BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 119 
main nerve fuses with the skin its course is continued along 
the mandibular arch by a number of cells of the nerve. These 
form the post-branchial branch, and innervate the musculature 
of the mandibular arch. Later, a pre-branchial nerve is deve- 
loped (Van Wijhe and others), which hooks over the angle of 
the mouth in the way that other pre-branchial branches hook 
over gill-clefts. 
Another apparent branch of the fifth is the nerve which 
Marshall has called a communicating nerve between the ciliary 
and Gasserian ganglia (fig. 51, ¢c. 6.). Its true nature has 
been worked out by Van Wijhe, who has shown that it really 
belongs to the ciliary ganglion. As I accept this statement 
I shall describe the nerve, as Van Wijhe has done, as part of 
the nerve of the second segment. 
The ophthalmicus profundus (fig. 51, oph. pro.) is also a part 
of the nerve of the second segment; this has been recognised by 
Marshall and Spencer, and also by Van Wijhe. 
The later fusions which occur between the fifth and seventh 
and the fifth and ciliary are in the early stages absent. In 
fact in its development the fifth has the typical charac- 
ters of the posterior root of a gill-bearing segment. 
It fulfils in every way, as Marshall found, the requirements 
of a segmental nerve as laid down by him, and it accords with 
our schema. It possesses a primitive branchial sense organ 
and an associated ganglion just abovea cleft, the mouth. 
It has the homologues of post-branchial and _ pre-branchial 
branches, and it develops a supra-branchial nerve in connection 
with the branchial sense organs over the snout (fig. 51, op. s.). 
The new additional light thrown on the nature of the mouth 
will be referred to in discussing the general morphological 
considerations arising out of these researches. Suffice it here 
to say that the facts given above seem to me to confirm 
Dohrn’s! conclusion that the mouth arose from a pair of 
coalesced gill-clefts. 
1 Dohrn, “Studien, &c.,” No. 1, ‘ Mittheil. a. d. Zool. Station zu Neapel,’ 
Bd. iii, p. 252. 
