BIOLOGY OF PHYSA 75 



tive to chemical stimuli. The snail does not give the food reac- 

 tion when food is gently touched to this region or held above 

 it. It would seem probable that this ganglion is more intimately 

 connected with the process of respiration than with food taking. 



V. Positive and negative responses to chemical stimuli. 



Physa does not respond to all chemical substances alike. 

 This may be conveniently demonstrated by applying chemicals 

 with a glass rod to the region about the mouth when the animal 

 is crawling upon the surface film. Physa, Lymnaea palustris 

 and L. stagnalis respond to chemical stimuli in the same way; 

 but this reaction may be more clearly demonstrated in L. stag- 

 nalis on account of its size and, perhaps, because this snail is 

 more sensitive. Snails should be used that are accustomed to 

 being handled and , ordinarily well fed. By this is meant that 

 the snail should have access to such food as water-soaked grass, 

 or maple leaves and algae, that it would get in the field. If 

 small pieces of bread, meat, apple, or sugar are gently applied 

 to the mouth region, the snail shortly gives the food response, 

 its head often reaching up out of the water. If the side of the 

 head is touched gently, the animal will turn toward the source 

 of the stimulation and give the food reaction. Food with a strong 

 odor such as pear and apple, causes these positive responses 

 when gently waved in the air a mm. or so above these parts. 



If a glass rod is dipped in five hundredths per cent, 

 hydrochloric acid and gently touched to the mouth region, the 

 snail momentarily withdraws its head from the film and does 

 not give the food reaction. If it is applied to the side of the 

 head, the snail turns away and does not give the food reaction. 

 Diluted onion juice, pieplant and the juice of a walnut husk 

 also cause a negative response. 



The snail responds to a neutral substance, such as chalk, 

 when it comes in contact with the head region, as it does to 

 any mechanical stimulus. The reaction of Pkysa to a food 

 substance may be observed if dry cracker crumbs are scattered 

 near the animal as it is moving slowly on the surface film. When 

 the crumbs are within a mm. or so of the snail's body they are 

 pulled with some force toward the animal. This pull is the 

 result of a surface tension which exists between two substances 

 that form like surface curves, such as two pieces of dry cracker, 



