AN ANALYSIS OF THE LEARNING PROCESS IN THE SNAIL 5 



3. The Response to the Food Stimulus 

 In order to utilize the snails it was necessary to know: 



(1) Whether the mouth reactions occur only as the result of 

 external stimulation, and 



(2) Their constancy as to the nimiber of reactions and the 

 duration of the response following a single stimulus. Should 

 they occur regularly as the result of external stimulation and 

 only then, they must be regarded as " involuntary " responses 

 or reflexes available for the purpose of the experiments. Upon 

 the constancy and duration of the response depends its value 

 for quantitative uses. 



A. 1 he response to the food stiniulus a reflex. — It was found 

 by observation not only that the mouth movements of Physa 

 were induced by food and mechanical stimulation of the mouth 

 region, but that they also sometimes occurred when no such 

 stimuli could be detected. Such apparently voluntary move- 

 ments might be due to the stimuli from microscopic particles 

 in the water. A check series of experiments in filtered *water 

 was deemed necessary in order to determine whether or not the 

 mouth reactions might normally occur without external stimu- 

 lation and if so under what conditions and with what frequency. 

 In these tests each snail, under the controlled conditions just 

 mentioned, was in turn held in the hand of the operator until 

 it suspended itself from the surface film. It was then permitted 

 to move about, and was touched only when necessary to keep 

 it from crawling down the sides of the dish. At such times it 

 was pushed gently toward the center of the tank with a sterile 

 glass rod. Each animal was thus kept crawling on the surface 

 film for an average period of forty-five minutes under constant 

 observation and a record was made of each mouth reaction. It 

 was found to be impossible to keep the snail absolutely free from 

 all chance stimulation of the mouth. It has been shown that 

 the animal uses the same method of locomotion in crawling on 

 the surface film that it does in moving over any substratum, 

 that is, it crawls along a mucous path which it secretes as it 

 moves and which remains behind it on the surface of the water. 

 (Parker, 1912), (Dawson, 1911). The mucus thus left on the 

 surface mav reach the mouth of the snail as it moves about. In 



*A Berkefeldt filter was used. 



