232 THE ORIGIN OF VERTEBRATES 



vertebrate from the Palaeostracan, but points indubitably to an origin 

 from a scorpion-like rather than a crustacean-like stock. To com- 

 plete the evidence, it ought to be shown that the ancient sea-scorpions 

 did possess an olfactory passage similar to the modern land-scorpions. 

 The evidence on this question will come best in the next chapter, 

 where I propose to deal with the prosomatic appendages of the Palse- 

 ostracan group. 



Summary. 



The vertebrate olfactory apparatus commences as a single median tube 

 which terminates dorsally in the lamprey, and is supplied by the two olfactory 

 nerves which arise from the supra-infundibular portion of the brain. It is 

 a long - , tapering- tube which passes ventrally and terminates blindly at the 

 infundibulum in Ammocoetes. The dorsal position of the nasal opening is not 

 the original one, but is brought about by the growth of the upper lip. The 

 nasal tube originally opened ventrally, and was at that period of development 

 known as the tube of the hypophysis. 



The evidence of Ammocoetes thus goes to show that the olfactory 

 apparatus started as an olfactory tube on the ventral side of the animal, which 

 led directly up to, and probably into, the oesophagus of the original alimentary 

 canal of the palaeostracan ancestor. 



Strikingly enough, although in the crustaceans the first pair of antenna? 

 form the olfactory organs, no such free antennas are found in the arachnids, 

 but they have amalgamated to form a tube or olfactory passage, which leads 

 directly into the mouth and oesophagus of the animal. 



This olfactory passage is very conspicuous in all members of the scorpion 

 group, and, like the olfactory tube of the vertebrate, is innervated by a pair of 

 nerves, which resemble those supplying the first pair of antenna? in crustaceans 

 as to their origin from the supra-cesophageal ganglia. 



This nasal passage, or tube of the hypophysis, corresponds in structure and 

 iu position most closely with the olfactory tube of the scorpion group, the only 

 difference being- that in the latter case it opens directly into the oesophagus, 

 while in the former, owing to the closure of the old moutli, it cannot open into 

 the infundibulum. 



The evidence of the olfactory apparatus, combined with that of the optic 

 apparatus, is most interesting, for, whereas the former points indubitably to an 

 ancestor having scorpion-like affinities, the structure of the lateral eyes points 

 distinctly to crustacean, as well as arachnid, affinities. 



Taking the two together the evidence is extraordinarily strong that the 

 vertebrate arose from a member of the palasostracan group with marked 

 scorpion-like affinities. 



