MOVEMENTS, ETC., OF FRESH-W ATKR PLANAKTANS. 579 



stimulus. With stimuli just effective iu calling forth the 

 reaction the turn is only slight, and since it affects only the 

 head end the direction of movement of the whole animal may 

 be scarcely changed at all. The amount of turning of the 

 anterior end is typically from 30° to 40°. 



There is in the negative reaction a pause at the instant of 

 stimulation, preceding the turning away. The first effect of 

 the stimulus is to cause the animal to stop its relatively rapid 

 movement. This pause may be so slight as to be almost 

 imperceptible in the case of comparatively weak stimuli, or, 

 on the other hand, may lengthen to a quite noticeable 

 interval when the stimulus is very strong. It is a character- 

 istic feature of both the positive and negative reactions of 

 planarians, and is evidently due merely to the fact that before 

 a reaction (i. e. something involving a change of motion) the 

 former movement must stop. 



The effect of localisation of mechanical stimuli in the head 

 region may next be considered. As has already been men- 

 tioned, stimulation of the sides of the head produces the 

 positive or negative reaction according to the intensity of the 

 stimulus. There are no special regions of specific sense- 

 organs connected with either of these reactions. The nega- 

 tive response is given after strong stimulation of any part of 

 one side or the other of the head and, so far as it is possible 

 to observe, just as decidedly after stimulation of one part as 

 of another. It is of interest to know what happens after 

 stimulation of the head in the median line. It is very 

 difficult to get a stimulus exactly in the median line, but one 

 may come very near it by stimulating the dorsal surface of 

 the head in the region between the eyes. The reaction pro- 

 duced is a longitudinal contraction of the antei'ior part of the 

 body, drawing the head back away from the stimulus, "^^riie 

 head is then turned to one side or the other as in the usual 

 negative reaction, and the animal starts ahead again in the 

 new direction. The side towards which the turn is made 

 after median stimulation is indeterminate — that is, there is no 

 tendency to turn in more cases towards one side than towards 



