108 JEAN DAWSON 



through an arc of one hundred and eighty degrees if another snail 

 comes in contact with it. This reaction to a contact stimulus is 

 not common in other mollusks and is perhaps due to the mantle 

 of Physa being reflected over its shell. On account of this reac- 

 tion, Physa are not usually found in contact with one another 

 even though thousands are living in an optimum habitat. Occa- 

 sionally, if the water lacks oxygen, they may be seen in masses 

 about the surface film, or in the angles formed by the sides of 

 a rectangular box or bottle. This latter reaction is marked in 

 Physa and is similar to that which Pearl (1903) found in plan- 

 arians and called goniotaxis. 



II. Variations in the reactions of Physa due largely to differences in 



experience. 



No exact statement can be made as to the delicacy with 

 which Physa reacts to a general disturbance of its environment 

 or to a localized mechanical stimulus, since there is a wide diflier- 

 ence in response among members of the same species and in 

 the same individual from time to time. Changes in the physio- 

 logical state may be causes of the differences in the snail's behavior, 

 but the cause underlying the diversity in the physiological 

 state must be sought and in a measure controlled before the 

 behavior of the snail can be analyzed with any degree of accuracy. 



There is sometimes a considerable difference in the reactions 

 among the Physa of the different aquaria. A snail in one 

 aquarium may be so sensitive that it reacts to the breath upon 

 the film while another snail in a different aquarium may be 

 touched with a pencil before it gives a like response. Here the 

 difference in responsiveness is due to the amount of handling to 

 which the snail or the containing aquarium has been subjected, 

 since all environmental conditions were essentially identical. The 

 snail that reacted so delicately to a disturbance of the film had 

 been kept alone and undisturbed for weeks, while the one that 

 failed to respond so delicately had been handled and had been 

 accustomed to the experience for some time. The experiment 

 was reversed. The tamed snail was kept by itself, wholly undis- 

 turbed for weeks, and the sensitive snail was handled, with 

 complete exchange of their physiological states as a result. 

 This phenomenon was observed early in the work. In subse- 

 quent experiments on their reactions to various agents the 

 snails were reduced to a like physiological state (i.e., they were 



