408 THOS. H. MONTGOMERY jr., 
as follows: 80, 90, 50, 170, 20, 20, 40, 50, 20 w; and in another in- 
dividual, in a region a little more posterior: 40, 60, 33, 26, 60, 40, 
40, 33, 20, 6?/,, 40, 20, 46, 13, 40 u. Thus such commissures are 
very numerous and not separated by regular distances. 
In the nerve cord there is no segmental grouping of the nerve 
cells of either type so far as could be determined, and certainly no 
regular ganglionic arrangements of them. The transverse commissures 
also are too close together and too irregularly disposed to be regarded 
as metameric. 
B. The Cephalic Ganglion. 
The anterior end of the nerve cord is slightly enlarged (N.C 
Figs. 2, 3, Pl. 37), more thickened from side to side than it is dorso- 
ventrally; this anterior enlargement may be called the cephalic ganglion 
(see also Fig. 6, Pl. 37; Figs. 16 and 17, Pl. 38, Ceph.G). It is not 
sharply demarcated from the nerve cord, and its posterior beginning 
may be said to be at that point where there cease to be nerve cells 
placed on the lateral aspects of the cord (about the plane of the 
posterior end of the eye capsule in the female, Fig. 2, Pl. 37). 
In the cephalic ganglion the nerve cells are numerous but limited 
to the median line, and extend forward as far as the plane where 
the cephalic nerves (Ceph. N Fig. 7) separate from each other. Thus 
in the head the fibre tracts appear to consist mainly of a large one 
on each side of the mid line, anteriorly the median fibre tract di- 
minishing as the nerve cell layer becomes higher. About 187 « from 
the most anterior end of the head the two lateral fibre tracts diverge 
from each other, the nerve cells ending also at this point; the separated 
fibre tracts, which may be called the cephalic nerves (Ceph. N Figs. 2, 
3, 7), pass forwards and slightly dorsad, and terminate in the heightened 
hypodermis as will be described later; in them are no chromophobic 
nerve cells. The posterior portions of these nerves are separated from 
the hypodermis by a layer of parenchym, and upon their dorsal and 
median surfaces there is a sheath formed of the tissue of the eye 
capsule (E. Cap Fig. 13). The parenchym layer which separates the 
posterior portion of each cephalic nerve from the hypodermis is inter- 
rupted at two points, one 127 w and the other about 153 «u from the 
anterior end of the head; at each of these points a branch nerve 
passes ventrad from the cephalic nerve to connect with the hypo- 
dermis; these small nerves may be termed the ventral nerves of 
the head, and of them the more anterior is the larger. They are 
