BRANCHIAL SENSE ORGANS OP ICHTHTOPSIDA. 179 



infra-orbital sense organs respectively (branchial sense organs). These 

 branchial sense organs, it is hardly necessary to state, are developed 

 from the dichotomously divided sensory thickening mentioned above. 

 The portio facialis of the ophth. superficialis (fig. 51, p.f.), is obviously 

 enough, as pointed out by Marshall, Balfour, and Van Wijhe, a 

 so-called dorsal branch ; that is, what we have here called a supra- 

 branchial. Van Wijhe has, and I fully agree with him, classed the 

 r. buccalis (fig. 51, r.b.) as a "dorsal branch," and gives these reasons: 

 (1) Its origin from the same rudiment as the former nerve ; (2) its 

 simultaneous appearance with that nerve ; (3) its similar development 

 and distribution to (branchial) sense organs. Van Wijhe, indeed, 

 regards the two as branches of one nerve, and as therefore equivalent 

 to one so-called dorsal branch. Dohrn 1 has advanced very weighty 

 reasons for the existence of a hyomandibular segment in front of the 

 hyoid and behind the mouth, but has not adduced the cranial nerves 

 in support of his view. I would here venture to suggest that an 

 additional ground for his view is to be seen in the existence of two 

 supra-branchial nerves in the facial. It would indeed be remarkable 

 if Van Wijhe were correct in regarding these two nerves as merely 

 branches of one nerve, for in no other single and simple cranial nerve 

 do we meet with more than one supra-branchial nerve. To my mind 

 the best explanation of the pi'esence of these two branches is that the 

 facial is composed of the fusion of two cranial segmental nerves, and 

 this apart from its fusion with the auditory. The reader may compare 

 Dohrn 's views on the nature of the hyomandibular with this explana- 

 tion. Except for this the facial seems to be a fairly typical cranial 

 nerve, and agrees well with the general schema. It should be noticed 

 that the supra-branchial branches grow forwards, for this point will be 

 referred to in discussing the vagus. Though I agree fully with Van 

 Wijhe's 2 view that there are two segments in the hyoid arch, and this 

 apart from the hyomandibular portion, I cannot treat the auditory 

 nerve here. The special modifications it has undergone will be best 

 considered after some of the other nerves have been discussed. In 

 their earliest appearance I believe the auditory and facial nerves are 

 not fused, and even in the later stages (figs. 21, 42), as already noticed 

 by Marshall in the chick, the ganglia of the two nerves are partially 

 separated, and the line of division is easily recognisable. For the 



1 Dohrn, " Studien zur Urgeschichte des Wirbeltliier-Korpers," No. vii, ' Mittheil. a. d, 

 Zool. Stat, zu Neapel,' vol. vi, part i. 

 a Op, cit., pp. 9 and 28. 



