CEPHALOPODA. 465 



mediately 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 natu- 

 rally 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 ex- 

 tended to the utmost by enlarging the pupillary aperture. 



The principal tunic of the eye is a tough exterior membrane 

 or sclerotic {Jig- 212), 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 what- 

 ever 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 mem- 

 brane 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 in- 

 terior of the globe. 



However much is still left to be ascertained by future observa- 

 tions, 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 



9 TT 



<V II 



