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 faeces 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 Lymnaea 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 Lymnaea 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. 



Lymnaeas 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 Lymncca. 1 

 Cooke 2 cites the case of a Dytiscus in an aquarium which killed and 

 devoured seven Lymncea stagnalis in the course of an afternoon. These 

 beetles also ate Lymnaa pcrcgra but seemed to prefer stagnalis, for 



l Amer. Nat.. XI, p. 445. 

 2 Mollusca, p. 59. 



