BIOLOGY OF PHYSA 105 



pheric air for some time manifest indifference to the influence 

 of gravity, and scatter over the sides and bottom of the bottle. 

 They have never been observed to " let go and drop like dead 

 weights " upon being denied atmospheric air. 



7. Physa has no power to change its specific gravity by ex- 

 panding or contracting the air in its lung ; a change in buoyancy 

 is brought about only by taking air into the lung or expelling it. 

 The snail may crawl upon the surface film even though its specific 

 gravity is greater than water, since the foot and the slime ribbon 

 offer an expanded surface. 



8. Physa lived in the laboratory in aerated water for sixty -two 

 days when denied atmospheric air. They were much more in- 

 active than the snails in the control experiment which had access 

 to atmospheric air. 



9. Physa is affected by the gaseous content of the water. 

 If water lacks air or contains much carbonic acid the 

 snails will come to rest near the surface film. They die 

 sooner in boiled water than in water that has not been boiled. 



10. Physa reacts positively to oxygen and negatively to car- 

 bonic acid. Experiments in the laboratory show that the 

 snail reacts very delicately to oxygen, gathering in those parts 

 of the water that have the highest oxygen content. They thus 

 corroborate the observations in the field. 



11. In very well aerated waters such as the creek-bed and 

 the optimum Crooked Lake habitats, Physa obtains almost 

 its entire oxygen supply through its integument, as it rarely, 

 if ever, comes to the surface for air. Moquin-Tandon (p. 81) 

 kept Lymnaea glabra, Planorhis rotunda and Physa sp. without 

 atmospheric air for four days. Saint Simon (1853) kept Physa 

 sp. and Planorhis contortus four and twelve days and Pauly 

 (1877) kept two specimens of Lymnaea stagnalis ninety-one 

 and twenty- two days without air. Walter (1906) says that, 

 " When Lymnaea elodes is prevented from coming to the sur- 

 face for atmospheric air it dies." Walter's results are not 

 conclusive as there is some reason to believe that his snails 

 died from lack of oxygen rather than from the absence of at- 

 mospheric air. V. Siebold (1858) noted that Lymnaea auric- 

 ularia, L. miicronata, Planorhis carinatus, P. laevus and L. 

 stagnalis were not seen to visit the surface to take atmospheric 

 air in shallow or running water. Pauly (1877) confirms the 



