72 Herbert Eugene Walter 



than when going into the greater. The average number of fail- 

 ures to respond to these contrasts of intensity reaches about two 

 out of three. 



Phagocata, at the critical line separating two contrasting inten- 

 sities, responds oftener when on the surface film than when glid- 

 ing over the bottom of the aquarium. 



The number of responses increases with the increase in the con- 

 trast between the two intensities employed, but the percentage of 

 response is greater, regardless of ratio, when one of the lights is of 

 low intensity (13.6 cm.) than when both are of higher intensity 

 (33+ cm.) 



2 PHOTOTAXIS 



The term "phototaxis" was introduced by Strasburger ('78) in 

 a study of certain swarm-spores, to indicate movements which 

 were parallel with incident light rays. The term has since been 

 extended by several authors to include similar movements on the 

 part of animals. Any organism is said to be positively phototac- 

 tic when it moves toward the source of light in the direction of the 

 rays and negatively phototactic when it goes in the opposite direc- 

 tion. 



The purpose of this section is to consider the phototactic move- 

 ments of planarians, as distinct from their photokinetic behavior, 

 (A) when the light remains constant, (B) when the light is changed 

 either (a) in intensity or (b) in direction, and (C) when phototaxis 

 is combined with responses of a different kind. 



A In Constant Directive Light 



Orientation. With the exception of Bdelloura all the planarians 

 studied are, under normal conditions, negatively phototactic so 

 far as their first movements in directive light are concerned. To 

 obtain quantitative data for this statement it was necessary to 

 construct an apparatus in which the worms to be tested could be 

 placed quickly and with as little mechanical disturbance as possible 

 in the center of a unit circle with the long axis at right angles to the 

 direction of incident light. The circle was marked off into degrees 

 so that by noting the place at which a worm made its exit a quan- 



