8 



ELIZABETH LOCKWOOD THOMPSON 



and immediately withdrawn. This stimulus served to start the 

 movements of the mouth parts, and an exact record of the num- 

 ber of reactions thus induced was kept for each individual. 

 Each complete opening and closing movement is spoken of as a 

 reaction. The number of reactions (whether one or several) 

 induced by a single stimulation of the mouth parts is termed a 

 response. During work a record was also made of the length 

 of time that the reactions persisted; that is, the duration of the 



response (Table II). 



TABLE II 



Showing the number of reactions of the mouth of two individuals of Physa gyrina 

 Say, resulting from a single application of food to the mouth. The tests were made 

 in filtered water on alternating days. The snails had been starved for 24 hours 

 preceding the tests. 



Snail No. 4 



* Cases in which lack of response was due to known mechanical disturbance. 



