every vertebrate, whatever may be the ultimate form of the external nasal orifices. The single median position of the olfactory organ in the Cyclostomata, in contradistinction to its paired character in the rest of the vertebrates, has always been a stumbling-block in the way of those who desired to consider the Cyclostomata as degenerated Selachians, for the origin of the olfactory protuberance, as a single median plakode, seemed to indicate that the nose arose as a single organ and not as a paired organ. [Illustration: FIG. 100.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4 MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.) _A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._, notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper lip; _l.l._, lower lip; _vel._, septum between oral and respiratory cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four epibranchial ganglia.] On the other hand, the two olfactory nerves of Ammocoetes compare absolutely with the olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of Ammocoetes arose from a pair of bilateral organs, which have fused in the middle line. {230}[Illustration: FIG. 101.--_Galeodes._ (From the Royal Natural History.)] {231}The comparison of this olfactory organ with the camerostome gives a satisfactory reason for its appearance in the lowest vertebrates as an unpaired median organ; equally so, the history of the camerostome itself supplies the reason why the olfactory nerves are double, why the organ is in reality a paired organ and not a single median one. Thus, in a sense, the grouping of the fishes into Monorhinæ and Amphirhinæ has not much meaning, seeing that the olfactory organ is in all cases double. [Illustration: FIG. 102.--_Thelyphonus._ (From the Royal Natural History.)] The evidence of the olfactory organs in the vertebrate not only confirms, in a most striking manner, the theory of the origin of the {232}vertebrate from the Palæostracan, but points indubitably to an origin from a scorpion-like rather than a crustacean-like stock. To complete 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 Palæostracan group. SUMMARY.