ORGANS OF SKXSK. 33 



breathing ami reproductive organs. A recurrent nerve from the 

 inferior buecal ganglion follows the oesophagus, ending in a 

 ganglion upon tlie stomach. 



A modification of this plan is found in the Nautili. Here a 

 thick transverse cord situated in front of the 1 (esophagus repre- 

 sents the cerebral ganglia; from its outer angles the optic and 

 olfactory nerves proceed, from its anterior edge those of the 

 bueeal mass. The pedal ganglia placed close to the cerebral 

 ganglia, are united by a slender commissure; from them are 

 supplied all the nerves of the foot or arms, of the funnel and of 

 the ears. 'The parieto-splanchnic ganglia, lengthened into a thick 

 cord, united at each end with the cerebral ganglia, form an 

 (jesophageal ring. Two large ganglia are found, one on either 

 side of the stomach. 



Organs of Sense. 



We have already, in speaking of the digestive system, shown 

 the probable situation of organs of. taste ; it remains to ns to pass 

 in review briefly those of Sight, of Hearing and of Smell. 



A. O/v//.'..s- of Sight. The sessile eye is lodged in orbital 

 cavities on either side of the head, in the dibranchiates ; in the 

 tetrabranchiates it is elevated upon peduncles. In the former 

 case the cephalic cartilage, as we have already seen, partty 

 encloses it, whilst in some instances special orbital cartilages are 

 also present ; the enclosure is completed by a fibrous capsule 

 continuous with the cephalic cartilage, which becomes transparent 

 over the eye and is likened to the cornea or even to the eyelid in 

 vertebrates. This transparent capsule presents several modifica- 

 tions ; it may be entire or with a small perforation as in the 

 Octopods. in Sepia, Loligo and other genera, constituting the 

 division Ulyonxidae of d'Orbigny ; or it may have a wide opening, 

 for the projection of the crystalline lens, as in Ommastrephes, 

 Loligopsis, etc. the Oigojjsidae of d'Orbigny. It is entirely 

 wanting in Nautilus. 



We find in the dibranchiates a large portion of the eye- 

 chamber occupied by the optic ganglion, by ocular muscles and 

 by a white glandular substance. The silvery tapetum lines but 

 does not adhere to the ocular capsule ; its two layers pass into 

 5 



