l62 LIGHT AND THE BEHAVIOR OF ORGANISMS 



dicular to the bottom of the aquarium, without any reaction 

 excepting perhaps a very sHght increase in the lateral move- 

 ment of the anterior end. 



The fact that these organisms turn directly toward the 

 light when the side is illuminated apparently shows that 

 they have the power of differential response to localized 

 stimulation, that they can orient without preliminary trial 

 movements. The fact that the anterior end is constantly 

 being turned from side to side, and that orientation may 

 take place if the ray direction is only slightly changed by 

 merely swinging this end a little farther toward the source 

 of light each time that it turns in that direction in the 

 regular process of lateral movement, shows that under cer- 

 tain conditions orientation takes place by the trial method. 

 The fact that the planulae can move toward any point 

 between two sources of light shows that neither the direc- 

 tion of rays through the body nor the angle the rays make 

 with the surface is of importance in orientation. If this 

 be true, the orienting stimulus must be due to difference of 

 intensity or a change of intensity on opposite sides of the 

 body, especially on the anterior end. The lateral swinging 

 movements serve to magnify the difference or change of 

 intensity on this end, and thus make it possible for the 

 animal to orient more accurately than it otherwise could. 



The turning of the anterior end appears to serve in direct- 

 ing the course much as a cane serves a blind man in keeping 

 him on the path. The man may go directly toward his 

 goal without deviation and still it is evident that he orients 

 and keeps on his course by the trial method. Every move- 

 ment of the cane is a trial movement. Likewise every 

 lateral movement of the anterior end in the planulae and 

 many other organisms, as well as separate movements of 

 the antennae, eyes, and other special organs in various 

 forms may be trial movements. It is therefore clear that 

 the mere fact that an organism moves directly toward a 

 source of stimulation is not sufficient evidence to show that 

 its orientation and direction of movement are not regulated 



