CEPHALOPODA. 321 



tached by loose cellular tissue to the vessels of the part. The pos- 

 terior collar gives off numerous nerves (m) of a flattened form, 

 which supply the muscles of the shell. The respiratory nerves 

 form a small ganglion (q) at the base of each pair of gills, from 

 which branches are sent to those organs, to the heart, and to the 

 appendages of the veins. A plexus of more delicate visceral nerves 

 (r) is continued backward along the interspace of the branchial 

 nerves, and the chief branches are connected with a small ganglion 

 situated between the cardiac and pyloric orifices of the stomach. The 

 posterior subcesophageal nervous mass combines the analogues of both 

 the branchial and pedial ganglions in the inferior Mollusca : the an- 

 terior ring answers to the ganglia in the higher Cephalopods, called 

 ' pes anserinus ' by Cuvier : the ophthalmic tentacula, which derive 

 their nerves (71, n) close to the origin of the optic ganglion, may be con- 

 sidered as analogous to the four tentacula in the Aplysia. The hollow 

 plicated process beneath the eye, which Valenciennes regards as the 

 olfactory organ, likewise receives its nerves from the extremity of 

 the supra-cesophageal chord. Three small nervous filaments, de- 

 scribed by the same author as passing from the extremity of the 

 supra-oesophageal chord to the adjoining part of the cephalic 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 Gasteropods, 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 surface 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 pigment, which is 

 doubtless perforated by the retinal papillae, or otherwise a perception 

 of light must take place in a manner incompatible with our know- 

 ledge of the ordinary mode in which the retina is effected by lumi- 

 nous rays. The crystalline lens would seem to be small, if it exists 

 independently of the vitreous humour : it was not present in the speci- 

 men dissected by me, and M. Valenciennes states that he was unable 

 to observe any of the humours of the eye.* 



* Loc. cit., p. 289. 

 Y 



