CHAPTER VI 
THE EVIDENCE OF THE OLFACTORY APPARATUS 
Fishes divided into Amphirhine and Monorhine.—Nasal tube of the lamprey. 
—Its termination at the infundibulum.—The olfactory organs of the 
scorpion group.—The camerostome.—Its formation as a tube.—Its deriva- 
tion from a pair of antenne.—Its termination at the true mouth—Com- 
parison with the olfactory tube of Ammoccetes.—Origin of the nasal tube 
of Ammoccetes from the tube of the hypophysis.— Direct comparison of the 
hypophysial tube with the olfactory tube of the scorpion group—Summary. 
In the last chapter I finished the evidence given by the consideration 
of the mesosomatic or opisthotic nerves, and the segments they 
supplied. The evidence is strongly in accordance with that of 
previous chapters, and not only confirms the conclusion that verte- 
brates arose from some member of the Paleostraca, but helps still 
further to delimit the nature of that member. It is almost startling 
to see how the hypothesis put forward in the second chapter, sug- 
gested by the consideration of the nature of the vertebrate central 
nervous system and of the geological record, has received stronger 
and stronger confirmation from the consideration of the vertebrate 
optic apparatus, the vertebrate skeleton, the respiratory apparatus, 
and, finally, the thyroid gland. All fit naturally into a harmonious 
whole, and give a feeling of confidence that a similar harmony will 
be found upon consideration of the rest of the vertebrate organs. 
Following naturally upon the segments supplied by the opisthotic 
(mesosomatic) cranial nerves, we ought to consider now the segments 
supplied by the pro-otic (prosomatic) cranial nerves, 7.¢. the segments 
belonging to the trigeminal nerve-group in the vertebrate, and in the 
invertebrate the segments of the prosoma with their characteristic 
appendages. There are, however, in all vertebrates in this foremost 
cranial region, in addition to the optic nerves, two other well-marked 
nerves of special sense, the olfactory and the auditory. Of these, 
the former are in the same class as the optic nerves, for they arise 
