84 JEAN DAWSON 



of the chemical, the jaw action became increased and continued 

 for the two hours it was under observation. The snail threw 

 its head back into its shell and upon the application of the 

 powder it acted disturbed, but it took it into the mouth re- 

 peatedly. The foot and head after a time seemed to shrink 

 and the mouth was sunken in somewhat. The snail was alive 

 several days afterward. The quinine seemed to act as a strong 

 stimulant poison, causing increased activity for some time. 

 and then, although the animal became quiet the jaw action 

 continued. In this case the food reflex seemed to have passed 

 entirely beyond the control of the animal after the first 

 few minutes, much as an engine might with the governor thrown 

 out of gear. 



VIII. General summary. 



Physa is almost omnivorous in its food habits, feeding upon 

 a variety of plant and animal forms, fresh or in various stages 

 of decay. Lymnaea stagnalis, L. palnstris and L. reflexa have 

 carnivorous and cannibaHstic traits while the genus Physa is 

 not known to take living prey; Planorhis trivolvis and P. 

 tricar nata on the other hand have a vegetable diet only. The 

 conditions of the habitats determine largely the food eaten by 

 the snail, the greater variety being found in still water. 



Physa as a scavenger is highly desirable in water. 



The mucus secreted by the snail's foot aids it in adhering to 

 a surface and serves as a lubricant, as a food, and as a means 

 of collecting food, since by remaining adhesive in water it en- 

 traps all kinds of microscopic plants and animals. Experi- 

 ments show that Physa even if in a starving condition does 

 not eat its egg masses. The physiological state of the snail 

 due to the amount of food taken affects the amount and viscosity 

 of the mucous flow of the foot. A fasting snail secretes but 

 little mucus and it is thick and impedes its progress, while the 

 mucus from the foot of a recently fed snail is rendered thinner 

 and permits greater activity. 



Drought and temperature affect food taking. The snail may 

 live for months without food during hibernation and desic- 

 cation. Nevertheless, Physa will starve in a week or two if 

 kept at room temperature without food. Lowering the tem- 

 perature decreases the activity and desire for food and vice versa. 



