As long as the branchial unit consisted of the gill-pouch the segments of the head-region were always referred to such units, hence we find Dohrn and Marshall picturing to themselves the ancestor of vertebrates as possessing a series of branchial pouches right up to the anterior end of the body. Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on this earth, or could be reasonably imagined to have lived on it. Yet Dohrn's conceptions of the segmentation were sound, his interpretation only was in fault, because he was obliged to express his segments in terms of the gill-pouch unit. Once abandon that point of view and take as the unit a branchial appendage, then immediately we see that in the region in front of the branchiæ we may still have segments {186}homologous to the branchial segments, originally characterized by the presence of appendages, but that such appendages need never have carried branchiæ. The new mouth may have been formed by such appendages, which would express Dohrn's suggestion of its formation by coalesced gill-slits; the olfactory organ may have been the sense-organ belonging to an antennal appendage, which would be what Marshall really meant in calling it a branchial sense-organ. THE FACIAL NERVE AND THE FOREMOST RESPIRATORY SEGMENT. This simple alteration of the branchiomeric unit from a gill-pouch to an appendage, which may or may not bear branchiæ, immediately sheds a flood of light on the segmentation of the head-region, and brings to harmony the chaos previously existing. Let us, then, follow out its further teachings. Next anteriorly to the glossopharyngeal and vagus nerves comes the facial nerve; a nerve which supplies the hyoid segment, or, rather, according to van Wijhe the two hyoid segments, for embryologically there is evidence of two segments. As already mentioned, the facial nerve is usually included in the trigeminal or pro-otic group of nerves, the opisthotic group being confined to the glossopharyngeal and vagus. This inclusion of the facial nerve into the pro-otic group of nerves forms one of the main reasons why this group has been supposed to have originally supplied gill-pouch segments, for the hyoid segment is clearly associated with branchiæ. When, however, we examine Ammocoetes (_cf._ Figs. 63 and 64) it is clear that the foremost of the segments forming the respiratory chamber, which must be classed with the rest of the mesosomatic or opisthotic segments, is that supplied by the facial nerves.