106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The second (2) and third (3) parapodial nerves contain both motor 
and sensory fibres. In Figure 32 the motor fibres are shown, and in 
Figure 33 the sensory fibres of the third parapodial nerve. The motor 
fibres turn back along the muscles that move the sete, and are lost 
among the muscle fibres. The cells of the sensory fibres lie far beneath 
the hypodermis. They send a process either to the hypodermis, or to 
the tissue around the openings through which the sete project. At the 
latter place the fibres apparently end in fibrillations. Figure 37 repre- 
sents a sensory cell of the anterior wall of the parapocium. The periph- 
eral process of this cell enlarges just beneath the cuticula into a small 
knob, from which a fine prolongation extends out through the cuticula 
Figure 38 represents a similar cell and nervous process in the posterior 
wall of the parapodium. In Figure 35 is seen a sensory cell from the 
base of the parapodium, and in Figure 36 one from the side of the body 
near the fourth segmental nerve. 
Figure 34 shows the manner in which the motor fibres end in the 
longitudinal muscles, and Figure 40 shows'the bushy endings of the 
fibres around the glands of the hypodermis between the bundles of 
circular muscles. 
PART II. DISCUSSION. 
1. TOPOGRAPHY. 
In methylen blue preparations it is usually not easy to determine the 
relation of the stained fibres to other organs, because of the difficulty of 
seeing structures which are not stained. For this reason I first made 
a study of the topography of the nervous system, tracing the nerves 
with considerable detail in preparations made by vom Rath’s method. 
By this means nerves consisting of but a few fibres can be traced through 
serial sections. The account of the topography given in Part I. is 
more minute, but otherwise agrees in the main with that given by 
Quatrefages ('50) for Nereis. There is one important point, however, 
in which I cannot agree with Quatrefages. He states that the segmen- 
tal nerve which he designates by the letter o (Planche 3) passes forward 
through the dissepiment to the preceding segment, thus making a ner- 
vous connection between two segments, in addition to that of the ventral 
nerve cord, From the diagram (Plate 1, Fig. 8) it will be seen that there 
is no segmental nerve passing from one segment to another in N. 
virens. The three nerves (I, IV, V) that arise near the intersegmental 
plane pass out parallel with that plane, two anterior to it and one 
