116 JEAN DAWSON 



Opened them against it as though trying to get air. After several 

 repetitions of the experiment they passed over the vaselined 

 surface without tapping, but it took longer for them to be- 

 come accustomed to this than to any other change in their 

 environment that had been tested. It is possible that vase- 

 line in contact with water retains a coating of air, and this, 

 together with the yielding of the vaseline, may have served to 

 cause a stimulus similar to that of the film. 



VI. Young Physa differs from the adult in the character of its responses. 



Yoimg Physa are much less sensitive to general disturbances 

 of their environment and localized mechanical stimuli than 

 are the adults. Large snails readily expel the air from their 

 lungs and quickly fall to the bottom of the pond, while the 

 little ones may be held between the thumb and finger without 

 doing so. This difference in sensitivity is also illustrated by 

 the following. A large number of Physa were collected with 

 a dip-net, brought to the laboratory and placed in a pail of water. 

 Many seeds of Alisma plantago were collected with them and 

 were floating together with a large number of Physa upon 

 the surface. The water in the pail was stirred briskly so that 

 the snails would fall to the bottom; but upon removing the 

 seeds from the surface the young snails were found among 

 them. The young snails had thus not reacted by falling as 

 had the adults. 



In the experiment in which Physa was seen to adjust itself 

 to a new experience, the adult snail wandered about aimlessly 

 for a longer time before it reached the film than the younger 

 one did. Perhaps owing to its greater sensitiveness, it received 

 a greater shock from the unaccustomed experience and in conse- 

 quence was longer in recovering from it than the younger one. 

 Repeated observation and experiment show that an increased 

 sensitiveness to general disturbances of the environment and 

 localized mechanical stimuli does not seem to come on gradually 

 as the snail grows, but there seems to be an abrupt change 

 when it is about five mm. in length. 



VII. Differences in reactions of Physa young and adult. 



Young Physa of various sizes were taken from the pond and 

 made to expel the air from their lungs. They were then placed 



