The Reactions of Planarians to Light 87 



Bdelloura continues activity in the light for a much shorter time 

 than Phagocata. 



When subjected to successive trials the period of a planarian's 

 activity decreases. 



Change in the intensity of light is less important than the effects 

 of fatigue in determining the time required for a worm to leave 

 a unit circle. When fatigued, worms often become indifferent 

 to light, coming to rest less frequently in an oriented position with 

 reference to the light than in an unoriented one. 



Fresh-water planarians come to rest as far away from the source 

 of light as possible and, if the directive stimulus does not prevent, 

 in the region of least illumination. 



Bdelloura Candida, on the contrary, comes to rest in regions of 

 greater rather than of less illumination; usually worms of this 

 species arrange themselves in compact rosettes with the anterior 

 ends pointed outward. 



B In Changing Directive Light 



The light acting upon planarians in their natural habitat must 

 necessarily be a variable factor of great complexity, since its inten- 

 sity changes constantly throughout the day, while the position of 

 the sun relative to various surfaces which reflect light is also 

 continually shifting. 



The fact that planarians, to a great extent, keep out of the light, 

 does not diminish the force of this statement, for whatever the 

 part played by light in their behavior, it must always be an exceed- 

 ingly varied and complex one. 



Changes in the Intensity. When a worm is gliding away from 

 a source of light it shows a more marked response to change of 

 intensity when the change is made suddenly than when it is made 

 gradually. In fact, it is possible by exercising patience and care to 

 change the intensity of directive light to a considerable degree so 

 gradually as to produce no corresponding response on the part of 

 the worm, whereas a comparatively slight change, if abruptly 

 effected, immediately results in the animal's performing some one 

 or more of the acts in its repertory of behavior, such as halting, 

 wigwagging, etc. 



