118 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
makes the statement that these elements of the abdomen innervate the 
abdominal muscles, while those of the thorax go to the ambulatory ap- 
pendages. The reason for the difference in the arrangement of the nerve 
elements will be immediately perceived. The muscles of the abdomen 
act bilaterally in unison, hence the union of the associated nerve elements. 
In the case of the ambulatory appendages there is little movement in 
unison, hence the corresponding independence of the fibres concerned. 
d. Fibres of Set ©, 
Lenhossék (’92) makes the general statement relative to the sensory 
fibres of the earthworm, that they do not cross the ventral nerve cord, 
but end in fibrillations on the side from which they enter the cord. 
The fibre © of Nereis is an interesting exception to this rule. Concern- 
ing the anastomoses of this system I need say but little. The fibres are 
80 large, the anastomoses so numerous and distinct, and the fibre in such 
excellent condition for study, that there is small chance for error. There 
is no vacuolation of the fibre nor other evidence to lead one to conclude 
that there has been a fusion of fibrillations in the manner suggested by 
Cajal (’96). I have seen no evidence of anastomosis between fibres 
except those of set Band set O, and here the anastomosis is always 
between fibres of the same set. 
I wish to call attention to one more point relative to these fibres. 
The small decussating branches cross the ganglion by a sinuous course, 
and yet where they cross each other they are invariably in contact. 
Why this should be so is difficult to say, unless the function of the fibres 
necessitates such contact. A similar relationship is also to be found be- 
tween fibres of other sets, as in the case of the fibres of sets A and B, as 
described above. Although physiologists do not recognize contact be- 
tween axis cylinders as a means of bringing fibres into functional re- 
lation, it seems to me quite probable that such a relation exists in 
some Cases, 
SUMMARY. 
1. The central nervous system of Nereis virens occupies a deeper 
position than does that of most Polycheetes. It is separated from the 
hypodermis by the circular muscles, and is enveloped by an elaborate 
protective tissue. 
2. The protective tissue consists of two parts ; an inner spongy layer, 
the neuroglia, of ectodermic origin, and an outer sheath, the neurilemma, 
of mesodermic origin, 
