The shifting of the nasal tube from a ventral to a dorsal position, as seen in Ammocoetes, is, perhaps, the most important of all clues in connection with the comparison of Ammocoetes to the Palæostracan on the one hand, and to the Cephalaspid on the other; for, whereas the exact counterpart of the opening of such a tube is always found on the dorsal head-shield in all members of the latter group, nothing of the kind is ever found on the dorsal carapace of the former group. The reason for this difference is made immediately evident in the development of Ammocoetes itself, for the olfactory tube originates as a ventral tube--the tube of the hypophysis--in exactly the same position as the olfactory tube of the Palæostracan, and later on in its development takes up a dorsal position. In fact, Ammocoetes in its development indicates how the Palæostracan head-shield became transformed into that of the Cephalaspid. In another most important character Ammocoetes indicates its relationship to the Cephalaspidæ, for it possesses an external skeleton or head-shield composed of muco-cartilage, which is the exact counterpart of the so-called bony head-shield of the latter group; and still more strikingly the structure of the cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one case, by the deposition of calcium salts, a hard external skeleton, capable of being preserved as a fossil, has been formed; in the other, by the absence of the calcium salts, a soft chondro-mucoid matrix, in which the characteristic cells and fibrils are embedded, distinguishes the tissue. The recognition that the head-shields of these most primitive fishes were not composed of bone, but of muco-cartilage, the precursor of both cartilage and bone, immediately clears up in the most satisfactory manner the whole {354}question of their derivation from elasmobranch fishes; for the main argument in favour of the latter derivation is the exceedingly strong one that bone succeeds cartilage--not _vice versâ_--therefore, these forms, since their head-shield is bony, must have arisen from some other fishes with a cartilaginous skeleton, most probably of an elasmobranch nature. Seeing, however, that the structure of their shields resembles muco-cartilage much more closely than bone, and that Ammocoetes forms a head-shield of muco-cartilage closely resembling theirs, there is no longer any necessity to derive the jawless fishes from the gnathostomatous; but, on the contrary, we may look with certainty upon the Agnatha as the most primitive group from which the others have been derived.