MOVEMENTS, KTC, UV FRESH-WATI'iR PLANAiaANS. 641 



of how a lower orguiiiisiu gvia its food, taking' advantage of 

 the good and rejecting the bad, and thus apparently choosing 

 one thing from several, is one of the most interesting and 

 important in comparative psychology. 



The food reaction of planarians consists of an extremely 

 well co-ordinated set of reflexes, which may be set into action 

 by stimuli of two sorts, — first, chemical ; and second, 

 mechanical. Both sorts of stimuli are, of course, given by 

 the food. The first and most important of till the reflexes in 

 the food reaction is the turning of the head towards the 

 source of stimulation, followed by movement in that direction. 

 This is the reaction which enables the animal to find food. 

 Evidently it is the same thing exactly as what has been 

 described as the positive reaction to mechanical stimuli; or, 

 in other words, the positive reaction to mechanical stimuli is 

 only a special case of the general food reaction. Its primary 

 function is evidently the getting of food, whatever the stimulus 

 which calls it forth. The reason for a food response following 

 mechanical stimulation is to be found in the fact that it most 

 frequently happens that many things (e. g., whole animals) 

 which are available for food are not emitting chemical sub- 

 stances into the water in sufficient quantity to cause an 

 effective stimulus. If the planarian did not give a positive 

 reaction after contact with such bodies they would be missed, 

 and no advantage taken of them as food. By reacting 

 positively to Aveak mechanical stimuli the animal is in a 

 position to take advantage of the presence of food of all sorts, 

 whether it is in condition such as to diffuse chemical sub- 

 stances through the water or not. This fact that the animals 

 react to food substances as a result of mechanical stimulation 

 affords a possible explanation of the "gripping " phase of the 

 general response. The purpose of this " gripping " may be 

 to bring the sense organs of the head, which are sensitive to 

 chemical stimuli, into very close contact with the substance 

 in order to determine whether it possesses the chemical 

 characteristics of food. In other words, this reaction is a 

 "tasting" reaction, which is made necessary by the fact that 



