76 



The Animal Mind 



Planaria maculata, a common freshwater planarian, re- 

 v, sponds to stimulation by two forms of negative reaction, a posi- 

 ' tive reaction, and a feeding reaction. The negative and posi- 

 tive responses are given either to mechanical or to chemical 

 stimuli, the former being produced by strong, 

 the latter by weak stimulation. Hence they 

 .do not suggest correlation with qualitatively 

 7 different sensation contents, but rather with 

 unpleasantness and pleasantness. The two 

 forms of negative reaction correspond to dif- 

 ferences in the location of the stimulus. If the 

 head end of the body is stimulated strongly on 

 one side, the head is turned aw r ay from that 

 side. If the posterior part of the body is 

 strongly stimulated, the animal makes power- 

 ful forward crawling movements. The signifi- 

 cance of local differences in stimulation for 

 response and for possible consciousness, again, 

 will more properly be discussed in a later 

 chapter. As has just been said, both weak 

 chemical and weak mechanical stimulation 

 cause Planaria maculata to give a positive 

 reaction by turning its head in the direction of 

 the stimulus, which need not be in actual con- 

 tact with the body (316). A planarian will 

 follow an object such as the point of a pin 

 moved in front of it, and one planarian will 

 follow the trail of another that happens to come within the 

 proper distance. Similarly, the neighborhood of food will 

 cause the animal to turn toward it. Bardeen has suggested 

 that the so-called "auricular appendages," two small movable 

 prominences on the animal's back near the head end, which 

 are specially sensitive to touch, may be " delicate organs 



FIG. 9. Plana- 

 rian, dorsal 

 view. After 

 Woodworth. 



