DIFFERENTIATIONS OF EOTODERM IN NECTURUS. 531 
geminus proper does not participate in the innervation of the 
lateral line system.” 
The innervation of the hyomandibular line shows that the 
surface covered by the sense-organs of this line has extended 
in a posterior as well as anterior direction, and that the length- 
ening of the main stem of the hyomandibularis has not kept pace 
with the growth of the sensory line. While in fig. 21 we find 
that the hyomandibularis closely underlies its sensory ridge, 
in fig. 81 the sense-organs that have developed from the ridge 
are seen to be connected with the main nerve by long slender 
stems that unite into four or five branches before reaching the 
common trunk. The present innervation of the mandibular 
line is not less misleading in its interpretation of development, 
and one would hardly fancy from the distribution of the nerve 
that the mandibularis primarily came in contact with its sensory 
line, not near the corner of the mouth, but at the posterior 
limit of the mandibular line, where this line meets the hyoman- 
dibular. The point where the main nerve now forks in two 
directions, once lay midway on the path of the undivided nerve, 
as shown by comparison with fig. 21. In the reconstruction, 
the nerves which collect the dorsal and ventral branches of the 
_ hyomandibular line, and the mandibular branch of the facialis 
are not united. In fact, however, the ventral branch of the hyo- 
mandibularis unites with the mandibularis, and then with the 
dorsal branch of the hyomandibularis, and the three nerves enter 
the ventro-lateral surface of the ganglion by a common root. 
From the point where the reconstruction leaves the nerves their 
course is inwards, and was difficult to represent in surface view. 
I have already called attention to the fact that the inner 
row of mandibular sense-organs at the margin of the lower lip 
is Innervated by the hyomandibularis (mandibularis internus), 
thus bearing testimony to the earlier union of the two sensory 
lines, hyomandibular and mandibular (fig. 21). The nerve of 
the posterior half of the mandibular line anastomoses with a 
branch from the hyomandibularis, so that these posterior organs 
now appear to receive their innervation from both mandibular 
and hyomandibular nerves. 
