186 THE ORIGIN OF VERTEBRATES 
homologous to the branchial segments, originally characterized by 
the presence of appendages, but that such appendages need never 
have carried branchie. 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. 
Tue FAactaAL 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 branchie, immedi- 
ately 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 glosso- 
pharyngeal 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 
branchie. 
When, however, we examine Ammoccetes (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. 
An examination of this respiratory chamber shows clearly that 
there are six pairs of branchial appendages or diaphragms, which are 
all exactly similar to each other. These are those already considered, 
the foremost of which are supplied by the IXth or glossopharyngeal 
nerves. Immediately anterior to this glossopharyngeal segment is 
seen in the figures the segment supplied by the VIIth or facial 
nerves. It is so much like the segments belonging to the glosso- 
pharyngeal and vagus nerves as to make it certain that we are dealing 
here with a branchial segment, composed of a pair of branchial 
appendages similar to those in the other cases, except that the 
