8o The Animal Mind 



maculata is directly sensitive to chemical stimulation, 

 though it responds 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 (417). 



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, contracting so as to squeeze it. The substance 

 being thus brought into contact with the pharynx, swal- 

 lowing movements are produced if the proper stimulus is 

 given. In Microstoma caudatum the organ of the chemical 

 sense has been held to be sensory epithelium in the floor 

 of the pharynx (398). 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 (20). Pearl, however, be- 

 lieves that mechanical and chemical stimulation must com- 

 bine. 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 stimulation are united, the reaction is 

 given whether the chemical is edible or not; Pearl found 

 it occurring in response to sodium carbonate (561). 



Evidence of the influence of physiological condition upon 

 the reactions of planarians is furnished by the fact that the 

 resting planarian shows a decidedly lowered susceptibility 

 to stimulation. Bardeen found that if the animal was not 



