36 • ELIZABETH LOCKWOOD THOMPSON 



period of nearly thirty-three hours (about five and a half hours 

 to each animal) the response occurred but eight times. Three 

 of these responses were due to observed accidental stimulation 

 and the rest were doubtless due to the same cause. On the other 

 hand, as the tables show, stimulation of the mouth region induces 

 the response in a sufficient percentage of trials to make it avail- 

 able for experimental purposes. The response is therefore a 

 reflex available for these purposes. 



The response of Physa to stimulation of the mouth region 

 unaccompanied by other stimulation will be referred to as the 

 normal food response. It consists of one or more opening and 

 closing movements of the mouth. Each complete opening and 

 closing movement is spoken of as a reaction. The response is 

 measurable in terms of the number of its reactions as well as in 

 duration. An inspection of the tables shows ver>'^ great fluctua- 

 tion in the numerical value of the response. 



B. Probable causes of fluctuation in response are not difficult 

 to find. They arise in part from the accidents of manipulation 

 and are in part due to the physiological state of the snails. It 

 is not an easy matter to apply a stimulus to the mouth of a 

 moving snail, delicately suspended from the surface film. The 

 difficulty is increased when two stimuli must be accurately placed 

 at the same time. If the pressure is applied with the pressure 

 fork strictly vertical, it is only that of the weight of the movable 

 part of the apparatus and should be constant. wSlight deviation 

 from the vertical is inevitable and must result in occasional 

 variation in pressure and in the manner in which the food reaches 

 the mouth. These accidents should shorten response. That 

 unskilled manipulation sometimes inhibits response entirely 

 is clear from those cases marked x in the tables. It is fair to 

 assume that they partially inhibit other responses. They depend 

 in part on the phy.siological state of the operator. Added to 

 these personal factors the tendencies to produce irregularity in 

 response are vibrations of an unstable building due to slamming 

 of doors, passing of streets cars and movements of people. Other 

 accidents prolong response. Probably at times portions of food 

 are retained in the mouth of the snail and the response may be 

 prolonged. At times accidental stimulation from unobserved 

 miucus secreted by the snail, is added to that of the food on the 

 hook and response is prolonged. 



