1 86 THE ORIGIN OF VERTEBRATES 



homologous to the branchial segments, originally characterized by 

 the presence of appendages, but that such appendages need never 

 have carried branchiae. 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 Eespiratory Segment. 



This simple alteration of the branchiomeric unit from a gill-pouch 

 to an appendage, which may or may not bear branchiae, 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 

 branchiae. 



When, however, we examine Ammoco^tes (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 Vllth 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 



