HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 105 
tact with it, while the other branch passes directly through it. Some- 
times the penetrating branch, instead of passing through the middle of 
the giant fibre, goes so far to one side that it does not become free from 
the sheath of the giant fibre, but is still wholly embedded in its 
substance. 
d. Fibres of Set O. 
The next fibre system (Plate 4, Figs. 27, 30), set O, is apparently 
centripetal, since no cell was found connected with it, and since what ap- 
pears to be the main fibre, entering the cord from the fourth segmental 
nerve (IV), immediately divides, forming the characteristic Y of centrip- 
etal fibres. One of the branches runs directly back and ends in fibrillations 
opposite the second nerve (II) of the succeeding segment. The other 
branch runs forward, and ends in a similar way opposite the second seg- 
mental nerve (II) of its own segment. Near its origin the second 
branch gives off a third which runs diagonally backward and across the 
ganglion, ending in a position symmetrical to the ending of the first 
branch, Since the counterpart of each of these three branches is found 
on the opposite side of the nerve cord, there must be six branches ending 
in each segment, on either side three, all of which are opposite the sec- 
ond segmental nerve (Fig. 27, II). The ends of the fibres are enlarged, 
and give off a few fibrillations. The three endings of each side of the 
body lie side by side, and are connected with one another by several 
ladder-like anastomoses (Plate 4, Figs. 29, 30). The fibres of this set 
are rather large, and lie almost wholly on the ventral side of the cord. 
The third or decussating branches, however, are rather slender, and in 
crossing the ganglion first curve up and then down. Where the two 
fibres cross each other they are always in contact. 
e. Peripheral Fibres. 
The following are some of the fibres found in the parapodial ganglion 
(compare Plate 1, Figs. 5, 8, Plate 5, Fig. 39): (a) Fibres entering the 
ganglion from the second (II, Figs, 5, 39) segmental nerve pass through 
the ganglion and out either by the first (1) or by the fourth (4) para- 
podial nerves. (5) Fibres entering from the segmental nerve divide 
into two branches, one of which passes out through the second (2), the 
other through the third (3) parapodial nerve. Neither of these classes 
of fibres gives off fibrillations in the ganglion. (c) A third kind of fibre 
enters the parapodial ganglion from the segmental nerve, and ends in 
the ganglion in fibrillations. 
