This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Cuvier, Meinoire sur la Poulpe, p. 36.
(1581). In Nautilus, the part indicated by Professor Owen* as appropriated to the sense of smell consists of a series of soft membranous laminae (fig. 289, I; fig. 291, h) compactly arranged in a longitudinal direction, and situated at the entry of the mouth, between the internal labial processes. These laminae are twenty in number, and are from one to two lines in breadth, and from four to five in length; but they diminish in this respect towards the sides. They are supplied by nerves (fig. 291,10) from the small ganglia (8) which are connected with the ventral extremities of the anterior subcesophageal ganglia, and from which the nerves of the internal labial tentacula are likewise given off.
(1582). The structure of the eyes in the two divisions of the Cephalopoda differs remarkably, and in both is so entirely dissimilar from the usual organization met with in other classes of animals, that we must invite the special attention of the reader to this portion of their economy.
(1583). In the Tetrabranchiata, of which the Nautilus is the only example hitherto satisfactorily investigated, according to Professor Owen's observations 1 the eye appears to be reduced to the simplest condition that an organ of vision can assume without departing altogether from the type which prevails throughout the higher classes; for although the light is admitted by a single orifice into a globular cavity, or camera obscura, and a nerve of ample size is appropriated to receive the impression, yet the parts which regulate the admission and modify the direction of the impinging rays were, in the specimen examined, entirely deficient. In this structure of the eye, observes Professor Owen2, the Nautilus approximates the Gasteropods, numerous genera of which, and especially the Pectinibranchiata of Cuvier, present examples analogous in simplicity of structure, and in a pedicellate mode of support and attachment to the head. Moreover, as the Pearly Nautilus, like the latter group of mollusks, is also attached to a heavy shell, and participates with them in the deprivation of the ordinary locomotive instruments of the Cephalopods, the anatomist whose remarks we quote hence deduces the more immediate principle of their reciprocal inferiority with respect to their visual organ, observing that it would little avail an animal to discern distant objects when it could neither overtake them if necessary for food, nor avoid them if inimical to its existence.
* Mem. on Nautilus, p. 41. 1 Loc. cit. p. 39 et seq. 2 Op. cit. p. 51.
(1584). The eyes of Nautilus (fig. 284, m) are not contained in orbits, but are attached each by a pedicle to the side of the head, immediately below the posterior lobes of the hood. The ball of the eye is about eight lines in diameter; and although contracted and wrinkled in the specimen examined, it appeared to have been naturally of a globular form, rather flattened anteriorly. The pupil was a circular aperture, less than a line in diameter, situated in the centre of the anterior surface of the eye. This small size of the pupil in Nautilus, which contrasts so remarkably with the magnitude of that aperture in the Dibranchiate Cephalopods, Professor Owen suggests is most probably dependent on the great degree of mobility conferred upon the eye of the Nautilus in consequence of its attachment to a muscular pedicle, which enables it to be brought to bear with ease in a variety of directions; whilst in the higher Cephalopoda, corresponding motions of the head and body, on account of the more fixed condition of the eye in them, would have been perpetually required, had not the range of vision been extended to the utmost by enlarging the pupillary aperture.
(1585). The principal tunic of the eye is a tough exterior membrane or sclerotic (fig. 291), thickest posteriorly, where it is continued from the pedicle, and becoming gradually thinner to the margins of the pupil. The optic nerves, after leaving the optic ganglions (2), traverse the centre of the ocular pedicles, and, entering the eye, spread out into a tough pulpy mass which extends as far forwards as the semidiameter of the globe. This nervous tissue, as well as the whole interior of the cavity, is covered with a black pigment which is apparently interposed between the impinging rays of light and the sentient membrane. The contents of the eye-ball, of whatever nature they had been, had escaped by the pupil. If the eye had ever contained a crystalline lens, that body must have been very small; as otherwise, from the well-known effect of ardent spirits in coagulating it, it would have been readily perceived. What adds, however, to the probability of this eye being destitute of a crystalline humour is the total absence of ciliary plicae, or any structure analogous to them. In some parts of the cavity a membrane could be distinguished which had enveloped the fluid contents of the eye; but it had entirely disappeared at the pupil, which had in consequence freely admitted the preserving liquid into the interior of the globe.
(1586). However much is still left to be ascertained by future observations, we learn from the above able exposition of the appearances detected on examining the solitary example of a visual organ of this description hitherto met with, that the eye of the Nautilus exhibits every indication of inferiority of construction when compared with that of the Dibranchiate tribes. Encased in no orbital cavity, and consequently unprovided with any other muscular apparatus than the fleshy pedicle whereby it is connected with the head - unprotected by eyelids and devoid of lacrymal appendages - without either transparent cornea, aqueous humour, iris, or crystalline lens - and, moreover, coated internally with a dark pigment, apparently situated in front of the nervous expansion which represents the retina, instead of behind it in the usual position of the choroid tunic - all these are facts calculated to arrest the attention of the physiologist, and excite the surprise of every observer who studies on a large scale this part of the animal economy.