BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 177 



olfactory nerve, nerve of the ciliary ganglion, fifth, seventh, eighth, 

 ninth, and vagus complex, all can, by the consideration of their actual 

 development, and of the condition of the various organs which are, or 

 would be if present, related to them, be reduced to the general schema. 



The divergencies between the various nerves are, as might be 

 suspected, naturally dependent on the presence or absence of gill- 

 clefts in connection with the segment to which the nerve belongs. 



For this reason I shall consider the nerves out of their natural order, 

 taking those of the true gill-clefts first. Their order of treatment will 

 thus be as follows : 



Nerve. Cleft. Segment. 1 



Seventh. Spiracle, and one absent. Fourth and fifth. 

 Ninth. First branchial. Seventh. 



Vagus. Second, third, fourth, and Eighth, ninth, tenth, and 



fifth branchial. eleventh. 



Fifth. Mouth. Third. 



Ciliary. Hypophysis. (?) Second. 



Olfactory. Absent. First. 



Auditory. Absent. Sixth. 



In the above list it will be noticed that the cleft of the fifth nerve 

 is described as the mouth. This view, which we owe to Prof. Dohrn, 

 seems to me to receive very considerable support from my researches. 

 I shall refer to the matter subsequently. 



For the ciliary, olfactory, and auditory nerves I have hesitated to 

 assign clefts, because the evidence for their existence is uncertain, and 

 the nature of the three nerves is more easily explicable if we regard 

 the clefts as absent or metamorphosed. Here it will suffice to say that 

 clefts have been assigned to these nerves by various zoologists, with 

 what justification we shall see later on. 



Doe sal Root of the Fourth and Fifth Segments, Seventh Nerve 



or Facialis. 

 As already described by Balfour 2 and Marshall, 3 the seventh nerve 

 arises from the neural crest in the region of the hind-brain and just 

 in front of the auditory capsule. 



1 The numbering of the segments is in accordance with those conclusions from my 

 researches which appear to me to be fairly certain. Probably the facial nerve is a complex of 

 two segmental nerves, apart from the auditory segmental nerve. If this be the case, then 

 there are eleven segments at least from the olfactory nerve to the fourth root of the vagus 

 inclusive. 



2 Balfour, * Comp. Embryol.,' voL ii, p. 377. 



3 Milnes Marshall, "Head Cavities and Associated Nerves in Elasmobranchii," 'Quart, 

 Journ. Micr. Sc.,' 1880 ; also, "Nervous System of Chick," 'Quart, Journ. Micr, Sc,,' 1878, 



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