HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 95 
tudinal muscles to the parapodial ganglion, which lies in the ventral 
base of the parapodium between the longitudinal muscles and the pore 
of the segmental organ. 
The third segmental nerve (III) is very small. It arises from the 
posterior end of the segmental ganglion and passes diagonally outward 
and backward to the posterior base of the parapodium, Like all the 
paired segmental nerves, it lies embedded in the hypodermis throughout 
its length. Next in size to the parapodial trunk is the fourth nerve 
(IV). It arises from the anterior third of the succeeding ganglion, and 
hence lies in the posterior part of the segment, near the intersegmental 
line, where the longitudinal muscles are attached to the hypodermis. 
It runs parallel with the first nerve (1) of the following segment, but 
continues in a direct course around the body almost to the mid-dorsal 
line. 
The fifth segmental nerve (V) is very slender. It lies close behind 
the fourth nerve, and very near the intersegmental plane. It extends 
as far as the base of the parapodium, 
d. Parapodial System. 
The innervation of the parapodium (Figs. 5, 8) is almost wholly from 
the parapodial ganglion, from which four nerves radiate toward the 
periphery. The most anterior one (1) is very slender and usually passes 
out in front of the pore of the segmental organ, It goes to the retractor 
muscles of the anterior side of the parapodium. The second parapodial 
nerve (2) is comparatively large, and innervates the ventral ramus. 
Near the ganglion it divides into an anterior and a posterior branch. 
The anterior one passes along the anterior wall of the ventral ramus 
to the anterior setigerous lobe. The other runs along the posterior wall 
to the posterior setigerous lobe, giving off a branch to the ventral cirrus 
and another to the inferior ligula of the ventral ramus. 
The third parapodial nerve (3) passes outward and upward on the pos- 
terior wall of the parapodium. About half way up the side of the para- 
podium it gives off a branch to the glandular region of the dorsal base 
of the parapodium. Then other branches are sent to the anterior setige- 
rous lobe of the dorsal ramus, the posterior setigorous lobe, the superior 
ligula, and the dorsal cirrus. 
A fourth parapodial nerve (4) goes to the posterior base of the 
parapodium. 
