94 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ning at the median plane, the first pair of nerves (y) goes to the 
esophagus; they pass forward near the median line, and then bend 
backward to be distributed to the ventral wall of the csophagus. The 
second and third pairs are respectively the commissures and the nerves 
of the posterior cirri; the other two pairs pass out laterally and forward 
to the walls of the cephalic segment. The ganglia of the first three or 
four body segments are displaced backward, as compared with those of 
the typical segment, and are consequently crowded together, thus mak- 
ing them appear as a single ganglion. Each segment in this region 
receives the typical number of nerves, though in regard to size the 
nerves of each segment do not bear quite the same relation one to 
another as they do in a typical segment. 
c. Typical Body Segment. 
The ganglia of the ventral chain (Plate 1, Fig. 8) are about half as 
long as the segment, and, if the segmentation of the longitudinal mus- 
culature be taken to indicate the boundaries of the metameres, they lie 
intersegmentally, with at least two thirds their length in the posterior 
segment. The longitudinal connectives are enclosed in a single sheath, 
so that there appears to be but one connective, At the intersegmental 
plane there is a constriction of the investing tissue of the ganglion, but 
this does not extend to the nervous part. Five larger pairs of nerves 
are given off from each ganglion, and there are, besides, many smaller 
ones, which consist of only a few fibres each. A few of the latter are 
given off ventrally, but most of them pass dorsally from the mid-dorsal 
line of the nerve cord, and probably are distributed chiefly to the 
digestive tube. 
The paired nerves are most conveniently characterized with reference 
to their position in the segment. Beginning at the anterior end, there 
is close behind the intersegmental constriction of the ganglion the first 
segmental nerve (I, Fig. 8). This nerve is rather slender and passes 
out at right angles to the ventral nerve cord. It lies external to the 
longitudinal muscles, and is partly buried in the hypodermis (1', Fig. 4). 
It may be traced with little change in calibre to the dorsal base of the 
parapodium, where it is lost either in the circular parapodial muscles, 
or in the hypodermal plexus, or most likely in both. 
The second segmental nerve (11) is the largest of the five, and is the 
parapodial trunk. It leaves the segmental ganglion near its posterior 
end and passes diagonally backward across and external to the longi- 
