78 Herbert Eugene Walter 



Change in Character of Course. When several specimens of 

 Phagocata were placed in a square aquarium which received light 

 solely from one side, their first movements were plainly negative, 

 that is, away from the light. After a brief interval, however, it 

 was seen that apparently as many worms were going toward the 

 light as in the opposite direction. In fact an actual count showed 

 that in a certain interval of time 43 worms passed a central point 

 going toward the light while 44 passed the same point in the oppo- 

 site direction. This apparent change in the character of the 

 course was probably due, not to any change in the degree of nega- 

 tiveness of the animal, but rather to the fact that the impulse to 

 keep moving in some direction is stronger than the impulse to neg- 

 ative phototaxis. Consequently when the limit of the aquarium 

 in a negative direction is reached a worm, since it normally travels 

 in straight lines or sweeping curves and does not turn around 

 and around in one spot, continues its locomotion in the direction 

 of least resistance, namely, back toward the light. It will be 

 remembered that Loeb ('93b) has called attention to this fact by 

 saying that planarians are not negatively "heliotropic" in a strict 

 sense because they do not remain as far away from the source of 

 light as they can get. 



Amono; various observations made with other ends in view, 

 there were numerous incidental cases of a normally negative 

 worm making an unexpected positive response even from the first 

 moment of being subjected to the light stimulus. This occasion?. 1 

 positiveness is clearly apparent from the general fact already 

 noted that four times out of a hundred the average negative pla- 

 narian turns toward the light. 



Two definite instances of a reversal in the character of response 

 may be cited. 



The first was the case of a Phagocata in the double aquarium, 

 which became increasingly positive through twelve successive trials. 

 Its average emergence from the circle for the first four trials was 

 45°, which is a normal negative result, since 90° represents com- 

 plete indifference. In the next four trials, however, the average 

 was 100°, that is, slightly positive, and in the last four, 124°, 

 which is decidedly positive, as shown diagrammatically in Fig. 6. 



