116 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
there are several facts which point to a connection with the forward 
locomotion of the animal. The worm advances by a rhythmical move- 
ment of the parapodia, which begins at the posterior end and passes 
toward the head. With this movement there is usually associated a 
serpentine motion of the body, which also passes from behind forward, 
Both movements are less vigorous near the head, and the serpentine dis- 
appears entirely between the twentieth and tenth segments. The size 
of the fibres of set A in a given region corresponds to the degree of 
activity of the locomotor movements of that region. Whether this 
fact is more than a mere coincidence I cannot say, but it would seem 
to be so. Besides, if there is a causal relation between the condition of 
these fibres and the locomotor movements, we may even account for the 
enormous size of the fibres on the ground of their functional importance. 
Another evidence of this correlation is the serial arrangement of the 
fibres, which may be connected with the progressive character of the 
motor excitation, and with the postero-anterior disposition of each fibre, 
the latter corresponding to the direction of the movement. 
Although these speculations concerning the function of giant fibres 
are purely tentative, they may serve as a basis for physiological 
experiments. 
c. Fibres of Set B. 
In describing the fibres of set B (Plate 4, Figs. 27, 28) I merely 
mentioned the fact of an anastomosis between the axis cylinders of the 
components of each pair. I wish here to discuss the subject more fully. 
The description of these fibres was by no means based wholly on meth- 
ylen-blue preparations. Indeed, all the facts, excepting that of anasto- 
mosis, were demonstrated on serial sections before an impregnation by 
methylen-blue was obtained. The fibres are so large that they can 
easily be traced through serial sections. This fact is important in con- 
sidering the value of the evidence for anastomosis. 
I have carefully examined seventeen pairs of these in serial sections 
cut in one or the other of the three cardinal planes of the body, and in 
addition eight pairs stained in methylen-blue and examined before cut- 
ting. Where the fibres of a pair crossed the ganglion they were always in 
contact with each other, and, with one exception, they ran parallel for a 
considerable distance. In the exceptional case the fibres crossed each 
other at an angle of about ten degrees, which still allowed a line of con- 
tact equal in length to one fourth the width of the ganglion. The fibres 
usually cross the ganglion at right angles to its longitudinal axis, but 
in one instance they crossed at an angle of about sixty degrees (Plate 1, 
7 
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