436 Ww. A. HASWELL. 
off the corresponding lateral nerve. Nerve-cells surround 
the fibrillated material of the brain, and are most abundant 
over the dorsal portion. 
The dorsal nerve trunks, four or, more rarely, six in 
number, run backwards beneath the muscular layers, and 
give off numerous branches, which branch and anastomose to 
form a plexus extending over the entire dorsal surface. 
From the same part of the brain (the ventral portion) 
which gives origin to the lateral nerves are given off several 
nerves, which run forwards in the region in front of the 
brain, breaking up into numerous branches, forming a rich 
plexus, which extends to the anterior extremity. 
Von Graff (15, p. 31) describes the brain of A. cinereus, 
but did not succeed in making out the arrangement of the 
longitudinal nerve-cords. ‘I'he brain is, in its upper part, a 
transversely-elongated four-sided mass, bearing anteriorly 
two swellings, from which arise two nerves that unite to form 
the “anterior commissure.’ Below the brain increases in 
bulk, mainly owing to the accumulation of nerve-cells at its 
posterior angles giving rise to the dilatations from which the 
longitudinal nerves are given off, and to the presence of a 
pair of lateral swellings from which the “ posterior roots” of 
the middle longitudinal nerves arise dorsally. The arrange- 
ment of the longitudinal nerves is supposed to correspond to 
that observable in C. roscoffensis as described originally 
by Delage (5). In this form there are six longitudinal 
nerves, which run parallel with one another to the posterior 
extremity of the body, where they lose themselves in a net- 
work of anastomoses. Delage terms these inner, middle, and 
outer pairs of longitudinal nerves. The last lie almost at 
equal distances from the dorsal and the ventral surfaces, 
while the other two are distinctly dorsal, lying immediately 
below the stratum of Zoochlorelle. ‘lhe two inner arise 
from the posterior angles of the more dorsally-situated part 
of the brain, while the others originate on either side from a 
common root which arises from the anterior (ventral) part. 
In addition the middle longitudinal nerve is connected with 
