BIOLOGY OF PHYSA 



109 



accustomed to being handled) before the experiments were 

 begun. Similar phenomena were observed in land pulmonates. 

 The snails that were kept in an unfrequented room, would 

 retreat into their shells if approached, while those that were 

 kept in the busy laboratory where they were handled more or less, 

 could be picked up without retreating into their shells. Indeed, 

 they often became so accustomed to being handled that they 

 would withdraw into their shells only after very rough treatment. 

 It was observed that, even in an environment (field or labora- 

 tory) free from human disturbance, great diversity in the char- 

 acter of the response to the same stimulus was manifest. This 

 difference was due, at least in part, to a difference in the amount 

 and kind of stimuli received by the snails in the different habitats. 

 That the environment of these snails was not uniform may be 

 seen by referring to Table IV, which gives the chief sources of 

 mechanical stimuli in some of the most favorable habitats. Some 

 of the reactions uniformly shown by Physa in its undisturbed 

 habitats are as follows: 



TABLE IV. 

 Sources of stimuli received by Physa in field habitats. 



