REACTIONS TO LIGHT IN PLANARIA MACULATA 107 
to the pigment, it is evident that it does not ascribe to the pigment 
the entire function of the localization of photic stimulation as 
is done by Hesse (97). 
2. Localization of photic stimulation in relation to the structure 
of the eye. If we are correct in the assumption that light must 
penetrate a given rhabdome along its longitudinal axis in order 
to stimulate it, then the structure of the eye should be such that 
whenever light enters it, under normal conditions, the longi- 
tudinal axes of some of the illuminated rhabdomes will be parallel 
to the stimulating rays of light. Let us consider this problem. 
The best way to describe the relation of the axis of the light 
rays entering the eye to the axes of the rhabdomes is to consider 
the actual condition of affairs when the eye is illuminated from 
each of several different directions. This is represented in figure 
18. By referring to this figure the following may be seen: Light 
which strikes the eye from directly in front of the animal illumi- 
nates those rhabdomes which lie on the outer posterior margin 
of the pigment-cup, and the longitudinal axes of all of these 
rhabdomes are directed approximately parallel to the rays of 
light (fig. 18, A, a, § and 9). The same holds true for light 
entering the eye from an oblique posterior direction (fig. 18, 
B, d, 1), and, approximately, for lateral illumination (fig. 18, 
A, b, 2-6). When the light comes obliquely from in front of 
the animal, however, the light rays strike certain rhabdomes 
parallel with their longitudinal axes (fig. 18, B, c, 7) and other 
rhabdomes at various angles to their longitudinal axes (fig. 18, 
B, c, 5,6, 8, 9). 
The same is true for the rhabdomes when the light enters the 
eye from different points in the transverse vertical plane. Light 
which enters the eye from directly above (fig. 18, C, a, 8) and 
obliquely below (fig. 18, D, d, 1) strikes all of the rhabdomes 
in both cases parallel to their longitudinal axes. Light from 
obliquely above (fig. 18, D, c, 4-8) and, to a less extent, light 
from the side (fig. 18, C, b, 2-4) passes along the longitudinal 
axis of certain rhabdomes and not of others. 
This seems to indicate that the structure of the eye is such 
that if ight from any given direction enters the eye, it illuminates 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 31, NO. 1 
