BIOLOGY OF PHYSA 77 



positive and some negative and that the quickness with which 

 the food is perceived depends upon the diffusion of its juices. 

 The snail usually turns away or gives a negative response to a 

 mechanical stimulus of medium strength, and turns toward or 

 responds positively to a gentler mechanical stimulus. The 

 strength of the mechanical stimulus received by the snail in its 

 undisturbed habitat depends upon the momentum mth which 

 it is moving when it comes in contact with a solid. A snail 

 moving slowly (about an inch in ninety seconds) strikes a solid 

 and receives a gentle mechanical stimulus which causes it to 

 respond positively; while the same snail moving with greater 

 rapidity (about an inch in twenty seconds) strikes the same solid 

 and receives a stronger stimulus with the result that a negative 

 response is evoked. 



In one case a Physa was moving slowly toward a piece of meat 

 ^^hich had been placed but recently in its path. The meat was 

 fresh and its juices were diffusing very slowly and in consequence 

 the snail actually touched it with its tentacles before it turned 

 toward the food. In this case it is impossible to say which 

 stimulus, chemical or mechanical, produced the reaction, as 

 both would produce the same result. 



A Physa was observed as it moved slowly toward a piece of 

 onion which had been freshly cut and washed to prevent too 

 rapid diffusion of its juices. The snail touched the onion with 

 its tentacles before it reacted to its presence. It first turned 

 toward the onion, paused, and then turned away. It w^ould 

 seem that in this case the snail responded positively to the 

 gentle mechanical stimulus and then negatively to the chemical 

 stimulus of the onion. The two responses are in different direc- 

 tions, with the result that the snail first approaches and then 

 retreats from the source of stimulation. 



If Physa was moving at a moderately rapid rate when it 

 came in contact with the meat, it received a sufficiently strong 

 stimulus to cause it to turn away, to pause and then turn 

 back. It would seem that the mechanical stimulus was not 

 only sensed first but obeyed, and then the chemical stimulus 

 was in turn sensed and obeyed. If the onion was used in 

 place of the meat in this experiment, the snail turned away, 

 paused and then continued to move away. The reaction 

 to the two kinds of stimuli were both in one direction; 



