44 JEAN DAWSON 



or no enemies; minimum amount of debris; protection from 

 waves and currents; moderate amount of water weeds; and well 

 aerated water. 



It is clearly not a question of food which causes these snails 

 to collect in numbers in these habitats, as snails are well known 

 to be almost omnivorous in their habits. Neither is it the kind 

 of food since the same kinds that occur in the most favorable 

 habitat, are found in greater abundance in neighboring places 

 that have no snail life. 



According to the work of Whipple and Parker (1902) and 

 Hoppe Seyler (1896) the shallow water of these habitats with 

 its water plants is often saturated and even supersaturated with 

 oxygen. This is brought about by a diffusion from the over- 

 lying atmosphere and is known as a breathing process. Water 

 plants are well known to add to the amount of oxygen of the 

 water when they are subjected to direct sunlight. According 

 to the above cited authorities water that is agitated takes oxygen 

 from the air by mechanical processes and liberates its carbonic 

 acid. The water that breaks over stones in the creek-bed habitat 

 and the water that is lashed by the waves through a gap in an 

 ice-rampart or into the open end of a spit-formed pool, is prob- 

 ably well aerated and contains little carbonic acid. Direct 

 sunlight favors a moderate amount of decomposition which in 

 turn directly affects the amount of oxygen" and carbonic acid. 

 It seems that Physa is living in parts of the water which have 

 the highest percentage of oxygen and the lowest percentage of 

 carbonic acid. Experimental evidence corroborating these ob- 

 servations will later be considered in detail. 



Aside from the question of enemies, the conditions common 

 to the optimum habitats resolve themselves into that of the 

 gaseous content of the water, and indeed later experiments 

 show that the snail reacts very positively to oxygen and is 

 strongly repelled by carbonic acid. ^^ 



