44 THE CHICAGO ACADEMY OF SCIENCES. 
der natural conditions the food of snails is so abundant and so uni- 
versally distributed that apparently there has been no occasion for 
natural selection to act in this direction. 
‘“Defecation is more frequently accomplished by snails while they 
are at rest than when they are in motion, and it is noticeably less in 
snails that have been placed in water which has been boiled. Snails 
in boiled water probably find very little to add to the contents of their 
digestive tracts, and this may be the reason why their feces are longer 
retained. 
“It was repeatedly noted during the course of experiments in 
locomotion that a snail would cease moving in a manner quite inex- 
plainable by the external factors known to be at work. After an in- 
terval of quiet defecation sometimes took place, a fact which appears 
to be an instance of interference with the action of external stimuli 
from within the organism itself. The physiological condition, or tonus, 
in which an animal happens to be when it is subjected to an external 
stimulus very largely determines the nature of its response. The 
greater the range of its physiological conditions the less it is possible 
to predict with accuracy what the animal will do under definitely 
known external stimuli.” 
As remarked by Walter, and confirmed by personal observation, 
there appears to be no struggle between individuals for the possession 
of food, each snail simply eating everything in its path without rela- 
tion to its neighbors. 
The jaws of Lymneza serve to bite the small pieces of food while 
the radula tears or rasps it into smaller fragments. It has been ob- 
served while watching Lymnza feed as it glided along the glass side 
of an aquarium, that the radula is thrust entirely out of the mouth, 
the motion approaching nearest to that of a cat lapping milk. The 
jaw and the radula appear to meet in the mouth, both seeming to gather 
the food. 
i. FOOD FOR OTHER ANIMALS. 
Lymneas as well as other fresh-water mollusks form a staple food 
for other animals such as fish, birds and some mammals. The white 
fish of the Great Lakes feed largely upon Physa and Lymnea.* 
Cooke? cites the case of a Dytiscus in an aquarium which killed and 
devoured seven Lymnea stagnalis in the course of an afternoon. These 
beetles also ate Lymnea peregra but seemed to prefer stagnalis, for 
1Amer. Nat., XI, p. 445. 
*Mollusea, p. 59. 
