REACTIONS TO LIGHT IN PLANARIA MACULATA ras) 
ment-cup of the eye, it suddenly reorients and again proceeds 
directly from the source of stimulation as represented in figure 
4, R. In referring to this figure one might ask why this ‘wander- 
ing reflex’ with the subsequent reorientation occurred only after 
the direction of the rays was changed at 6 and not after the pre- 
ceding changes. The answer to this lies in the fact that after 
the preceding changes in the direction of the rays the animal was 
not allowed to proceed far enough, for, as previously stated, the 
animal always proceeds a certain distance before the wandering 
commences. In practically all of the numerous records made of 
the orientation to light in Planaria maculata this wandering re- 
action and reorientation occurred one or more times. 
The wandering of specimens from the path of orientation and 
the subsequent reorientation as soon as the animal turns enough 
to let the light rays enter the pigment-cup suggests strongly 
that once an animal is oriented it receives no orienting stimula- 
tion unless it leaves the path of orientation. This will be con- 
sidered in detail in another section. 
The second type of réaction alluded to above has been desig- 
nated the ‘twisting reflex.’ Whenever a planarian is proceeding 
from a source of light, it pauses at irregular intervals and twists 
the anterior end so that the ventral surface tends to be directed 
upward. Under strong illumination this response is exhibited 
at intervals of approximately 3 to 4 cm. 
It is interesting to note that decapitated specimens never give 
the twisting reflex. When the anterior ends of such specimens 
are allowed to regenerate, this reflex is not exhibited until after 
the anterior end is almost fully developed. From a study of the 
sections of such animals, considerable evidence was obtained in- 
dicating that it is necessary for the regeneration of the ‘brain’ 
to be practically complete before the twisting reflex occurs. 
Neither the twisting reflex nor the wandering reflex apparently 
plays any significant part in the normal process of orientation, 
but, as we shall see later, both play a paramount réle in the 
orientation of forms with one eye removed. 
2. Character of turning in animals during orientation in different 
intensities of illumination. In the description of the orientation 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 31, No. 1 
