BIOLOGY OF PHYSA 101 



The snails did not collect but, if anything, seemed to avoid the 

 disturbed water. It seems, therefore, that Physa do not react 

 positively to motion of this sort in the water, and that it is the 

 aeration of the water that causes them to collect. 



It is probable from the above observations and experiments 

 in the laborator}^ that Physa in the field collect in certain 

 habitats because they react positively to oxygen, and always 

 go into waters which contain the highest percentage of oxygen 

 (whenever suitable protection offers) and away from water 

 which contains carbonic acid gas. 



4. The reaction of Physa to shallow waters may he due to greater 

 oxygen content. The extreme sensitivity of Physa to oxygen 

 suggested a possible explanation of the fact that the snail is 

 always found in very shallow waters in the field. The waters 

 in Physa' s most favorable habitats always have two and often 

 three means of aeration. 



1. According to Whipple and Parker, shallow water takes 

 up air until it is saturated. 



2. Submerged water plants are well known to add oxygen 

 to the water, 



3. According to water analyses quoted by the above authors, 

 water may be saturated before, and supersaturated after being 

 mixed with air in its passage over a fall. Water may also 

 become supersaturated in lakes and rivers by wave action. 

 In the optimum habitat of Crooked Lake all three of these 

 means of aeration are present. The snails, however, were not 

 found in abundance in water over fourteen to eighteen inches in 

 depth, although other conditions remained the same. It was not 

 because the water at this depth did not contain sufificient oxygen, 

 but because the thinner sheet of water nearer the shore had 

 probably a greater oxygen content and the snails collected there, 

 responding as delicately to oxygen in the field as they did in the 

 laboratory. The greater the distance from the surface of the 

 water the greater the percentage of carbonic acid gas found in 

 it and Physa reacts as negatively to this gas as it does positively 

 to oxygen. Thus it is seen that Physa' s negative response to 

 carbonic acid and its positive response to oxygen may explain 

 its gathering in the shallow marginal waters. 



^ . The reaction of Physa away from an optimum pond weed hab- 

 itat may be due to diminution of oxygen and an accumulation 



