(1524). The most important piece met with in the cartilaginous skeleton of the Cuttle-fish encloses and defends the brain, and therefore is most appropriately called the cranial cartilage, being the correspondent, both in position and office, with the cranium of a vertebrate animal. This rudimentary cranium embraces the oesophagus with a cartilaginous ring, encases the brain, affords passage to the optic nerves, and gives off orbital plates for the protection of the eyes. The cranial cartilage likewise gives a firm origin to the muscles of the locomotive tentacula appended to the head, and, moreover, contains within its substance an auditory apparatus, presenting the earliest condition of an organ of hearing such as is met with in the vertebrate division of the animal kingdom; in every respect, therefore, it claims to be considered as the first appearance of a skull. Another broad cartilage is imbedded among the muscles at the base of the funnel; and two distinct plates, situated in the lateral fins of such species as possess appendages of that description, offer, undoubtedly, the rudiments of those portions of the skeleton that sustain the locomotive limbs of quadrupeds.

(1525). But while we thus see in the Cephalopoda the earliest form of an internal osseous skeleton, we cannot be surprised to find these mollusks still retaining, at the same time, the tegumentary calcareous shell or epidermic skeleton of inferior animals.

(1526). On slitting up the mantle of a Calamary (Loligo) along the mesial line of the back, it is found to contain a large cavity, wherein is lodged a long plate of horn, called the gladius, which in shape might be not inaptly compared to the head of a Roman spear. This enclosed horny substance, notwithstanding the dissimilarity of texture, is, in fact, strictly analogous to the enclosed shell of the Slug, described in a former page; and its growth is effected in the same manner, namely by an exudation of corneous material from the floor of the chamber that contains it; and this horny secretion, hardening as it is deposited layer by layer, adds to the dimensions of the gladius as the growth of the animal proceeds. Several of these plates may be produced in succession; and in old individuals it is not uncommon to find two or three enclosed in the same cavity, and placed one behind the other - that nearest the visceral aspect of the chamber being the most recently formed. These rudimentary shells have no connexion whatever with the soft parts of the Calamary, to which, in fact, they are so little adherent that they fall out as soon as the sac wherein they are secreted is laid open.

(1527). In the Cuttle-fish (Sepia officinalis) the dorsal plate (os Sepiae) is found in the same situation as the gladius of the Calamary, from which, however, it differs remarkably both in texture and composition. The cuttle-bone, with the appearance of which every one is familiar, is principally composed of calcareous substance, and, were we to judge of its weight from its bulk, would seem calculated materially to interfere with the movements of an aquatic animal destined to swim about, and consequently needing whatever assistance might be derived from lightness and buoyancy. Did a creature so apparently destitute of natatory organs possess a swimming-bladder like that of a fish, to assist in supporting it in the water, we should conceive such an apparatus to be far more adapted to its predatory habits than a shell so bulky as that which it is destined to carry.

(1528). We have, however, already seen, in the case of the Nautilus, that it would be by no means impracticable to convert a shell into a float nearly equalling a swimming-bladder in efficiency; and on more accurate examination it becomes obvious that even in the bone of the Cuttle we have a provision of a similar nature, though the end arrived at is obtained in a very different manner. On making a section of a cuttle-bone, it will be found to be composed of numerous stages of very thin calcareous plates placed at some distance above each other, and kept apart by the interposition of vertical laminae of the same substance, having, from the tortuosity of their meanderings, the appearance of millions of microscopic pillars. Thus organized, the shell in question becomes sufficiently light to float in water, and consequently, from its buoyancy, no doubt assists, instead of impeding, the movements of the mollusk. This admirable float, like the horny gladius of Loligo, is lodged in a membranous capsule and enclosed in the back of the Sepia, having no connexion whatever with the sides of the cavity wherein it is placed, being so loosely adherent that it readily falls out on opening the sac.

(1529). The cuttle-bone is formed in the same manner as other shells, by the continued addition of calcareous laminae secreted by that side of the containing capsule which is interposed between the shell and the abdominal viscera; and these layers, being successively added to the ventral surface of the shell, thus gradually increase its bulk as the Cuttle-fish advances to maturity. Neither in the mode of its growth nor in its texture, therefore, does the os Sepias resemble bone, properly so called; it receives neither vessels nor nerves, but is in all respects a dermal secretion, imbedded in the mantle, and formed in the same manner as the dorsal plate of the Slug.

(1530). We now come to consider the long-disputed question relative to the nature of the shell of the Argonaut. The Poulpe that inhabits the elegant abode represented in a preceding figure (fig. 283), when removed from its testaceous covering, has the general form of an Octopus. Its body (fig. 285) is enclosed in an ovoid muscular sac (d); and the head is surmounted by eight long sucker-bearing arms, of which six (e, f) taper gradually from their origins to their extremities, while the other two, formerly regarded as sails, and which we shall continue to designate by their ordinary name, vela, expand into broad membranes (6).