Sensory Discrimination : the Chemical Sense 77 



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 positive reaction when given to food may be really a 

 response to mechanical stimulation (10). As Pearl, however, 

 found that chemicals, diffused in the water, would produce 

 positive responses (316), it is probable that Planaria maculata 

 is directly sensitive to chemical stimulation, though it re- 

 sponds thereto in the same way as to mechanical stimulation. 

 A land planarian, Geodesimus bilineatus, is reported by 

 Lehnert to perceive food at distances from four to five times the 

 length of its body, and he does not describe the positive reaction 

 as given in response to any other than food stimulation (231). 



The food-taking reaction in Planaria maculata is made 

 under the influence of combined mechanical and chemical 

 stimuli, in contact with the pharynx or the ventral side of the 

 animal. When an object which has occasioned the positive 

 reaction is reached, the head folds over it and grips it, contract- 

 ing so as to squeeze it. The substance being thus brought into 

 contact with the pharynx, swallowing movements are pro- 

 duced if the proper stimulus is given. Bardeen was inclined 

 to think that contact with a soft substance constituted the 

 proper stimulus, as he found that hard particles placed on the 

 pharynx were not swallowed (9). Pearl, however, believes 

 that mechanical and chemical stimulation must combine. 

 The former alone does not suffice, for swallowing movements 

 are not evoked when one planarian crawls over another; the 

 latter alone is insufficient, for placing the animal in a sugar 

 solution has no effect. If chemical and mechanical stimula- 

 tion are united, the reaction is given whether the chemical is 

 edible or not ; Pearl found it occurring in response to sodium 

 carbonate (316). 



Evidence of the influence of physiological condition upon 

 the reactions of planarians is furnished by the fact that the 



