The Reactions of' Planarians to Light 4 1 



This led him to state ('97, p. 552) that "die Starke der Reaction 

 auf Lichtwirkung nicht der Starke der Lichtwahrnehmung ent- 

 spricht," and he ascribes this difference in behavior to a differ- 

 ence in the"GefLihIston"ofthetwo species. Among other observa- 

 tions described by Hesse, the tw^o following are of importance 

 in this connection, namely, that a sudden introduction of light 

 caused an almost immediate turning away on the part of the worm, 

 and, that worms with eyes could not be made to remain in the 

 light when escape was possible. In his opinion this apparent 

 perception on the part of the worm is due not to the animal's 

 abihty to distinguish light but rather to unpleasant chemical reac- 

 tions set up within the organism as the result of light stimulation. 

 And, lastly, Hesse showed that the general position of the eyes 

 of a planarian, together with the arrangement of their sensory 

 portion, partly enclosed as it is within pigment cups, affords a 

 device whereby the worm can be oriented to light. By means of 

 this simple apparatus it receives a localized stimulus, which enables 

 it to distinguish the direction from which the light corhes. If 

 light, striking the eye of a worm, fell upon sense cells which were 

 unscreened in any way by pigment, there would result a general 

 stimulation without localization of the stimulus and consequently 

 orientation could not be effected. 



Parker and Burnett ('00) sought, by quantitative methods and 

 with more accuracy than Loeb or Hesse, to establish the part 

 played by the eyes in light responses of planarians. They came 

 to the same general conclusion as these authors since they found 

 that Planaria gonocephala without eyes reacts to light essentially 

 as normal animals do, except that the reaction time is somewhat 

 longer. They also showed that worms when pointed toward the 

 source of lio-ht travel at a slower rate than when headed in the 

 opposite direction. With regard to the mechanism of the light 

 response they say ('00, p. 383): "We have seen nothing in our 

 experiments that supports the opinion suggested by Hesse (p. 551) 

 that reactions such as we have described are due to the direct 

 influence of light on the internal parts of the planarians, and 

 we are more inclined to the view that these reactions are initiated 

 by the effect of light on the integument of the animal, /. ^., are 



