284 THE CEPHALOPODA. § 247. 



The Optic nerves enter the posterior part of the orbit through a kind of 

 Foramen opticum, after which they swell into a large kidney-shaped ganglion 

 in which a portion of the nerve-fibres are completely interlaced with those 

 from the opposite side/''' Leaving this Ganglion opticum, the nerve 

 divides into numerous filaments which traverse the cribriform sclerotica, 

 and then unite with the other elements of the Retina. The external layer 

 of the retina is composed of these filaments ; beneath it, is a pigment layer 

 of a reddish-brown color, and pierced by numerous fibres given oif rectan- 

 gularly from the external layer. The internal layer is composed of gran- 

 ules, among which the fibres of the optic nerve probably terminate.*'"' The 

 external layer is continuous as a thin membrane upon the ciliary body, 

 and even upon the septum of lens.*"' 



The two optic ganglia are enveloped by a peculiar white substance com- 

 posed of fat-cells, which, perhaps, serves only as a fat-cushion.*^-' The 

 eye is moved by several straight and oblique muscles, which arise from the 

 cartilaginous portion of the orbit, and are inserted, usually, upon the mid- 

 dle of the bulb. 



But with Nautilus, the eyes difi"er in many respects from those of the 

 other Cephalopoda. They are supported upon a muscular stalk and pro- 

 ject from the head : while with the other Cephalopoda, excepting Loligopsis, 

 they are sunken deeply in the head.*'"' From the rudimentary lower lid a 

 narrow furrow passes over the anterior surface of the eye even to the small, 

 circular pupil. As yet, neither cornea nor lens has here been found. *'^' 



9 For the interlacement of the nerve-fibres in the a long time upon dead specimens, it may be sus- 



Ganglion opticum., — see, especially, Wharton pected that the exceptionable peculiarities ob- 



Jones, and John Power, loc. cit. served by Owen, and Valenciennes, are referable 



W The intimate, very complicated structure of the to the want of fresh specimens. It is, at first, 



retina, has been described principally by Trevi- singular that Owen (On the Nautilus, p. 39, PI. I. 



ranus (loc. cit. p. 155), Wharton Jones (loc. cit), v. w., or Isis, p. 32, Taf. I. 1, fig. 1, v. w., or Ann. 



and Paccini (Nuove ricerche microscop. suUa tes- des Sc. Nat. p. 139, PI. I. fig. 1, v. w.) speaks of a 



situra intima della retina nell' Uomo, nei Vertebrati, ridge, and Valenciennes (loc. cit. p. 289, PI. IX. 



nei Cefalopodi e negli Insetti. Bologna, 1845, jj. 55, fig. 1, No. 3) of a furrow, running from the border 



fig. 13, 14). The mysterious phenomenon, that, of the lower lid to the pupil. As the cornea is 



according to the older anatomists, the surface of wanting, it might almost be supposed, from exaniin- 



the retina exposed to the light is covered with a ing Valenciennes'' figure (PI. VIII. fig. 2, P.), that 



pigment-layer, rests only on an imperfect knowl- Nautilus belonged to the Oigopsidis of D'Or- 



edge of the structure of this organ, as has been bigny, except, that with this animal, instead of a 



shown by Wharton Jones (loc. cit.), and Valen- complete absence of the anterior part of the ocular 



tin (Repert. f. Anat. II. 1837, p. 109). capsule, there exists only a fissure, regarded by one 



11 See Krohn, and Wharton Jones, loc. cit. of the authors in question as a ridge, and by the 



12 Mayer (Analekt. &c. p. 53) regards this sub- other as a furrow. The lens, not perceived by 

 stance as a semi-adipose gland with several excre- either Owen, or Valenciennes, escaped perhajis 

 tory ducts, — a kind of lachrymal gland the product through this fissure, after having been detached 

 of which is poured into the conjunctival sac ; but by maceration. 



Kolliker (Entwick. d. Ceph. p. 103) could find As for the pigment layer, spoken of by Owen, 



nothing glandular in its structure. as situated upon the concave surface of the retina 



13 For the pedunculated eyes of Loligopsis, see of Nautilus, this problem ivill be explained, from 

 Rathki, in the Mem. d. St. Petersburg, loc. cit. researches upon fresh specimens in the same way, 

 PI. I., and F^russac, loc. cit. as with the other Cephalopoda. 



14 The eyes of Nautilus having been studied for 



