42 



CONARIO-HYPOPHYSIAL TRACT. 



divisions and functions of the brain are simplified, and so are 

 more clearly manifested. The cartilaginous defensive case 

 protects only the homologue of the " sub-" or, rather, " neur- 

 cesophageal " ganglion, which is more distinctly divided 

 into a fore and hind mass. The first of these supplies the 

 anterior or cephalic muscular and tegumentary parts, the 

 second the posterior or corporal ones ; and from this division 

 or cerebral centre are derived the nerves of the acoustic 

 organs developed or imbedded in the corresponding sup- 

 porting cartilage*. 



The super- (hsem-joesophageal body develops no peripheral 

 lobe, is in the form of a thick cord which sends forward nerves 

 to oral parts suggestive of an olfactory function, and, laterally, 

 the large short cords, swelling into ganglions, subserving the 

 retinal supply of the pedunculate eyes. 



The brain- space traversed by the gullet is wider than in 

 the Dibranchiates, the annectant tracts between the " supra-" 

 and " suboesophageal " masses are longer; but their resem- 

 blance to the oesophageal cords in the Articulates is still 

 closer in the modifications of the cephalopodal type of the 

 nervous system, especially of its encephalic centres, which are 

 seen in Aplysia and all lower Mollusca. 



John Hunter, in his descensive comparison of the brains of 

 animals, stopped at the Snail. In my description of these 

 Preparations I quoted from the scrap of manuscript which he 

 had attached to the numbers of the preparations and names 

 of the dissected animals. Being arranged in the ascending 

 series, the MS. begins : — " The First Class has a brain in the 

 form of a ring, through which passes the oesophagus. It 

 consists of a pulpy substance, somewhat transparent, which is 



* Macdonald, ''Anatomy of the Nautilus umhilicatus,'' Phil. Trans. 

 1855, p. 279. 



