HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 115 
contraction. When the habits of the animal are considered, it is pos- 
sible to understand what the function of such a contraction brought 
about by the stimulus of light might be. The worm lives in the mud 
in burrows, and frequently rests with the anterior end above the surface, 
while the remainder of the body is in fhe burrow. Under such circum- 
stances the longitudinal contraction would cause the animal to retreat 
into the burrow, for longitudinal contractions are in general accompanied 
by the pointing of all the parapodia towards that end of the body from 
which tho stimulus comes. For example, if the stimulus is applied at 
the anterior end, the parapodia are all thrown forward, and the longitu- 
dinal contraction of the body immediately follows. This will cause the 
anterior end to move towards the tail while the latter remains station- 
ary, since the position of the parapodia prevents movement of the body 
in the opposite direction. Now, if the shadow cast by a predatory 
animal were to bring about this movement, the mechanism would be of 
vital importance to the worm. Perhaps the importance of the function 
and the great extent of the movement brought about help to account 
for the large development of the giant fibres. The objection may be 
urged that since the phenomena which I have described for Nereis have 
not been found elsewhere, they cannot be of general importance, even if 
the condition be admitted for Nereis. But the exceptional conditions 
under which such phenomena can be observed render it probable that 
they may have been overlooked even when present. 
It must be remembered that, in order to demonstrate the passage of 
one fibre through another, there must be a differential staining of the 
substance of the two fibres. Only in preparations fixed and stained by 
the method of vom Rath, and not in all of these, have I obtained such a 
differentiation. Successful preparations, however, leave no doubt con- 
cerning the actual relation of the fibres, for I have carefully compared 
series of sections cut in each of the three cardinal planes, and always 
with the same result, 
If, then, the giant fibres are nervous in function, the neuron theory 
of Waldeyer (91) will require considerable modification. The nervous 
element is not always unicellular, but may consist of a number of cells 
united in function. The nervous connection between fibres is not always 
through fibrillations; it may be directly between the axis cylinders 
themselves, 
b. Fibres of Set A. 
Since little is known about the relations of the fibres of set A to other 
fibres, we cannot say much about their probable function, Nevertheless, 
