24 ELIZABETH LOCKWOOD THOMPSON 



The food-pressure series thus shows the following character- 

 istics: (1) An absence of reaction at the beginning, (2) a simi- 

 larity in the reaction and response curves, (3) in general, a gradual 

 rise of the response curve, (4) a less average number of responses 

 per 50 trials and a less average number of reactions per response 

 as compared with the normal food series, with the exception 

 of Snail No. 4, (5) an increase in the number of reactions per 

 minute, i. e. an acceleration of the individual response, (6) a 

 fairly constant reaction duration. This is characteristic of all 

 the series. 



D. Experiments ivith pressure alone on trained snails: asso- 

 ciation. — The third part of the problem called for the applica- 

 tion of the secondary stimulus alone. Any mouth reactions 

 which might follow (beyond those attributed to chance stimu- 

 lation of the mouth), would according to the Pawlow theory be 

 evidence that an association had been formed between the essen- 

 tial, primary or " unconditioned " stimulus and the secondary — 

 or in this case, between food and pressure. The series of tests 

 was carried on in exactly the same manner as the experiments 

 just described. The same apparatus was used but no food was 

 placed upon the food hook. vSince the food hook was shorter 

 than the pressure fork (fig. 1, Plate I) it did not touch the mouth, 

 which therefore received neither food nor mechanical stimulation. 

 The pressure fork was applied to the usual place on the ventral 

 surface of the snail. The following table (IV) shows the results 

 obtained. 



