220 THE ORIGIN OF VERTEBRATES 
same position is always found in the dorsal head-shields of all the 
Cephalaspidee and Tremataspide, 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, con- 
taining 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 in- 
fundibular tube terminates blindly and the notochord ends. After 
transformation, when the Ammoccete 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 cesophagus and 
was innervated, though single and median, by a pair of olfactory 
nerves which arose from the supra-cesophageal 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 antenne, often called the antennules, 
are olfactory in function, and these are free-moving, bilaterally 
