94 JEAN DAWSON 



and up the bank; this would be even more impossible in the 

 Elodea habitat since the water was deeper , swifter, and the 

 banks were further apart. No snails were observed crawling 

 up the bank; and it would be difficult to see how they would 

 know where the banks were, unless they reached them by mere 

 chance. The same difficulty attends lung respiration in water 

 too deep for thread spinning in which there are no plants or de- 

 bris upon which the animal may crawl to the film. In most of 

 the habitats where Physa are found living, however, the snails 

 have abundant opportunity to reach the air, although they do 

 not always do so. 



Relative buoyancy of Physa. Lung respiration causes the 

 specific gravity of a pulmonate snail to fluctuate from time 

 to time. If the snail is dislodged it falls to the bottom, if its 

 lung is relatively empty; if the lung is full of air the snail is 

 buoyed to the surface of the water. The buoyancy of a snail 

 appears when watching a water-soaked leaf on the bottom of the 

 aquarium upon which the snails are feeding. The leaf does 

 not move if few Physa crawl upon it and begin to feed; but if 

 eight or ten snails are upon i':, the leaf rises to the top and 

 remains there as long as the snails continue to feed. As one 

 after another of the snails crawl away the leaf gradually sinks 

 to the bottom. These changes in specific gravity are evident 

 to a casual observer, but the question arises whether the animal 

 changes its specific gravity by compressing the air in the lung 

 sac. The following observations and experiments bear upon 

 this point: Several Physa, while crawling on the film, were 

 gently turned about by means of their mucous ribbons when- 

 ever they approached the side of the dish to crawl down into 

 the water. After they had crawled about actively for an hour 

 or more, they apparently grew exhausted but seemed unable 

 to rest with their feet spread out on the film and their shells 

 suspended beneath the water. They either clung together 

 in masses on the film, an unusual thing for Physa to do, or 

 slowly expelled the air from the lungs and sank to the bottom. 

 \ They were accustomed to being handled and were unmolested 

 \ at the time they expelled the air from their lungs. In no case 

 ' were the snails able to leave the film and go down into the water, 

 'but by expelling some of the air from their lungs. Planorbis 

 trivolvis sometimes expels air from its lung so gradually that 



