REACTIONS TO LIGHT IN PLANARIA MACULATA 109 
These observations strongly support the assumption that if 
light strikes a rhabdome parallel with its longitudinal axis stimu- 
lation results; whereas, if light does not strike a rhabdome along 
this axis stimulation does not follow. This will explain why it 
is not necessary for the pigment-cup to act as a localizer of 
photic stimulation. The latter is localized not by the pigment- 
cup, but by the position of the longitudinal axes of the various 
rhabdomes. 
NATURE OF THE STIMULUS DURING ORIENTATION 
At the present time there are two chief theories advanced to 
account for the orientation of organisms to light—the continuous- 
action theory and the change-of-intensity theory. The literature 
bearing on these two theories is so extensive that we shall limit 
the present review to a very brief outline. 
Loeb, who is the chief upholder of the continuous-action theory, 
maintains that an organism orients to light because of unequal — 
chemical changes induced by the light in symmetrically placed 
photoreceptors; that effects of these chemical changes are trans- 
mitted eventually to the locomotor organs, thus producing un- 
equal ‘tension or energy production’ in the musculature of the 
two sides, and that this results in a turning of the organism. 
According to him, after the organism has become oriented, the 
light produces equal chemical changes in the photoreceptors 
and the organism proceeds directly toward or away from the 
light owing to continuous and equal action of the light on sym- 
metrically located photoreceptors. 
In reference to his theory, he says (’16, pp. 258-259): 
The reader will perceive that according to the writer’s theory two 
agencies are to be considered in these reactions: first, the symmetrical 
arrangement of the photosensitive and the contractile organs, and, 
second, the relative masses of the photo-chemical reaction products 
produced in both retinae or photosensitive organs at the same time. 
If a positively heliotropic animal is struck by light from one side, the 
effect on tension or energy production of muscles connected with the 
eye will be such that an automatic turning of the head and the whole 
animal towards the source of light takes place; as soon as both eyes are 
illuminated equally the photochemical reaction velocity will be the 
