102 W. H. TALIAFERRO 
D. Localization of photic stimulation 
Considerable evidence has been noted indicating that the 
rhabdomes of the eye of Planaria maculata are arranged in two 
localized sensory regions. If this be actually true, it is obvious 
that there must be some localizing device whereby light from 
one direction will stimulate one of these regions and yet not 
stimulate the other. Otherwise, a general illumination of the 
eye would result in the stimulation of all the rhabdomes, some 
of which cause the animal to turn in one direction and others 
in an opposite direction. 
1. Mechanism of localization of photic Lene In a paper 
referred to several times in the preceding pages, Hesse (’97) 
advances the theory, without experimental evidence, that the 
pigment of the turbellarian eye acts as a localizer of photic 
stimulation. This he illustrates very clearly by means of a 
series of diagrams reproduced in figure 15. 
He maintains that although light can enter the pigment-cup 
from different angles, it illuminates different portions of the 
interior from each angle. As he points out, in forms in which 
there is only one retinula, this would simply result in the illumi- 
nation of different parts of the same retinula. In forms, how- 
ever, whose eyes contain numerous retinulae, this mechanism 
would result in the illumination of different sets of retinulae. 
Many of the results presented in the preceding pages are in 
harmony with Hesse’s interesting theory which contains the 
most valuable suggestion yet made concerning the function of 
the pigment in flatworm eyes. Some of the results obtained in 
our work are, however, not in harmony with this theory. 
The experiments which we have described dealing with the 
reactions to light in forms with one eye and the posterior part 
of the other removed bear directly on the question as to whether 
or not the pigment acts as a localizer of photic stimulation as 
Hesse maintains. It will be remembered that such forms when 
illuminated laterally on the side containing the anterior part 
of the eye turn directly away from the light and proceed in a 
more or less direct path, as is the case with normal specimens 
and specimens with one eye (fig. 14, B). 
