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, Monorhinæ and Amphirhinæ, according as they possess a median unpaired olfactory opening, or a paired opening. The Monorhinæ 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 Ammocoetes, 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 {220}same position is always found in the dorsal head-shields of all the Cephalaspidæ and Tremataspidæ, as will be explained more fully in Chapter X. All the evidence points to the conclusion that the olfactory apparatus of the vertebrate originated as a single median tube, containing the special olfactory sense-epithelium, which, although median and single, was innervated by the olfactory nerve of each side. The external opening of this tube in the lamprey is dorsal. How does it terminate ventrally? The ventral termination of this tube is most instructive and suggestive. It terminates blindly at the very spot where the infundibular tube terminates blindly and the notochord ends. After transformation, when the Ammocoete becomes the Petromyzon, the tube still ends blindly, and does not open into the pharynx as in Myxine; it, however, no longer terminates at the infundibulum, but extends beyond it towards the pharynx.