. It is a curious fact, that at the time the animal throws off its shell, it also disgorges its bony stomach and secretes a new

one.

The teeth of the cuttle-fish are arranged not very differently, being situated in the centre of the lower part of the body, where, in fact, its mouth is posited; they are two in number, and horny, and in their figure exactly resemble the bill of a parrot.

The teeth of the echinus genus (sea-hedgehog) are of a very singular arrangement. A round open