The Problem of Orientation 73 



movement of the head to one side noted. In the 

 two specimens employed the first detectible turn was 

 away from the light 27 times and towards the light 

 23 times. After a few extensions the worm in 

 nearly all cases soon turned and crawled away from 

 the light. The first detectible movement of the 

 earthworm seems, therefore, to be nearly as likely 

 to be towards the light as away from it. The slight 

 preponderance of negative turns may be due to the 

 fact that some of the smaller trial movements were 

 overlooked, to a slight direct orienting effect of the 

 rays, or merely to chance." 



In the leech Glossophonia, which crawls by a sort 

 of looping motion, the anterior part of the body is 

 frequently raised, extended, and moved about as if 

 the animal were feeling its way. "If the animal 

 turns it in the direction of a strong light it is quickly 

 withdrawn and extended again, usually in another 

 direction. If the light is less strong it waves its 

 head back and forth several times and sets it down 

 away from the light; then the caudal end is brought 

 forward, the anterior end extended and swayed 

 about and set down still farther from the light than 

 before. When the leech becomes negatively ori- 

 ented it may crawl away from the light, like the 

 earthworm, in a nearly straight line. The extension, 

 withdrawal and swaying about of the anterior end 

 of the body enable the animal to locate the direc- 

 tion of least stimulation, and when* that is found 

 it begins its regular movements of locomotion. Of 



