It is characterized by two most important events. In the first place, up to this time the oral chamber has been cut off from the respiratory chamber by a septum—the velum—so that no food could pass from the mouth to the alimentary canal. At this stage the septum is broken through, the oral chamber communicates with the respiratory chamber, and the velar folds of the more adult Ammocœtes are left as the remains of the original septum. The other striking change is the growth of the upper lip, by which the orifice of the nasal tube is transferred from a ventral to a dorsal position. Fig. [100], taken from Kupffer's paper, represents a sagittal section through an Ammocœtes 4 mm. long; l.l. is the lower lip, u.l. the upper lip, and, as is seen, the short oral chamber is closed by the septum, vel. Opening ventrally is a tube called the tube of the hypophysis, Hy., which extends close up to the termination of the infundibulum. On the anterior surface of this tube is the projection called by Kupffer the olfactory plakode. At this stage the upper lip grows with great rapidity and thickens considerably, thus forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the full-grown Ammocœtes, it becomes the dorsal opening of the nasal tube, as already described. Here, then, in the hypophysial tube we have the original position of the olfactory tube of the vertebrate ancestor, and it is significant, as showing the importance of this organ, to find that such a hypophysial tube is characteristic of the embryological development of 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.
Fig. 100.—Ganglia of the Cranial Nerves of an Ammocœtes, 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 Ammocœtes compare absolutely with the olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of Ammocœtes arose from a pair of bilateral organs, which have fused in the middle line.
Fig. 101.—Galeodes. (From the Royal Natural History.)
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.
