214 WILLIAM BRODBECK HERMS 



author (Cohn, '66, p. 164) advocated the view that the direction 

 of the rays rather than intensity was the more important factor. 

 '' Weitere Versuche haben jedoch erwiesen, das nicht die Intensitat 

 sondern die Richtung der Lichtstrahlen es ist, welche die Bewe- 

 gungen der mikroskopischen Organismen beherrscht." 



From that time on investigators have favored one or the other 

 of the two views. Davenport ('97, p. 211) designates the effects 

 produced by the direction of the ray as phototactic and that pro- 

 duced by the '' difference in illumination of parts of the organism" 

 as photopathic. 



It is quite evident that both phototaxis and photopathy play 

 an important role in the movements of the flesh-fly larvae. The 

 path of these individuals is largely pre- determined by the direction 

 of the rays, its response being phototactic in that respect, while, 

 on the other hand, the creeping animal keeps its head as far as 

 possible in the shadow of its own body, — a photophathic response. 

 This statement, based on the evidence already discussed, is in 

 direct opposition to the conclusions of Holt and Lee ('01, p. 462) 

 who say: ''Experimental study and a reviev^ of the literature on 

 the subject have convinced us that the phenomena thus far 

 reported do not demonstrate either that direction of ray and inten- 

 sity of light operates separately, or that any distinction should 

 be made between phototaxis and photopathy as independent 

 forms of irritability," and further (p. 479) ''The direction of the 

 rays has, in itself, no effect whatsoever, on the movements of the 

 organism." The conclusions reached by Walter ('07) on plana- 

 rians is more or less in agreement with the conclusions of these 

 authors. Walter states that, in general, intensit}^ rather than direc- 

 tion is "the operative factor in light reactions," but he modifies 

 this by the further statement that "At the same time there is 

 much evidence that the intensity utilized by the organisms, is 

 intimately associated with, and powerfully modified by the direc- 

 tion of the light." These conclusions of Walter are also in accord 

 with those of Loeb ('93, p. 101) on another species of planarian 

 (Planaria torva), which is not phototactic, but reacts in a very 

 striking manner to changes in light intensity ("unterschiedsemp- 

 findlich"). On the larvae of Calliphora vomitoria, however, 



