ORGANS or SENSE.—The Cephalopodous class is the only one in the Invertebrate series in which distinct organs of sight, hearing, smell, and taste, have been detected, although the en joyment of these senses is evidently byno means limited to this class. Considerable differences, however, present themselves in the relative complexity, and even as to the existence of the different Organs of Sense in the two orders of Cephalopods : thus, of the senses which relate to distant objects, the Orga.n of Hearing appears to be wanting in the Nautilus, and the Organ of Vision is coinparatively imperfect, while those which take cognizance of proximate objects are more distinctly and extensively developed.
Organ of Sight.—In the Nautilus the eyes are supported on short pedicles which project outwardly from the sides of the head. They are of a spherical form, slightly flattened ante riorly ; are large as compared with the pe dunculated eyes of Gasteropods, but are of small size as compared with the complex visual organs of the Dibranchiates. They presented, in Mr. Bennett's specimen, the simplest con dition of an organ of vision, consisting only of a darkened globular cavity or camera ob scura, into which light was admitted by a single orifice, and a nerve expanded at the opposite side to receive the impression; the mechanism for regulating the admission of the impinging rays was wanting, and every trace of that which modifies their direction had disappeared. The form of the eye was maintained by a tough unyielding sclerotic coat (k, fig. 231), which became thinner towards the anterior part of the eye, where it was perforated by a circular aper ture less than a line in diameter (m). The nerves continued from the small oval optic ganglion (2) expand, and immediately line the sclerotic as far as the middle of the globe, forming a strong re ticulate retina (o), which, together with the rest of the cavity, is lined by a black pigment (n). There was no appeamnce of vitreous humour or crystalline lens; but both parts would no doubt be found to exist in the recent state.
In the Dibranchiata the eyes are sessile, but in some species project beyond the sur face of the head more than in others ; their complicated structure is truly one of the most remarkable features of the organization of this singular class.
The eyeball in the Cuttle-fish is inclosed in a capsule consisting posteriorly of a thick carti lage (a, a, fig. 234), in its lateral circumference of a strong white fibrous membrane (b,b), and anteriorly of the cornea (o).
The whole of the inner surface of the cap sule is lined by a thin serous membrane, as far as the margin of the thick posterior cartila ginous orbit, to which it is attached, and is thence reflected forwards (c, c) upon the mus cles of the eye-ball, also upon the long narrow anterior and inferior ocular cartilage (d, d), and upon the exterior fibrous layer of the sclero tica ; it is reflected inwards over the anterior thickened margin of the sclerotica, where the large anterior aperture of that membrane re mains unclosed by the cornea, and consequently passes along its inner surface like the mem brane of the aqueous humour ; it seems to us, however, not to pass over the anterior part of the capsule of the crystalline lens, but into the groove (p, p) which divides that body into two parts. The serous layer above described can not be detached from the cornea, but ceases to be demonstrable as a distinct membrane where the external fibrous coat is attached to the cornea. The space between the eye-ball and its capsule, which is thus circumscribed, is filled with a watery fluid, which is most abun dant in the Calamaries. The cornea is sepa rated by the same fluid from the eye-ball ; but its tension and slightly convex figure is main tained by it, as by the aqueous humour in the eye of the vertebrate animal. The motions of the eye-ball are facilitated by the secretion of the serous sac, as the movements of the heart in the pericardium, and in other instances in which serous membranes are developed.