1 98 The Animal Mind 



A point which has been regarded as of much importance 

 in deciding between the theories of Loeb and Jennings on 

 orientation to Ught is the actual occurrence or non-occur- 

 rence of random movements. Thus Holmes (334) beheves 

 the negative orientation of earthworms to light occurs by 

 the checking of random movements of the head towards the 

 light. In the crawHng 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 withdrawn.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 explains the observation of Parker 

 and Arkin that the head of the worm is much more apt to 

 turn from the light than toward it (552), 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 tweiity-three times toward the Hght. A similar method 

 of orientation by "trial and error" was observed in the 

 leech and in fly larvae by Holmes (334). 



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

 worm Perichceta bermudensis, declares that if the light is 

 strong enough there are no random movements of the 

 head at all, but the first movement is a direct reflex away 

 from the hght. 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. Locomotion 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" (288). 



