BRANCHIAIL SENSE ORGANS IN ICHTHYOPSIDA. 135 
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. 
Van 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 me the 
existence of two supra-branchial nerves for the seventh. Alone 
it is not convincing evidence, but taken in connection with 
Dohrn’s facts! 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 attach any weight to the reasons which Van 
Wijhe urged for this opinion, which was based on the presence 
of two head cavities in the hyoid arch. Van Wijhe does not 
appear to have attached much importance to the evidence 
offered by the nerves, for he did not regard the auditory nerve 
as in itself of segmental value, and he never suggested the 
homology of the auditory organ with the branchial sense 
organs. 
DEVELOPMENT OF THE AvupIToRY NERVE. 
In Elasmobranchii the facts of development for this segment 
are exactly comparable to those described for the olfactory 
segment. The arrangement is here the same. There is no 
gill-cleft, and of course, as a consequence of the absence of that, 
we cannot expect to find a post-branchial nerve. 
1 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 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, 
