8 4 



Herbert Eugene Walter 



The five worms coming to rest on the lighter side of D were 

 carefully examined and found to be mutilated or fragmented 

 individuals, while the same was not true of the others. 



The reason why the worms in B failed to arrive in the darkened 

 area is probably that, being started near the middle boundary 

 line, their first movements were normal, i. <?., away from the light, 

 and carried them into the area of greatest intensity, whence they 

 were unable to escape. In this case the effect of the directive 

 light seems to have more than counterbalanced the locomotive 



Light vertical 



■■■■■■ 1 



6 :.'.■■ 

 '3\ ■'.'■ 



." - 2 . 

 I ■ '• ; ' 



1 .' ■ 



2-i ■: 

 .7. -.".'■ 



, 2 

 I 



I * 



■in ■mm ' 



B 



D 



Fig. 8 Planaria gonocephala. The arrows represent the direction of the light. The dotted areas 

 were surrounded by black backgrounds, except for a space on the side toward the light, and the clear 

 areas similarly by white backgrounds. The figures represent the number of planarians that came to 

 rest in any particular locality. 



energy exerted by the worms. Had the species experimented 

 upon been Phagocata gracilis, instead of Planaria gonocephala, 

 the result might have been different, for in the former species, as 

 already shown (p. 78), the phototactic response is secondary to 

 the tendency to a general wandering. 



It was frequently observed that worms when fatigued after a 

 period of activity apparently lost their phototaxis, with the result 

 that the final movements of a tired worm would sometimes be 

 made toward the light. Such behavior is probably not to be con- 

 sidered as a reversal of phototaxis, but rather as indifference to 



