2o6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



source of light, not when it is turned in the opposite direc- 

 tion. Under such conditions they will of course never turn 

 * sharply toward the source of light. The swinging of the 

 anterior end may however be considered as a trial move- 

 ment and orientation consequently indirect. 



(6) The swinging movements of the anterior end increase 

 the possible accuracy of orientation in case of direct as well 

 as indirect orientation. By means of them the animal takes 

 its bearing, if this anthropomorphic term may be permitted. 



(7) The stimulations which lead to orientation are ordi- 

 narily unquestionably due to change of intensity on the 

 sensitive surface. 



(8) Light may possibly have some effect on orientation 

 by acting constantly as a directive stimulus, but there is 

 no evidence indicating that it has. There is no evidence 

 showing that these animals are tropic in accord with Loeb's 

 definition. 



4. Planaria 



It is well known that nearly all the planarians respond 

 to stimulation by light, but it is frequently assumed that 

 they orient only very indefinitely and that the effect of 

 light consists largely in making them more or less active. 

 Loeb (1906, p. 136) says, " If fresh-water Planarians are 

 put into such a circular glass dish, they show very little or 

 no tendency to move in the direction of the rays of light, 

 creeping along in an irregular manner and gathering not at 

 the negative or positive side of the jar, but on both sides 

 . . . where, on account of the refraction of light, the 

 intensity is a relative minimum." 



More recent investigations have however shown that 

 many of these forms orient fairly accurately, and it is pri- 

 marily these investigations that concern us at present. 



In the planarians the power of differential response to 

 localized stimulation seems to be more highly developed 

 than in the earthworms, and orientation in light seems to be 

 brought about largely by such responses. There are how- 



