REACTIONS TO LIGHT IN PLANARIA MACULATA 113 
12. Specimens with one eye removed orient accurately to 
. light, when illuminated on the normal side, by turning directly 
away from the source of light. 
13. Such specimens do not orient to light when illuminated on 
the ‘blind’ side unless the head is moved so that light enters the 
remaining eye. If, however, the head is moved so that light 
enters the remaining eye (wandering and twisting reflex), accurate 
orientation may follow. 
14. The rhabdomes in the eye are arranged in two localized 
sensory regions; illumination of the rhabdomes of the posterior 
and ventral edge of the pigment-cup is followed by the animal’s 
turning toward the side containing the eye, while illumination 
of the remaining rhabdomes is followed by the animal’s turning 
in the opposite direction. 
15. Specimens possessing only the anterior portion of one eye, 
when illuminated from in front, do not turn sharply toward the 
side containing the eye, as do specimens possessing one entire 
eye. The loss of this reaction in such specimens is probably 
due to the loss of the rhabdomes situated on the posterior margin 
of the pigment-cup. 
16. Removal of the posterior portion of the eye does not 
impair the capacity of the remainder of the eye to function in 
a normal manner. 
17. Specimens possessing only the posterior portion of one 
eye react to light as do specimens with both eyes removed. 
Histological examination of such specimens shows, however, 
that removal of the anterior portion of the eye severs the con- 
nection between the remaining rhabdomes and the ‘brain.’ 
18. The observed reactions in Planaria can be explained with- 
out assuming that the pigment-cup acts as a localizer of photic 
stimulation as suggested by Hesse (’97). It is possible, however, 
that the pigment has a limited localizing function in the individual 
retinula. 
19. Light must strike a given rhabdome parallel with its 
longitudinal axis in order to cause stimulation of the rhabdome. 
Thus the position of the longitudinal axis of the rhabdomes 
results in a localization of photic stimulation. 
