The Origin of Vertebrates

by Walter Holbrook Gaskell

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 Ammocœtes, 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 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 Ammocœte 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.

This position of the nasal tube suggests that it may originally have opened into the tube of the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in value by the fact that in the larval Amphioxus the tube of the central nervous system is open to the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is situated at the base of a pit, known as the olfactory pit because it is supposed to represent the olfactory organ of other fishes.

Following the same lines of argument as in previous chapters, this suggestion indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates consisted of a single median olfactory tube or passage, which led directly into the œsophagus and was innervated, though single and median, by a pair of olfactory nerves which arose from the supra-œsophageal ganglia. Let us see what is the nature of the olfactory organs among arthropods, and whether such a suggestion possesses any probability.

The Olfactory Organs of the Scorpion Group.

At first sight the answer appears to be distinctly adverse, for it is well known that in all the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often called the antennules, are olfactory in function, and these are free-moving, bilaterally situated, independent appendages. Still, even here there is the striking fact that the nerves of these olfactory organs always arise from the supra-œsophageal ganglia, although those to the second pair of antennæ arise from the infra-œsophageal ganglia, just as the olfactory nerves of the vertebrate arise from the supra-infundibular brain-mass. Not only is there this similarity of position, but also a similarity of structure in the olfactive lobes of the brain itself of so striking a character as to cause Bellonci to sum up his investigations as follows:—

"The structure and connections of the olfactive lobes present the same fundamental plan in the higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form, with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does of separate groups, may each one be called an olfactory glomerulus."