THE GEOTROPISM OF FRESHWATER SNAILS. QI 



As may be seen from the table, the snail did not respond eleven 

 times out of thirty-five trials, when it was lighter than water. 

 This means that it was still floating when the complete two- 

 minute interval was over. The failure of response to gravity 

 in these cases need not be interpreted as "indifference " to gravity, 

 because it was often observed that the snail had opened its air 

 cavity at the time of observation. Evidently it was taking in 

 more air. The geotropic response failed, therefore, because the 

 lung was not full of air. All internal conditions being equal, 

 the snail tends to crawl down, if its lung is full of air. If it 

 crawled downward, it crawled vertically, orienting itself with its 

 anterior end accurately in that direction. 



Observation 3: Limncea columella, again, when lighter than 

 water, failed to respond to gravity nearly half the time, as Table 

 V. has shown. 



TABLE V. 



Geotropism of Limncea columella on a Vertical Side of a Beaker Full of Water in Total Darkness. 

 When it was Lighter than the Water. Table Shows Results After Two Minutes. 



Only a limited number of observations could be made at a 

 sitting, as the animals ceased to respond at all. This was 

 probably a fatigue effect. 



It was observed three or four times that the snail crawled down 

 with its shell in a horizontal position ; and two or three times with 

 the anterior end of its shell pointed up. It thus crawled down 

 even against mechanical disadvantage. 



The writer was unable to obtain satisfactory observations on 

 this snail's negative geotropism. It crawled up to the surface 

 for air. But when it was subjected to experimentation, it 

 retreated into its shell and did not readily come out. 



The writer by the above observations confirms Walter's con- 

 clusion regarding the effect of taking air on the geotropism of 

 Physa and other species. 



