Sensory Discrimination: The Chemical Sense 79 



The two forms of negative reaction correspond to differ- 

 ences in the location of the stimulus. If the head end of 

 the body is stimulated strongly on one side, the head is 

 turned away from that side. If the posterior part of the 

 body is strongly stimulated, the animal 

 makes powerful forward crawling move- 

 ments. The significance 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 macu- 

 lata to give a positive reaction by turning 

 its head in the direction of the stimulus, 

 which need not be in actual contact with 

 the body (561). 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 "deli- 

 cate organs capable of stimulation by slight 

 currents in the water set up by the minute organisms 

 that prey" upon the animal's food; so that the pos- 

 itive reaction when given to food may be really a re- 

 sponse to mechanical stimulation (20). As Pearl, however, 

 found that chemicals, diffused in the water, would produce 

 positive responses (561), it is probable that Planaria 



FIG. 9. Pla- 

 narian, dorsal 

 view. After 

 Woodworth. 



