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, 7.2. 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-cesophageal 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, Monorhine and Amphirhine, 
according as they possess a median unpaired olfactory opening, or a 
paired opening. The Monorhinz 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 
