HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 117 
Fig. 7). Thus, one of the fibres partially retraces its course in order to 
maintain a course parallel with its fellow. Sagittal sections (Plate 3, 
Figs. 22, 23) show that the fibres are always flattened on their apposed 
faces. That part of the sheaths which forms the dividing wall is usually 
very thin, and in some cases seems to be wholly wanting. In the prepa- 
rations which are best preserved, however, the dividing wall can always 
be seen. I have not been able to demonstrate satisfactorily anastomoses 
in preparations made by the more usual histological methods. In 
methylen-blue preparations the fibres do not appear to be in contact, but 
this is due to a shrinking of the axis cylinder within the sheath produced 
during the fixing of the stain. The anastomoses, however, do exist, and 
are clearly shown in methylen-blue preparations (Plate 4, Fig. 28) ; they 
proceed from small elevations on the opposed faces of the fibres. From 
what has gone before, it is evident that the anastomosing bars simply 
pierce the thin membrane that separates the two fibres, and that they 
practically lie wholly within the fibre sheaths. Hence they cannot be 
rogarded as fibrillations fused by the action of the methylen-blue. The 
fibrillæ of the axis cylinder pass out into the anastomosing bars, but 
whether they pass completely across from one fibre to the other I can- 
not say. There is, however, a distinct interdigitation of the fibrille of 
the opposite fibres. The appearance of the preparations gives one the 
impression that there has been a breaking of the fibrillæ of the anasto- 
moses due to the shrinking of the fibres, The anastomoses are not 
always as evident as they are in the case reproduced in Figure 28, but 
there is always some indication of them. This may‘ consist simply of the 
pointed elevations arranged in pairs opposite each other on the fibres. 
Since the cells of set B are situated in a central organ, they are prob- 
ably motor, and since the fibres are united in bilaterally symmetrical 
pairs, they probably act in concert. Such animals as Annelids differ 
from more complex organisms in that many of their movements are in 
unison on the two sides of the body. The longitudinal contractions and 
expansions of the body are examples. In Nereis the movement of all the 
parapodia backward or forward, when the animal is touched at one end 
or the other, is another instance. When such movements are 80 fre- 
quent and of such vital importance, one may well expect to find an inti- 
mate association of the related nerve fibres. 
Allen (96) describes decussating nerve elements in the abdominal 
ganglia of the lobster so closely united that he was unable to resolve 
them into their constituent parts. He finds, however, that similar ele- 
ments in the thorax are not so intimately related. At another place he 
