BIOLOGY OF PHYSA 79 



and crawls over it. But where the snail moves very slowly 

 downward, it enters the mud for some distance before it gets a 

 strong enough stimulus to cause it to turn around. These 

 examples are sufficient to show that the rate of movement of 

 the animal so affects the usual stimuli received that the result- 

 ing behavior brings the animal into a new relationship with its 

 environment, which may even prove dangerous to it. This may 

 be true when the snail is crawling out of the water in the way 

 described above. The chances are that it may slide back into 

 the water, but it has been seen at times to remain stuck fast 

 by its mucus to the dry surface and so to perish. See Walter 

 (1906 p. 29). 



The nature of the substance giving the stimulus must also 

 be considered in accounting for the reaction. The stimulus 

 received from a soft substance is weaker than that received from 

 a hard one, if the rate of motion be the same. This may be seen 

 when a snail touches with its tentacles the side of a glass dish 

 as it moves along. The stimulus is sufTficient to cause it to turn 

 away sharply. If it strikes the soft parts of a fellow snail or 

 other like substance, however, when moving at the same rate, 

 it reacts positively because it receives a more gentle stimulus. 



The experiments on reactions to chemical stimuli were made 

 upon snails from the field. It is clear, that generally speaking, 

 a chemical causing positive reaction may be compared to the 

 gentle mechanical stimulus, and one which causes the negative 

 reaction may be compared to a strong mechanical stimulus. 



VII. Influence of physiological state on food taking. 



All experiments upon food reactions thus far described, were 

 performed upon snails fresh from the field. The snails used 

 in the foregoing experiments are believed to have been in like 

 physiological conditions, since they were taken from the same 

 environment and used at once. Early in the work it was dis- 

 covered that reactions of snails obtained from different sources 

 gave very conflicting results, and that no conclusions could 

 be drawn unless the physiological condition of the snail at the 

 time of the experiment was known. 



In the following experiments the snails are divided into three 

 groups according to the relative amount of food they had taken, 

 (i) The first group includes Physa taken from the field where 

 food was present. These have not been fasting, nor are they 



