igS LIGHT AND THE BEHAVIOR OF ORGANISMS 



such response at all, it is but little developed and is of very 

 little importance in the general reactions to light. 



(lo) Neither the direction of the rays through the organ- 

 ism, nor the angle with the surface, nor the symmetry of the 

 sensitive surface, nor absolute difference of intensity on the 

 body, is of importance in orientation excepting in so far as 

 they may influence change of intensity on the anterior end. 



(ii) There is no evidence indicating that the orienting 

 reactions in fly larvae are tropic in accord with Loeb's 

 definition of this term. 



3. Earthworms 



The light reactions of various earthworms have been 

 studied by a number of investigators, several of whom 

 directed special attention to the process of orientation. 

 Parker and Arkin (1901), Miss Smith (1902), and Adams 

 (1903) made observations on the direction of movement of 

 the anterior end when illuminated from one side, and found 

 that it turned from the light more often than toward it, 

 indicating, since these organisms are ordinarily negative, 

 that orientation is direct. Holmes, however (1905), is of 

 the opinion that the animals actually start to turn toward 

 the light just as often as from it, but that the movements 

 toward the light are inhibited owing to the greater exposure 

 as the end expands. This causes marked movements only 

 in the direction from the source of light. He believes that 

 Parker and Arkin and others may have failed to take into 

 consideration the slight preliminary movement which occurs 

 before the actual extension takes place, and that this may 

 account for the preponderance of negative turning recorded 

 by these investigators. Holmes, taking account of all the 

 minute preliminary movements, says (p. lOi): "In the two 

 specimens employed the first detectable 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 detectable 



