REACTIONS TO LIGHT IN PLANARIA MACULATA 95 
In order not to confuse the reactions resulting from the illum- 
ination of one eye with those following from the illumination 
of two eyes, all animals used in these experiments had one eye 
removed. The same specimens were used as in the preceding 
section. 
Let us consider first the reaction of a specimen with the left 
eye removed when the eye is illuminated from different angles 
in the horizontal plane of the animal. 
If the beam of light comes from a point directly in front of the 
animal, it turns the anterior end to the right, i.e., toward the 
normal or stimulated side (fig. 11, A, b). The same reaction 
follows if the source of light is shifted slightly to the left of the 
median line (fig. 11, A, a). As, soon however, as the light is 
shifted far enough to the left of the median line as to place the 
pigment-cup between the source of illumination and the sensory 
rhabdomes, no reaction follows (fig. 11, a-b). When the rays 
of light illuminate the eye from any point to the right of the 
median line, the animal turns in an opposite direction, i.e., it 
turns away from the normal or stimulated side (fig. 11, b-c). 
Here, just as with the previous case, as soon as the source of 
illumination is carried far enough to place the pigment-cup be- 
tween it and the rhabdomes no reactions follow. 
The same type of response is obtained when the specimen is 
illuminated from various points in the transverse vertical plane 
of the eye. If the rays of light illuminate the animal from a 
point directly above the eye, the specimen reacts by turning to- 
ward the normal or stimulated side (fig. 11, B, b) If the source 
of illumination is shifted to the left of the vertical line the animal 
responds in the same manner (fig. 11, a-b). If, however, the 
rays of light illuminate the animal from any point to the right 
of the vertical line, the specimen responds by turning away from 
the normal or stimulated side (fig. 11, B, b-c). As in the previous 
luminated from different points in the transverse, vertical plane of the eye. Sec- 
tion taken along z-y of A. If the light enters the eye from any point between the 
arrows a-b, the specimen turns toward the normal side. If the light enters the 
eye from any point between the arrows b-c, the specimen turns away from the 
normal side. No reaction follows if the eye is illuminated from any point along 
the broken line a-c. 
