CHAPTER VI 



THE EVIDENCE OF THE OLFACTORY APPARATUS 



Fishes divided into Ainphirhina? and Monorhinfe. — Nasal tube of the lamprey. 

 — Its termination at the infundihulum. — The olfactory organs of the 

 scorpion group. — The camerostome. — Its formation as a tube. — Its deriva- 

 tion from a pair of antenna?. — Its termination at the true mouth. — Com- 

 parison with the olfactory tube of Ammocoetes. — Origin of the nasal tube 

 of Ammocoetes 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 Pakeostraca, 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, i.e. 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 



