BRANCHIAL SENSE ORGANS OP ICHTHYOPSIDA. 203 



in<y olfactory nerve and organ of a Teleostean, Rhodeus amarus, is 

 figured, and in fig. 3 a similar stage in Rana temporaries In both 

 cases there is an epiblastic thickening, with which is united the 

 rudiment of a ganglion, and there is also the rudiment of a nerve, 

 the future • olfactory nerve (olf.n), just splitting off from the skin. 

 The development here is precisely similar to the development of the 

 fifth nerve in the frog as described by Spencer, or to that of the vagus 

 in the same animal as described in the preceding pages. 



It is hardly necessary to say that these facts confirm what has been 

 said of the nature of the nose in Elasmobranchii. 



Nerve op the Sixth Segment — Auditory Nerve. 



In a former paper 1 I suggested the homology of the auditory organ 

 with the so-called organs of the lateral line or branchial sense organs. 

 Subsequent investigation has only confirmed this suggestion. 



Gegenbaur originally ranked the auditory nerve as a dorsal branch 

 of the seventh. On embryological grounds Marshall and Balfour had 

 also been led to the conclusion that the auditory nerve was not in itself 

 entitled to segmental rank, but was in its development only a dorsal 

 sensory branch of the seventh. Marshall, indeed, held that there 

 was not room for another segmental nerve between the seventh and 

 ninth. 



Recent researches have led different zoologists to the opinion that 

 the hyoid arch is composed of two originally distinct arches. 



Yan Wijhe considers that the obliterated cleft was behind the facial 

 nerve, while Dohrn holds that it was in front of the hyoid cleft. The 

 possibility that both are right appears to me not unlikely. Dohrn 

 sees remains of a former cleft in the hyo-mandibular and in the 

 thyroid body. The only evidence afforded by the nerves in support 

 of this appears to be the existence of two supra-branchial nerves for 

 the seventh. Alone it is not convincing evidence, but taken in con- 

 nection with Dohrn's facts 2 it is, I think, of importance. 



That a cleft formerly existed behind the hyoid cleft and in front of 

 the first branchial is not admitted by Dohrn, and he has declined to 



1 Beard, "On the Segmental Sense Organs, &c.,'' 'Zool. Anzeig.,' Nos. 161, 162, 1884. 



2 Dohrn even goes further, and postulates a separate spiracular visceral arch just behind 

 the mandibular arch. Thus, according to Dohrn, there are four arches included between 

 the fifth nerve and the seventh nerve, viz. mandibular, spiracular, hyomandibular, and 

 hyoid. So far as my researches extend, I have found nothing in the nerves that would 

 suggest a spiracular arch. However, bearing in mind what has taken place in the case of 

 the vagus, I should hesitate to cast even a doubt on the truth of his view. 



