T lie Reactions of Plaiiarians to Light 



7S 



in order to obtain approximately the amount of unquestionable 

 negativeness. By following this method in the case just given, 

 the per cent of negativeness would be 92 instead of 96, but since 

 this method assumes that normally negative worms are never posi- 

 tive, which is contrary to the evidence to be given later, the most 

 accurate estimate of negativeness would probably fall somewhere 

 between these two percentages. 



Bdelloura, on the other hand, behaves in the same way only 

 three times out of ten, therefore showing itself to be positively 

 phototactic. 



This difference in orientation becomes more marked if the total 

 number of degrees, that is, the amount of positiveness and nega- 

 tiveness of emergence from the circle is used as the basis of reckon- 

 ing, instead of only the number of times of emergence. Such a 

 quantitative computation is shown in Table XII. 



TABLE XII 



Amount and kind of orientation to directive light exhibited by various species of planarians in mb trials 



Number of 

 trials 



Total de- 

 grees positive 



Total de- 

 grees negative 



Percentage of 

 degrees neg. 



Percentage of 

 degrees pos. 



Negative worms (Dendrocoe- 



lum, Planaria, Phagocata). 



Positive worms (Bdelloura) . . . 



386 



566 



397 



10157 

 50 



94-7 

 II .2 



5-3 



Although the actual number of trials for Bdelloura in this table 

 is small, they are characteristic of what was observed in a large 

 number of unrecorded instances. 



The amount a planarian may deviate from the direction in 

 which it is pointed, depends upon the direction of the light imping- 

 ing upon it. A negative species deviates from a straight course 

 least when headed away from the source of the light and most 

 when headed toward it, while an intermediate degree of deviation 

 occurs when the direction of the light is at right angles to the long 

 axis of the worm. In the case of Bdelloura the converse is true, 

 as shown in Table XIII. 



Rate of Locomotion. In obtaining the rate of locomotion of 

 worms subjected to directive light, the double aquarium apparatus 



