THE EVIDENCE OF THE OLFACTORY APPARATUS 219 



in the vertebrate from the supra-infundibular nerve-mass, and in the 

 invertebrate from the supra-CESophageal ganglia. The latter arise in 

 the vertebrate from the infra-infundibular nerve-mass, and, as the 

 name implies, are situated in the region where the pro-otic nerves 

 are contiguous to the opisthotic, i.e. at the junction of the prosomatic 

 and mesosomatic nerve-regions. 



The chapter dealing with the evidence given by the olfactory 

 nerves and the olfactory apparatus ought logically to have followed 

 immediately upon the one dealing with the optic apparatus, seeing 

 that both these special sense-nerves belong to the supra-infundibular 

 segments in the vertebrate and to the supra- oesophageal in the 

 invertebrate. 



I did not deal with them in that logical sequence because it was 

 necessary for their understanding to introduce first the conception of 

 modified appendages as important factors in any consideration of 

 vertebrate segments ; a conception which followed naturally after the 

 evidence afforded by the skeleton in Chapter III., and by the branchial 

 segments in Chapter IV. So, too, now, although the discussion of 

 the prosomatic segmentation ought logically to follow immediately 

 on that of the mesosomatic segmentation, I have determined to devote 

 this chapter to the evidence of the olfactory organs, because the 

 arguments as to the segments belonging to the trigeminal nerve- 

 group are so much easier to understand if the position of the olfactory 

 apparatus is first made clear. 



In all vertebrates the nose is double and opens into the pharynx, 

 until we descend to the fishes, where the whole group Pisces has 

 been divided into two subsidiary groups, MonorhinaB and Ainphirhime, 

 according as they possess a median unpaired olfactory opening, or a 

 paired opening. The Monorhinoe include only the Cyclostomata — the 

 lampreys and hag-fishes. 



In the lampreys the single olfactory tube ends blindly, while in 

 the hag-fishes it opens into the pharynx. In the lamprey, both in 

 Petromyzon and Ammoccetes, the opening of this nasal tube is a 

 conspicuous object on the dorsal surface of the head in front of the 

 transparent spot which indicates the position of the right median 

 eye. It is especially significant, as showing the primitive nature of 

 this median olfactory passage, that a perfectly similar opening in the 



