232 THE ORIGIN OF VERTEBRATES 
vertebrate from the Paleostracan, 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 Pale- 
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 aise from the supra-infundibular portion of the brain. It is 
a long, tapering tube which passes ventrally and terminates blindly at the 
infundibulum in Ammocetes. 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 Ammocetes 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 wsophagus of the original alimentary 
canal of the paleostracan ancestor. 
Strikingly enough, although in the crustaceans the first pair of antenne 
form the olfactory organs, no such free antenne are found in the arachnids, 
but they have amalgamated to form a tube or olfactory passage, which leads 
directly into the mouth and esophagus 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 antenne in crustaceans 
as to their origin from the supra-cesophageal ganglia. 
This nasal passage, or tube of the hypophysis, corresponds in structure and 
in 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 esophagus, 
while in the former, owing to the closure of the old mouth, 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 paleostracan group with marked 
scorpion-like affinities. 
