THE GEOTROPISM OF FRESHWATER SNAILS. 



8 9 



geotropism observed when the lung is empty, is predominant in 

 the majority of cases, that is, in over 90 per cent. Cases of 

 "indifference" to gravity numbered less than 10 per cent. That 

 Physa should become positive to gravity after taking in air 

 supply seems to the writer to be quite natural, since the animal 

 is primarily positively geotropic as will be shown later. More- 

 over, it is worth mention that each individual occasionally showed 

 a peculiar "habit." It crawled up as usual, turned downward 

 when it had reached the surface, arid crawled down, making no 

 effort to get air. The number of observations of this phenomenon 

 for each individual is given for reference in Table II. in the second 

 horizontal column and indicated by a star. 



TABLE II. 



GEOTROPISM OF Physa AT THE ANGLE OF 90 OF INCLINATION OF A SUPPORT IN 

 WATER, AFTER ANIMALS HAVE TAKEN AIR IN THE "LUNG-SAC." 



Observations on Planorbis and Limnaea. With single 

 individuals of these genera, the same tests as above were made 

 and still more conclusive results were obtained. Since these 

 snails were comparatively large forms, they were favorable for 

 observation. If they were dislodged they fell to the bottom, 

 provided their lungs were "relatively empty." If the lungs on 

 the contrary were full of air, dislodged snails floated on the surface 



