HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 93 
ventral edge of the brain immediately ventral to the ninth, and passes 
out directly to the commissural ganglion. 
The eleventh (XI) and twelfth (XII) nerves are the two optic nerves. 
They converge from the eyes toward the centre of the brain. 
The thirteenth (XIII) nerve arises back of the posterior eye, and 
goes directly to the ciliated groove. The fourteenth (XIV) is a rather 
diffuse nervous connection between the brain and the dorsal surface of 
the head. The region innervated lies nearly midway between the pos- 
terior eye and median plane, but slightly nearer the latter. 
Besides these fourteen paired nerves (I-XIV) there is a single median 
nervous connection between the dorsal surface of the head and the 
brain. This is similar to the diffuse fourteenth nerve, but is smaller 
and lies slightly anterior to it. Its position is shown in Figure 1. 
From the commissural ganglion a nerve (a) passes forward to the 
proboscis, where it unites with the fifth nerve of the brain (V) in a 
ganglion. Another nerve (5) passes backward along the side of the 
head. Four or five small connectives, not shown in Figure 1, unite 
the commissural ganglion with the optic ganglion, which lies in con- 
tact with the ventral side of the anterior eye. Lastly, the circum- 
cesophageal commissure passes from the commissural ganglion around the 
cesophagus to the sub-cesophageal ganglion, traversing on its way a 
ganglion which lies beneath the anterior pair of tentacular cirri. From 
this anterior cirrus ganglion two large nerves go each to an anterior 
cirrus, and, from the anterior side of it, a smaller one () to the pro- 
boscis. On its posterior side the anterior cirrus ganglion is connected 
by a small nerve (@) with the posterior cirrus ganglion, which lies 
beneath the posterior pair of tentacular cirri. The latter ganglion gives 
off two large nerves, one to each of the two posterior tentacular cirri, 
and also sends a nerve (¢) backward along the side of the head. The 
posterior cirrus ganglion is connected with the sub-cesophageal ganglion 
by a large nerve trunk (n. pa-coms.), which lies posterior to and parallel 
with the circum-cesophageal commissure. This trunk gives off several 
branches from a region midway between the posterior cirrus ganglion 
and the sub-cesophageal ganglion. 
b. Sub-cesophageal Ganglion. 
The sub-cesophageal ganglion (gn. sb-oe., Fig. 1) lies on the posterior 
boundary of the cephalic segment. It gives off three pairs of nerves 
besides the commissures and the nerves of the posterior cirri, Begin- 
