162 THE NEGATIVE GEOTROPISM OF THE PERIWINKLE 



IV. \'ariation in Negative Geotropism due to the Effects 

 OF Desiccation 



The technique of desiccation was very simple. A number 

 of periwinkles were placed in an open dish and exposed to the 

 air under conditions approximating those of normal intertidal 

 exposure. 



Groups were periodically removed and subjected to a 

 negative geotropism experiment similar to that recorded above. 



For several days after the desiccation commenced the 

 animals moved about in the dish; then the opercula became 

 sealed with mucus and all activity ceased. Each individual 

 used was subjected to a vital test before any experiment was 

 performed. 



It is important to note a difference in the method of cal- 

 culation of results in the desiccation experiments from that 

 used for the other experiments. Only the animals actually 

 exhibiting a response to gravity were considered. If an in- 

 dividual remained motionless or active on the bottom of the 

 jar it was disregarded from the calculations of averages; hence 

 the twofold character of the tables showing both the percentage 

 of animals responding and the average amount of response 

 for those that did show any activity. 



Table 2 shows the effect of a short period of desiccation 

 on negative geotropism. This experiment was conducted in 

 the hot sunshine of an August afternoon, the desiccation taking 

 place in the open; thus there was an approximation of the 

 most severe conditions under which periwinkles would find them- 

 selves during intertidal exposure. 



