BIOLOGY OF PHYSA 107 



13. A preliminary examination of the water of an optimum 

 pond weed habitat for its gaseous content, showed an abnormal 

 amount of carbonic acid and a small amount of oxygen. If 

 the observation is confirmed this condition of the water ex- 

 plains the reaction of Physa away from such habitats. The 

 amount of oxygen and carbonic acid may be explained in 

 the follo^^'ing way: Plants, crowded together are shaded so that 

 photosynthesis is interfered with and, in consequence little car- 

 bonic acid is consumed and but little oxygen liberated . Respiration 

 is normal and a large amount of oxygen is consumed ani car- 

 bonic acid liberated from the large mass of water plants. Birge 

 (1908, pp. 1286-7). 



14. The lung of the newly hatched Physa is either filled with 

 water or has not as yet been inflated. The young show, if their 

 lungs are empty or filled with water, the same geotactic response 

 as do the older snails. When but a few hours old they go to 

 the film and fill their lungs with atmospheric air. Baker (1902) 

 says regarding Lymnaeidae, "It is interesting to note that the 

 young animals breathe through the water for a long time, but 

 finally acquire the normal characteristic of the family, which is 

 breathing the air directly." 



V. vSOME PSYCHIC PHENOMENA OF PHYSA. 



I. Sensitivity of Physa. 



II. Physa varies in its reactions, due to differences in experience. 



III. Reaction of Physa to a new experience. 



IV. Adjustment of Physa to a new experience. 



V. Physa forms associated memories of its liabitat. 



VI. A young Physa differs from the adult in the character of its responses. 



VII. Differences in the reactions of Physa, young and adult. 



VIII. Summary and discussion. 



I. Sensitivity of Physa.* 



The response of Physa to oxygen and to gravity is rivaled 

 in delicacy b}^ its reaction to a disturbance of the general environ- 

 ment. The snail " hugs " the substratum if the aquarium in 

 which it is living is slightl}' jarred . Physa gyrina living in water 

 two inches deep, has been seen to react thus, while upon the 

 bottom, to the slightest disturbance of the surface film such as 

 is made by the breath. 



Physa also reacts delicately to a localized mechanical stimulus. 

 It is a common sight to see it whirl its shell quickly about, often 



*In this work, the writer has made no attempt to measure, in mathematical terms, 

 the delicacy of the reactions of Physa to stimuli. 



