i> 
HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 107 
posterior. The segmentation of the longitudinal muscles is marked by 
an interdigitation of the fibres of one segment with those of the next. 
These interdigitations lie in the plane of the constriction of the body 
which gives the external appearance of segmentation. The line of 
attachment of the longitudinal muscles to the hypodermis (Plate 1, 
Fig. 4) and the constriction in the protective tissue of the segmental 
ganglion (Plate 1, Fig. 8) also lie in this plane, which, as will be seen 
from Figures 4 and 8, thus separates the fifth (V’) and first (1’) seg- 
mental nerves throughout their length. The segmental dissepiment is 
concave anteriorly. Its ventral median edge is attached in the con- 
striction of the segmental ganglion, and is therefore in the intersegmental 
plane. But its lateral border is attached to the hypodermis, between 
the dorsal and ventral longitudinal muscles, anterior to the interseg- 
mental plane and even anterior to the position of the fourth (IV’) seg- 
mental nerve in that region (Fig. 4). Hence, if the position of the 
dissepiment were taken to determine the boundary of segments, the 
fourth (IV’) and fifth (V’) segmental nerves would appear to pass 
backward from the segment in which they arise to the one succeeding it. 
But I have found no segmental nerve passing forward through the 
dissepiment as described by Quatrefages, nor indeed passing out of the 
segment in either direction, if we determine the boundary of segments 
by the segmentation of the musculature. 
When compared with other annelids, we find that Nereis presents a 
generalized condition with respect to its nervous system. It indeed agrees 
very well with the description given by Lang (’88-’94) of the nervous 
system typical of Chetopods. In comparison with other Polychetes, 
however, Nereis shows a rather high degree of development, indicated 
by the deep position and elaborate protective tissue of the ventral nerve 
cord. In the majority of Polychetes the ventral nervous system lies 
embedded in the hypodermis, or intimately connected with it. In a 
few genera, however, such is not the case. Wawrzik (’92) shows that in 
Hermione and Aphrodite the ventral cord is entirely free from the hy- 
podermis, and in this respect he classes these genera with the Oligo- 
chetes. Nereis would also belong to this class, since the ventral cord 
lies internal to the circular muscles, as it does in the Oligochetes. 
2. PRorectivE Tissue. 
The nature and origin of the protective envelopes of the nervous sys- 
tem of Polychetes have been the subject of considerable discussion. 
The differences of opinion are probably due chiefly to real differences in 
