92 W. H. TALIAFERRO 
is illuminated from the ‘blind’ side, is by means of the twisting 
reflex. When an animal, moving at right angles to the direction 
of the rays of light, gives the twisting reflex it very often bends 
the side containing the remaining eye so far toward the light 
that the rhabdomes lying along the ventral surface of the pig- 
ment-cup are stimulated. (fig. 10, B and insert C). If this takes 
place, the animal sharply orients by turning toward the stimulated 
or normal side (fig. 10, B, 1/—4). 
The results presented above show that specimens with one 
eye orient normally when the side containing the eye is illumi- 
nated; but that when the ‘blind’ side is illuminated, there is no 
true orientation unless the animal moves in such a manner as 
to allow the rays of light to illuminate some of the rhabdomes of 
the eye of the normal side. 
The most interesting feature in regard to the process of orien- 
tation in such specimens is that when they are illuminated on 
the ‘blind’ side they very often turn toward the side containing 
the eye. Since turning is due to stimulation of the eye, such speci- 
mens turn toward the eye which is stimulated instead of from it 
as ordinarily occurs in normal orientation. In them, also, stimula- 
tion seems limited to certain portions of the eye and this seems 
to indicate that the direction of turning depends upon the local- 
ization of the stimulus within the eye. From a structural stand- 
point it indicates that the rhabdomes of each eye are arranged 
in two (or more) definitely localized sensory regions—the stimu- 
lation of one region resulting in the animal’s turning away from 
the side containing the eye and the stimulation of the other 
resulting in a turning in the opposite direction. If there are 
such regions in the eye can these regions be accurately outlined? 
This problem will be taken up in the next section. 
As has been pointed out above, Mast (10, p. 132) found that 
Planaria with one eye removed by gouging it out or by cutting 
off the anterior end obliquely move continuously from the 
wounded side, and that, later during the process of regeneration, _ 
they have a tendency, when stimulated by light, to move toward 
the side containing the newly regenerated tissue. In our experi- 
ments we did not obtain reactions of this sort. Mast maintains 
