580 EAYMOND PEARL. 



the ofclier, as has been found by Prandsen (: 01) to be the 

 case with Li max. This is what would be expected in the 

 case of the flat-worm, because it is a perfectly bilaterally sym- 

 metrical organism. Probably what actually determines which 

 way the organism shall turn after attempted median stimula- 

 tion is the fact that the stimulus really acts a little to one 

 side or the other, and the turning is really the negative 

 reaction. 



By repeatedl}^ stimulating the anterior end of a worm with 

 moderately strong mechanical stimuli its reactions may be 

 modified. In the beginning of such a series of stimulation 

 the worm turns away farther and farther from each suc- 

 ceeding stimulus, at the same time remaining at the same 

 place in the dish, i. e. not making any progressive move- 

 ments. This process tends to make the animal describe a 

 circle away from the stimulus, about its posterior end as a 

 fixed point. It never completely describes a circle, how- 

 ever, but after several stimuli have been given, to which it 

 has responded progressively more vigorously, it finally 

 jerks back with a strong longitudinal contraction, and turns 

 the anterior end through a considerable arc, so that it points 

 in an entirely different direction. This final strong reaction 

 in the majority of cases turns the anterior end towards the 

 side from which the stimulation is coming, or, in other words, 

 in an exactly opposite direction to that of the previous 

 reactions. This reaction appears as if, after the animal has 

 tried in vain to get away from an uncomfortable stimulus by 

 its ordinary reaction, it finally tries a wild jump in the oppo- 

 site direction. This curious change in the i-eactions induced 

 by a repetition of strong stimuli I have observed many times. 

 It indicates the effect of the organism as a whole on its 

 refiexes. As von Uexkiill (: 00, p. 73) has well brought 

 out, we must consider that in the case of a higher organism, 

 like a dog, the animal moves its legs, while with a lower 

 organism whose activities are reflex — for example, the sea- 

 urchin — it is really the " legs" (i. e. locomotor organs) which 

 move the animal. In the flat-woi-m the movements of the 



