78 W. H. TALIAFERRO 
a planarian bends its body in a lateral direction, this can be 
conceived to take place either by a lengthening of the side toward 
the light or by a contraction of the side opposite the light, or 
possibly both. If the animal bends by lengthening one side, 
this is probably due to a contraction of the dorsoventral muscles 
of that side. This contraction would tend to flatten the body 
in a given region and hence elongate its contour. On the other 
hand, a contraction of one side would most likely be due to a 
contraction of the longitudinal muscles of that side. Pearl (’03) 
is of the opinion that in planaria while turning away from mechan- 
ical stimulation, this turning is due to a contraction of the dorso- 
ventral muscles and, in consequence, to an elongation of the 
side away from the bending. In numerous experiments along 
this line, the author has been unable to satisfy himself as to 
which is true in orientation to light. The nerve fibers leading 
from the eye of a planarian must be connected indirectly with a 
rather complex system of muscles along either or both sides of 
the animal. In an animal under comparatively weak stimula- 
tion; the nerve impulse most likely is transmitted to a rather 
localized region of the musculature, viz., to a region near the 
cephalic lobes. The fact that the point of bending gradually 
moves posteriorly as the stimulation increases strongly suggests 
that in such cases there is a greater and greater spread of the 
nerve impulse along the musculature of a given side as each 
successive increase in the stimulation takes place. This assump- 
tion of course, would not explain the reaction of bending first 
toward and then away from the light under the influence of very 
intense illumination. It is likely that the latter involves a dif- 
ferent neuromotor mechanism, possibly analogous to protopathic 
stimulation in the vertebrate eye. In the human eye, for ex- 
ample, an intense illumination often involves a protopathic sen- 
sation entirely aside from the usual sensation of light. In this 
case the protopathic sensation involves a different mechanism 
than the usual sensation of light; it may, in fact, be invoked in 
a totally blind person (Sherrington, ’98, page 967). 
The reaction of bending the anterior end first toward and 
then away from the light is very similar to that described by 
