Spatially Determined Reactions 171 



earthworm Allolobophora fatida (81). Yet the movements 

 of earthworms are oriented by light ; as we have seen, they 

 tend to move away from a source of light. This orientation 

 Holmes believes to take place by the checking of random 

 movements of the head in the direction of the light. In the 

 crawling movements stimulated when light is thrown upon 

 the worm, the head is turned from side to side. If it happens 

 to be turned toward the light, it is withdrawn. Holmes ex- 

 plains the observation of Parker and Arkin that the head of 

 the worm is much more apt to turn from the light than toward 

 it (312), by saying that account was probably taken here only 

 of the first decided turn made. He himself experimented by 

 lowering a worm, crawling on a wet board, while its body was 

 in a straight line and contracted, into a beam of light at right 

 angles to the body, and noting the first movement of the 

 head. This was found to be twenty-seven times away from 

 the light and twenty-three times toward the light. A similar 

 method of orientation by "trial and error'' was observed in 

 the leech and in fly larvae by Holmes (185). 



E. H. Harper, on the other hand, working on the earth- 

 worm PericluBta bermudensis, declares that if the light is 

 strong enough there are no random movements of the head at 

 all, but that the first movement is a direct reflex away from 

 the light. When the light is only moderate, the appearance 

 of random movements is due to the fact that the worm is less 

 sensitive in a contracted than in an expanded state. Loco- 

 motion consists in a series of contractions and expansions, 

 and "as each extension begins in a state of lower sensibility, 

 the anterior end may be projected toward the light, only to 

 be checked when its increase of sensibility with extension 

 makes the stimulus appreciated" (161). A similar sugges- 

 tion that orientation may occur either by a definite reflex or 

 as the outcome of random movements, according to the ani- 



