HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 103 
united with the giant fibre, I cannot be sure that there is such a con- 
nection. I have found no other cells connected with these giant fibres, 
The median giant fibre divides in the sub-cesophageal ganglion into 
several branches, which continue forward parallel with one another along 
the median plane. One of them I was able to trace to one of a group of 
large cells lying between the ventral ends of the cirecum-cesophageal con- 
nectives. The other cells of the group are connected with similar fibres, 
but I could trace only one continuously from the cell to the giant fibre. 
The three giant fibres extend back into the last segment of the body 
without branching or changing their relative sizes or positions. Occa- 
sionally the median fibre in passing through a ganglion divides and 
allows the passage of a bundle of fibres between the two parts, which 
then immediately reunite, and the fibre continues on as before. This 
condition occurs frequently, but appears to be wholly accidental, since it 
is very irregular in the frequency of its occurrence, as well as in the size 
of the loop produced, and also in the relative sizes of the two divisions 
of the fibre. In one instance I found a similar condition in one of the 
lateral giant fibres, but it was not very well marked. 
The giant fibres are pierced by many smaller ones, which pass directly 
through them (Plate 1, Fig. 2). In the case of the lateral giant fibres 
this occurs most frequently near the places where the segmental nerves 
are given off from the ganglion. Sometimes the small fibres branch 
within the large one, the branches then continuing on through the giant 
fibre. In preparations stained with osmic acid, the small fibres stain 
much more deeply than the giant fibres, thus becoming very distinct. 
In a part of a methylen blue preparation which had not taken the stain, 
the small fibres traversing the giant fibres could be readily seen because 
they were more highly refractive than the giant fibre. 
I cannot say that in successive segments the giant fibres are pierced 
by corresponding sets of smaller fibres, but there is at least one set 
which regularly traverses the giant fibre on passing out into the fourth 
(IV) and fifth (V) segmental nerves. This fibre will be described as 
set B, 
b, Fibres of Set A. 
Along the inner margin of the lateral connectives there lies a set of 
fibres (Plate 2, Fig. 18, Plate 5, Fig. 31) which in transverse section 
are almost as large as the median giant fibre, and resemble it in their 
resistance to stains. They differ from giant fibres, however, in the fol- 
lowing particulars (compare Plate 4, Fig. 27, 4):—(1) They are ar- 
ranged segmentally, one pair of fibres originating in each segment ; 
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