66 



Herbert Eugene Walter 



intensity in the field below. The complete plan of the appa- 

 ratus is given in Fig. 3. 



The principal variations in the behavior of Dendrocoelum and 

 Phagocata upon reaching the critical line separating the areas of 

 two intensities are indicated diagrammatically in Fig. 4. 



The dotted line represents the boundary separating two areas 

 of different light intensities. The arrows represent the types of 

 paths made by Dendrocoelum and Phagocata. For the sake of 

 simplicity the worms are represented as going in one direction; 

 that is, into one of the two contrasting intensities, but the same 

 types of paths resulted as well when the opposite direction was 

 taken. The angles made in crossing the 

 critical line were also more varied than those 

 represented in the diagram. 



Type A represents a passage without re- 

 sponse; 5, an angular change of course made 

 at the critical line; C and F, a loop-like return 

 effected after a short excursion into the new 

 intensity, and G, a sharp turning aside, while 

 H indicates a halt at the critical line, as if a 

 ^'^'^'^"^ barrier had been encountered. Finally/) and 

 E represent a temporary pause on the part 

 of the worm accompanied by wigwag move- 

 ments of the anterior end of the body. In the 

 case of D the wigwagging is immediate, but 

 E typifies a case when there occurred in the 

 response an interval of such a nature that the 

 significant movements were not made until 

 the worm had advanced at least its own length 

 into the new area. 

 Of these types all, with the exception o^ A, are to be regarded 

 as reactions to differences in intensity encountered. The most 

 questionable are the infrequent types C and F, which may be 

 otherwise explained as arcs in a curving course which might have 

 occurred in a field of uniform intensity. By far the commonest 

 type was D, plainly the least doubtful of the series. 



Fig. 4 



