BIOLOGY OF PHYSA 95 



it requires the closest observation to determine how it descends 

 through the water. 



The experiment was tried of keeping Physa on the bottom 

 to see whether they could ascend through the water without 

 a thread by means of the expansion of the air in their lungs. 

 No snails were observed to have any such power. 

 • Lymnaea stagnalis is frequently seen to be just heavy enough 

 to barely touch the bottom with its spire. It may then sway 

 around and round as on a pivot. Sometimes it is able to reach 

 the bottom with its head, but it often is obliged to let air out 

 of its lung in order to do this. A slight difference in buoyancy 

 is apparent when its foot and head are extended as compared 

 with times they are retracted. A snail may come up the side 

 of an aquarium and crawl upon the film when its specific gravity 

 is somewhat greater than the water. Physa has been seen 

 to be so nicely balanced that, when the ribbon of mucus was 

 gently lifted, the snail slowly sank without giving up any air. 

 The condition of the snail's lung may be tested by gently turn- 

 ing over the snails that are crawling on the film. If they are 

 lighter than water they float shell upward; if heavier, they 

 sink. The snail does not crawl long, however, upon the film 

 with its lung partly empty, since the chances are that either 

 the lung region or the head will shortly receive the respiratory 

 stimulus from the film. I have never observed that Physa 

 employs any other means of altering its specific gravity than by 

 changing the amount of air in the lung. This means is in prin- 

 ciple what is now commonly held to be employed by Teleost 

 fishes. 



IT. Integumental, or cutaneous respiration. 



I. Physa may live without atmospheric air. Physa living 

 upon the bottom of streams have little or no opportunity of 

 reaching the surface film if the current is swift. Under these 

 conditions repeated observations lasting over periods of half 

 a day show that they do not take atmospheric air. This 

 together with the fact that the snails avoid the waters which 

 have the most carbonic acid and seek those that are the best 

 aerated, leads to the view that integumental respiration is a 

 continuous process. 



Physa that had expelled the air from their lungs were placed 

 in an aquarium with a few water-soaked leaves for food. A 



