REACTIONS TO LIGHT IN PLANARIA MACULATA 111 
that stimulation during orientation is due necessarily to either 
the actual amount of energy received or to the time-rate of change 
in intensity. In regard to the second and third questions, how- 
ever, we can draw definite conclusions. 
In the first place, as has been noted (p. 74), when an animal 
is proceeding away from the source of illumination, it tends to 
wander to the right or left (wandering reflex). When the animal 
has thus turned its head laterally to the extent that the rays of 
light enter the mouth of the pigment cup, it re-orients. This 
behavior strongly suggests that once the animal is oriented it 
receives no orienting stimulation until it leaves the path of 
orientation. In the second place, it was shown above (p. 95), 
under the experiments designed to map the regions of the eye, 
that no stimulation is received (or more exactly no reaction 
follows) as long as the pigment-cup is between the source of 
illumination and the rhabdomes. Now, an examination of the 
relative positions of the eyes shows very clearly that once an 
animal is oriented and is proceeding away from the light, no 
light can strike the rhabdomes unless it does pass through the 
pigment. From these observations we must conclude that when 
a planarian is moving away from the light the pigment-cup 
effectively shades the sensory portion of the eye. Therefore, as 
there is no continuous illumination of the sensory organs involved 
in orientation, there can be no continuous stimulation of these 
organs. 
The fact that specimens with one eye removed orient accurately 
to light (fig. 9) shows clearly that the symmetrical arrangement 
of the two eyes is not essential for orientation. The extent of 
turning is probably reflexly determined by the portion of the 
eye stimulated and is independent of the duration of the 
stimulation. 
It may, therefore, be concluded that, while our evidence is 
not conclusive in regard to the nature of the stimulus, orientation 
~ in Planaria maculata is not in accord with the ‘continuous-action’ 
theory as defined by Loeb. 
