CEPHALOPODA. 585 



cartilage, he considers as acoustic nerves ; but these nerves are given off 

 from the sub-oesophageal ganglion in the higher Cephalopods, and in 

 all the Gastropods, in which the organ of hearing has been observed. 



The eyes, as before stated, are attached each by a short pedicle to 

 the side of the head, over-arched by the projecting margin of the hood. 

 The form of the ball is sub-hemispherical, being flattened anteriorly 

 along its inferior border, there is a slightly elevated ridge, from 

 which a smaller ridge is continued to the middle of the anterior sur- 

 face of the eye, which is perforated by a round pupillary aperture, 

 about a line in diameter. The sclerotic tunic, which is a tough, 

 ligamentous membrane, is thickest posteriorly, and becomes gradually 

 thinner to the margin of the pupil. The optic filaments form a 

 pulpy mass at the floor of the eye, from which the retinal expansion 

 is continued as far forwards as the semidiameter of the globe : it is 

 lined, like the rest of the interior of the eye, by a thick black pig- 

 ment, which is doubtless perforated by the retinal papillae, or other- 

 wise a perception of light must take place in a manner incompatible 

 with our knowledge of the ordinary mode in which the retina is 

 effected by luminous rays. The crystalline lens was not present in 

 the specimen dissected by me, and M. Valenciennes states that he was 

 unable to observe any of the humours of the eye.* In the Nautilus 

 umbilicatus, Mr. Macdonald found " a mere viscous matter protecting 

 the retina from the sea-water." 



The cavity in the cephalic cartilage, apparently that described in 

 my Memoir f as containing a sinus of the cephalic veins, but which 

 M. Valenciennes regards as the organ of hearing, is defective in the 

 structures which the uniform analogy of that organ in the molluscous 

 subkingdom leads us to conclude are essential to it : there were no 

 traces, for example, of otolithes.J In the Naut. umbilicatus there are 

 "two spheroidal acoustic capsules placed one on each side, at the 

 union of the supra- and sub-oesophageal ganglia, and measuring each 

 about -j^^th of an inch in diameter. Each capsule rests internally 

 against the nervous mass, and is received on its outer side into a little 

 depression in the cephalic cartilage. It is enveloped in a kind of 

 fibrous tissue^ and filled with a cretaceous pulp, consisting of minute 

 otoconial particles, varying much in size, and sometimes combined in 

 stellate cruciform or other figures." § 



With respect to the organ of smell, the structure and position of 

 the soft close-set membranous lamellae at the lower and inner part of 



* Loc. cit. p. 289. t CCCLXXXVII. p. 16. 



% " Elle ctait remplie d'une pulpe homogene " (coagulated blood ?), " et ne con- 

 tenait aucune sorte de concretions." — Loc. cit. p. 291. Siebold suggests (xxiv. 

 p. 384.), that the otolithes may have been originally uncalcified, as in Eledone, and 

 have become dissolved, § CCCCVII. p. 312. 



