REACTIONS TO LIGHT IN PLANARIA MACULATA 103 
The turning in all of these cases follows the illumination of 
the rhabdomes lying in the center of the pigment-cup (fig. 16, 
A and C, x). After specimens with normal eyes, either one or 
two, are oriented, the rhabdomes, owing to the shadow cast by 
the pigment-cup, are no longer illuminated (fig. 16, B). But 
in specimens with the posterior portion of the eyes removed, the 
A 
var 
~ 
oa, 68 
(ed D 
= 0 ¢| @6% 
Fig. 15 Different illumination of the planarian eye with light coming from 
different directions. The optic cups (pigment-cups) are drawn too large in pro- 
portion to the head. The arrows indicate the direction of the light; the portions 
of the interior of the cup which are not illuminated are shaded (after Hesse, 
97, p. 575). 
B 
rhabdomes are quite as fully exposed to the light after they are 
oriented as when they are laterally illuminated (fig. 16, C and D). 
The results indicate very plainly that stimulation follows the 
illumination of the rhabdomes of the center of the pigment-cup 
only when the light strikes the rhabdomes from a lateral direction, 
not when it strikes them from behind, as it does in the oriented 
specimens with the posterior portion of the eyes removed. This 
seems to show that it is not necessary for the pigment to act as 
