REACTIONS TO LIGHT IN PLANARIA MACULATA 97 
cup (fig. 12, B, a-b). The remaining rhabdomes, in this case, 
are shaded by the dorsal edge of the pigment-cup. We may 
consequently conclude that illumination of the rhabdomes lying 
along the outer posterior edge and the outer ventral edge of the 
eye-cup is followed by turning toward the side bearing the eye. 
Which rhabdomes, then, are illuminated when the animal turns 
away from the side containing the eye? 
In an animal with the left eye removed this follows as a result 
of illuminating the specimen laterally from any point to theright 
of the sagittal plane of the eye. Now, when light enters the eye 
from any point in the horizontal plane to the right of the median 
line, various rhabdomes lying along the center and anterior 
edge of the pigment-cup may be illuminated (fig. 12, A, b-d). 
In the same manner, light which enters the eye from some point 
in the transverse vertical plane of the eye but to the right of the 
point directly above, always illuminates some of the rhabdomes 
lying along the center of the dorsal margin of the pigment-cup 
(fig. 12, B, b-d). These results indicate that illumination of the 
rhabdomes of the center and anterior and vertical edges of the 
pigment-cup is followed by the animal’s turning away from the 
side containing the eye. The region which causes the animal 
to turn away from the side containing the eye cannot, however 
be outlined as definitely as that which causes it to turn in the 
opposite direction, because in some cases light entering the eye 
may illuminate certain rhabdomes of both regions. Thus, by 
referring to figure 12, it will be seen that light entering the eye 
between the points 0 and c, although it causes the animal to 
turn away from the side containing the eye, illuminates certain 
rhabdomes of both regions. 
The observations recorded above indicate that illumination of 
the rhabdomes lying along the ventral and posterior edge of the 
pigment-cup is followed by the animal’s turning toward the side 
containing the eye, while illumination of the remaining rhab- 
domes, i.e., those of the center and anterior edge, is followed by 
the animal’s turning in the opposite direction (fig. 13). Further 
evidence that this conception of localized sensory regions in the 
eye is a correct one will be taken up in the next section. 
