2 20 THE ORIGIN OF VERTEBRATES 



same position is always found in the dorsal head-shields of all the 

 Cephalaspidse and Tremataspidse, 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 oesophagus 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 antenna 3 , often called the antennules, 

 are olfactory in function, and these are free-moving, bilaterally 



