VERMES, FLY LARVAE, AND ECHINODERMS 205 



also it Is Impossible to say whether the activity Is caused 

 by a change of Intensity or by constant Intensity. 



The Idea of Verworn, Holt and Lee, Loeb, and Torrey 

 that when an organism Is oriented both sides are equally 

 stimulated and consequently move at equal rates, and that 

 when it is not oriented the two sides are unequally stimu- 

 lated and therefore move at unequal rates, thus causing 

 orientation, has no experimental support in the reactions 

 of the earthworms. 



Summary 



(i) All the earthworms that react to light are ordinarily 

 negative. There is, however, some evidence that some at 

 least are positive in very low light Intensity. 



(2) They orient fairly accurately under some conditions 

 and move away from the source of Illumination. 



(3) The entire surface of the earthworms is probably 

 sensitive to light, but the anterior end Is much more sensi- 

 tive to light than any other part of the body. Our evidence 

 Indicates that in AUolobophora orientation is entirely con- 

 trolled by the sensory elements In the first five or six seg- 

 ments. The anterior end is most sensitive when extended, 

 as shown by Harper. 



(4) These animals have the power of differential response 

 to localized stimulation. Under certain conditions, if one 

 side Is Illuminated, they always turn toward the shaded 

 side without preliminary movements and therefore orient 

 directly. 



(5) They frequently swing the anterior end frorn side 

 to side continuously during the process of locomotion. If 

 light is thrown on one side under such conditions they turn 

 the oral end sharply in the direction opposite that In which 

 It chances to be when it receives the stimulation. They 

 may therefore turn toward the source of light first and 

 become oriented only after several more preliminary move- 

 ments. Or they may be In such a state that they are only 

 stimulated when the anterior end Is extended toward the 



