104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
(2) Each fibre is connected with a single cell; (3) They do not extend 
through more than two segments; (4) They are not pierced by other 
fibres, nor (5) do they show the reticulum found in giant fibres ; (6) They 
are branched. The cell (Plate 4, Fig. 27) of which this fibre is a process 
lies on the ventral side of the ganglion near the origin of the third seg- 
mental nerve (III). The general direction of the process is forward, but 
at the outset it crosses and recrosses the median plane, decussating twice 
with its companion of the other side, one decussation being immediately 
behind and the other in front of the origin of the second (IT) segmental 
nerve. After the second crossing the two fibres run side by side close 
beneath the median giant fibre, until they pass the first point of decus- 
gation of a similar set of fibres in the next anterior segment. Here they 
diverge and apparently break up into fibrillations or branches too small 
to be traced in preparations stained in the ordinary way. I have not 
succeeded in staining this fibre with methylen blue, This system is well 
developed in every segment from the last one of the tail to within twenty 
segments of the head, where the fibre gradually becomes smaller until, 
in the first three or four segments, it cannot be distinguished among 
the other fibres of the cord. 
c. Fibres of Set B. 
Next in size come the cells and fibres of set B (Plate 4, Figs. 27, B, 
28). The cells lie ventrally about midway between the origin of the 
first (I) and second (II) segmental nerves. From each cell a process 
extends forward and gradually rises into the middle of the ganglion. 
Opposite the origin of the fourth (IV) segmental nerve, the fibre turns 
squarely across the ganglion, running parallel to its mate, with which 
it decussates, and then divides into two branches, both of which go to 
the periphery ; one through the fourth (IV), the other through the fifth 
(V) segmental nerve. The two fibres of a pair lie in contact for some 
distance where they cross from one side of the ganglion to the other 
(Plate 1, Figs. 6, 7), and they anastomose at several points along the 
line of contact (Plate 4, Fig. 28). The fibres of sets A and B are in- 
timately associated at the point where they cross each other (Plate 1, 
Fig. 6, Plate 3, Figs. 22, 23), for they are not only in contact, but the 
smaller fibres lie in a deep indentation on the larger one. The relation 
of fibre B to the lateral giant fibre is still more intimate. Immediately 
after branching, one or both branches pass directly through the lateral 
giant fibre before passing out of the ganglion (Plate 1, Fig. 2). Some- 
times one branch may pass around the giant fibre, but still be in con- 
