SAKYO KANDA. 



The results, given in Table I. confirm Walter's conclusion. 

 However, there are marked individual differences. Number 5, 

 for instance, was exceptionally positive to light, while Numbers I , 

 2, and 3 were always negative. Nevertheless, 74 per cent, were 

 negative to light, and only 20 per cent, positive. From these 

 results, the conclusion may be drawn that Physa gyrina Say is 

 generally negatively heliotropic. 



Dawson (2, pp. 60-61), on the other hand, has observed that 

 darkness interferes with the activity of Pliysa. The writer's 

 observations seem to be in accord with Dawson's. Five of the 

 individuals above mentioned were kept in water under frequent 

 observation one afternoon (from 2.40-4 P. M.) in darkness. 

 Three of them were observed to crawl to the surface and then 

 down again only two or three times; and two of them only once 

 during the period, one 55 minutes and the other 70 minutes after 

 being covered. The rest of the time they did not move at all. 



2. Geotropism of Physa and Other Species, with the Lung Empty 

 and with the Lung Filled with Air. 



As already mentioned, Walter and Dawson disagree on their 

 observations concerning positive geotropism in Physa and other 

 species after the air supply is cut off. Neither has, however, 

 furnished the quantitative evidence from which the conclusion 

 was drawn. The writer, therefore, has made some quantitative 

 observations on this point. 



(a) Observations on Physa. Five selected individuals of Physa 

 were placed in a beaker containing about 300 c.c. of water. The 

 beaker, which had vertical sides and a horizontal bottom, was 

 placed as near as possible in optimum daylight. The water which 

 was used for this purpose was taken from the dish in which 

 Physa were kept, and was filtered when necessary. The following 

 observations have two aspects, (i) response of Physa to gravity, 

 when the lung is empty, and (2) when the lung is full of air. The 

 results are given in Table II. 



The negative geotropism of Physa, when its lung is empty, is 

 precise and marked. It is impossible to mistake it. It is also 

 evident, on the other hand, that the positive geotropism of the 

 animal after filling the lung, though not as precise as the negative 



