104 JEAN DAWSON 



open a fraction of a second ; others kept it open ninety-six seconds. 

 Walter apparently does not take the size of the aperture of the 

 siphon into consideration, and this would doubtless modify 

 the length of time taken to fill the lung. He also cites the 

 case of a snail that "took breath" several times in quick suc- 

 cession and "sets it down as an expression of individuality 

 on the part of this individual snail, due to some internal cause." 

 I have observed this behavior quite frequently in Physa and 

 believe that it may be seen at any time in any snail that re- 

 ceives oxygen stimuli at short intervals. 



6. Davenport and Perkins (1897) found that slugs were geo- 

 tactic. Walter (1906) finds that pond snails " are negatively 

 geotactic in direct proportion to their lack of oxygen" and "are 

 positively geotactic when supplied with sufficient oxygen." 

 He says, " When several snails (Lymnaea elodes) were put in 

 a square bottle, stoppered under water to exclude bubbles of 

 free air and then placed on one of its sides, they soon congre- 

 gated on the upper surface and remained there. In this case 

 negative geotaxis was stronger than any other stimulus which 

 could cause them to travel back down in the way they normally 

 would if they had received a fresh supply of oxygen." Again 

 he says: " Upon placing the flask in an inverted position they 

 crawled up to the highest point, but, receiving no air, they 

 seemed to experience great difficulty in passing beyond the 

 'pole' and down the other side, although the substratum was 

 continuous, presenting no barriers. After reaching the highest 

 point in the flask and finding themselves unable to renew their 

 air supply, their ordinary behavior, to which there were some 

 exceptions, was to let go and drop like dead weights." Physa 

 is negatively geotactic if its lung is empty but manifests indifter- 

 ence to gravity if its lung is full.. It crawls upon or under the 

 film, up or down the sides of the aquarium, upon a thread and 

 upon the bottom. If Physa is placed in a bottle in which the 

 air supply is cut off, it will if in need of air, come to the highest 

 p^oint. If the habitat is strange and the snails are not allowed 

 to take air before the bottle is corked they do not collect at 

 the highest point; if accustomed to living in the bottle before 

 it was corked, they will gather about the highest point and 

 the time they remain will depend upon the degree of familiarity 

 with their habitat. Physa, after they have been denied atmos- 



