The Reactions of Planarians to Light 147 



eral activity termed photokinesis, while the other, connected indi- 

 rectly at least with the direction from which light impinges upon an 

 organism, is called phototaxis. In the case of planarians these 

 two phases of light stimulation have been shown to be intimately 

 associated and both operative. Carpenter ('05) pointed out in 

 the case of the pomace fly that phototaxis occurs only when pre- 

 ceded by photokinesis or some other reaction, and such an inter- 

 relation of the two is undoubtedly of wide occurrence. The 

 object of this section is to inquire into the causes underlying pho- 

 totaxis. Loeb ('93b) has shown that phototaxis is the result of 

 orientation. It does not necessarily follow, however, that orien- 

 tation invariably results in phototaxis. In fact Dearborn ('00) 

 found that crayfishes would orient to an electric light introduced 

 into the water near them without making any considerable loco- 

 motor movements in consequence. 7 



To the question of how orientation of organisms to light is 

 caused, three possible explanations may be presented: 1, Chance 

 result of photokinesis; 2, reflex response to directive stimuli; 

 3, voluntary action. Since the first hypothesis seems entirely 

 inadequate to account for the uniformity of orientation in pla- 

 narians, and the third alternative is out of the question with refer- 

 ence to these animals, a consideration of the reflex responses to 

 directive stimuli may be taken up at once. 



There are two general theories which attempt to explain the 

 way in which orientation occurs through reflex responses to stimuli. 

 These theories are first, the trial and error theory of Jennings and 

 Holmes, and secondly, the tropism theory of Verworn and Loeb. 

 By the trial and error theory orientation, with its consequent pho- 

 totaxis, is interpreted as the result of repeated attempts on the 

 part of an organism to become adjusted to any given stimulus. 

 Those attempts which fail to result in adjustment to the stimulus 

 are "errors," and as such are followed by other attempts until 

 finally some one secures the necessary adjustment. Trials oi 

 this kind may be made in different ways according to the organism 



7 Throughout the following discussion orientation will be understood as a position assumed with refer- 

 ence to the light while phototaxis will be made to include motion toward or away from the source of the 

 light. 



