78 JEAN DAWSON 



the mechanical stimulus was obeyed first, then came a pause, 

 and the chemical stimulus was sensed and the resulting reaction 

 was in the same direction as that to the mechanical alone. The 

 experiment was again tried when the snails were moving very 

 actively. In this case they turned sharply away from both 

 meat and onion, and made no pause whatever. The mechanical 

 stimulus alone was obeyed and it is a question whether the 

 chemical stimulus was even sensed. 



It w"ould seem from the above experiments that Physa senses 

 and responds to a mechanical stimulus quicker than to a chem- 

 ical stimulus. These are the usual reactions to chemical and to 

 the mechanical stimuli of the ventral surface of the foot and 

 head of the snail. It is necessary to employ the word usual 

 since there is some variation of reaction depending on different 

 "physiological states" as set forth by Jennings (1904) and 

 Pearl (1903). In interpreting the results of the stimuli given 

 in the experiments or received by the snail from its environ- 

 ment, one must gauge the momentum of the moving snail. If 

 this is not considered it may lead to an exaggerated notion of 

 the amount of variation that occurs in responses to stimuli 

 from time to time in a given animal observed. I have found 

 it very difficult to gauge the strength of a stimulus when the 

 animal is moving. Indeed, I find it almost impossible to give 

 a gentle touch enough to cause the snail to turn toward the 

 source of stimulation when it is moving at a lively rate. This 

 difference in the strength of the stimuli depending upon the rate 

 at which the animal is moving, is well illustrated in the snail's 

 relation to the surface film. When the snail is moving up through 

 the water, it strikes the film with a blow that varies with its 

 velocity. In the great majority of cases the animal receives a 

 stimulus of sufficient strength to cause it to throw the head back, 

 thus touching the new surface, the under-side of the film, upon 

 which it then moves away. Sometimes a snail moves very 

 slowly up through the water, in which case the film is so gently 

 touched that it goes on through the film and up into the air. 

 This same phenomenon may be seen in the snail's relation to 

 the mud on the bottom of its habitat. Ordinarily the animal 

 crawls down into the water at a rate fast enough to get a con- 

 siderable stimulus upon reaching the substratum; unless the 

 mud is exceedingly soft, and even in this case it turns presently 



