In all cases where there is similarity of topographical position in the organs of the vertebrate and arthropod we may expect also to find similarity of structure. At first sight it would appear as though such similarity fails us here, for a cross-section of the olfactory tube in Petromyzon represents an elaborate organ such as is shown in Fig. 98, very different in appearance to the section across the olfactory passage of a young Thelyphonus given in Fig. 97. {226}[Illustration: FIG. 97.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE OF A YOUNG THELYPHONUS. 1 and 2, sections of first and second appendages.] [Illustration: FIG. 98.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE OF PETROMYZON. _cart._, nasal cartilage.] {227}As is seen, it is difficult to see any connection between these folds of olfactory epithelium and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of the lamprey, remarkable changes take place at transformation, and examination of the same tube in Ammocoetes demonstrates that the elaborate structure of the adult olfactory organ is actually derived from a much simpler form of organ, represented in Fig. 99. Here, in Ammocoetes, the section is no longer strikingly different from that of the Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the rest of the vertebrates put together. [Illustration: FIG. 99.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE OF AMMOCOETES. _cart._, nasal cartilage.] Still, even now the similarity between the two organs is not complete, for the tube in the lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids, and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does the development of the individual recapitulate the development of the race, that this very discrepancy becomes converted into a triumphant vindication of the {228}correctness of the theory advocated in this book, as soon as we turn our attention to the development of this nasal tube in the lamprey.