Organ of Hearing.—This organ has hitherto been found only in the Dibranchiate division of the Cephalopods. It consists, as in the Cyclostomous or lower organized cartilaginous Fishes, of an acotistic vestibule, containing a limpid fluid and a calcareous body or otolithe suspended in a delicate sacculus to the filaments of the auditory nerve, but without the semi circular canals, cochlea, or other parts which progressively complicate the Organ of Hearing in the higher animals.
. The vestibular cavities (a, a,fig. 23.5) are situated, not at the sides, but at the base of the cranium in that thick and dense part of the carti lage which supports the sub oesophageal cerebral masses.
In the Cuttle-fish the cavities are of a sub-quadrate form, separated only by a thin septum (c); and they are every where closed, except at the entrance of the nerve. From their inner surfaces project several obtuse moderately elongated processes (b, b,,fig. 235)1- of a soft elastic texture, which support the central sacculus (d) and otolithe (e), and doubtless serve to convey to it the vibrations which affect the body generally. The sinuosities in the intervals of these pro cesses seem to be the first rudiments of those which in the higher classes are extended in the form of canals and spiral chambers within the substance of the dense nidus of the labyrinth. The otolithe in the Sepia officinalis is of an ir regular flattened quadrangular figure, with two of the angles produced so as somewhat to re semble the human incus : the surface next the parietes of the sacculus is convex and smooth, the opposite one concave and broken : it is white and transparent. (In fig.235, the oto lithe is seen as exposed in the sacculus on the right side.) In the Octopus vnlgaris the vestibules are nearly spherical, and their parietes are smooth ; the otolithes are of an hemispherical figure at tached to the dorsal part of the membranous sac, of a white colour on the adherent-surface, and yellow on the opposite side : the rest of the sacculus is filled with a transparent gelatinous fluid. The auditory nerve divides into three branches, which spread over the sacculus, and convey to the sensorium the vibrations which affect the.otolithe and its sac.
In the Eledone cirrosa the otolithe is shaped like the shell of a limpet, with the apex rounded and curved backwards ; of a pink colour on the sides, but of a white semitransparent texture internally.
The otolithes in all the Dibranchiates effer vesce with acids, like other substances com posed of carbonate of lime ; and in the Poulp, Eledone, and all the Decapods, except the Cuttle-fish, they are the only earthy substances which enter into the organization of these animals.
Organ qf Smell.—The sense of smell has been attributed to the Cephalopods by all natu ralists who have written on their habits ; from Aristotle,— who mentions the strong-scented herbs which the Greek fishermen attached in his day to their baits, in order to prevent their being destroyed by the Mollia,—down to Cuvier, who expressly asserts that they are at tracted by the odour of different substances. But no organ expressly appropriated to the ex ercise of the olfactory sense has been deter mined in the Dibranchiate Cephalopods.
In dissecting the Nautilus Pompilius, our attention was directed to a series of soft mem branous lamin (h,fig. 231) compactly arran
ged in a longitudinal direction, and forming a circular body very closely resembling the lami nated olfactory organ in Fish. The position of these laminw, as well as their form and arrange ment, supported the belief that they exercised the functions of an olfactory organ ; being situated just before the entrance of the mouth, between the internal labial processes : nerves were also traced to them from the inferior labial ganglions. From analogy we are inclined to suppose that the external lips in the Dibranchi ate order may be the seat of the olfactory sense.
Organ of Taste.—From the elaborate struc ture which the tongue displays in both orders of Cephalopods, there can be no doubt but that these destructive creatures fully relish the prey that they devour, and, in correspondence to their particular tastes, are led to select those species the limitation of whose increase is assigned to their charge.
The anterior soft papillose lobes of the tongue of the Nautilus are shewn in the sub joined figure (fig. 236), in which they are denoted by the letter c; e indicates the middle spiny plate, f the posterior coarser papillose surface, and g the faucial folds. The nerves of this part are derived from the brain itself, or supra-cesophageal mass.
Organ of Thach.—With respect to the sense of touch, the exposed part of the integument of the Nautilus presents numerous papillary eminences ; and several of the naked Cepha lopods are remarkable for tbe irregular surface of the skin, which seems designed to increase its natural sensibility. Thus, in the Cranchia scabra, flattened processes terminating in nu merous pointed denticulations, Aproject from the surface of the mantle; in the Sepia papil luta the integument is beset with branched papillw; in Sepia mammillata with more sim ple obtuse eminences ; in Sepia tuberculata, with tubercles; in Ociopus aculeatus, with pointed tubercles, &c. That these projections serve to warn the creature of the nature of the surfaces which come in contact with its body is highly probable; and it_is not at all uncommon to find in those species, which have smooth skins over the body generally, that there are tubercles in the immediate neigh bourhood of the eyes, as in the Octopus vulgaris, Octopus Lichtenaultii, Octopus Wes terniensis, &c.
In the Nautilus, the more exposed pedun culate eyes are expressly provided with re tractile sensitive tentacles on each side, as has been already mentioned.
With respect to the organs destined for the active exercise of touch or exploration, we must suppose that the numerous tentacles with which the Nautilus is so remarkably provided, from the softness of their texture, their an nulated surface, and liberal supply of nerves, serve in this capacity as well as instruments of prehension and locomotion. The less nu merous but more highly developed arms of the Dibranchiates doubtless exercise the same faculty, especially at their attenuated flexile extremities.
The internal fringed circular lip surrounding the mandibles, in both orders of Cephalopods, presents another example of the dermal co vering so disposed as to be the seat of delicate sensation.