BY F. ROLAND HAYES, B. SC. 171 



\ II. Summary and Conclusions 



There exists in Littorina a clearly definable, specific, phy- 

 siological gradient corresponding to the position in the littoral 

 zone, and the variation in negative geotropism may be taken as 

 an index of this gradient. Thus the average figure representing 

 the negative geotropism of an individual collected in the upper 

 littoral zone, will be higher than that for an animal collected 

 at a lower level. 



Confirmatory evidence of the existence of this gradient 

 was found in an analysis of the effects of desiccation and im- 

 mersion on negative geotropism. Desiccation for periods of a 

 few hours might be likened to ordinary intertidal exposure of 

 individuals from the upper zones, resulting in increased nega- 

 tive geotropism following subsequent immersion. 



It was found that the greatest negative geotropism value was 

 obtained after desiccation for 1 .5 hours. Desiccation for longer 

 periods resulted in a condition which may be compared to that 

 of an animal left above high tide level by neap tides. The 

 result was a profound diminution of geotropic response serving 

 therefore in nature as an effective, automatic check upon the 

 activity of animals already at the upper danger line of their 

 habitat. 



Subjection to constant immersion will produce a change 

 in the physiological life history, and a variation can be demon- 

 strated in the negative geotropism which acts as an index to 

 this change. The evidence from immersion experiments is in 

 entire harmony with the other experimental results correlating 

 negative geotropism with tidal level, as it might be expected 

 that constant immersion of an intertidal animal would produce 

 a reaction designed to restore it to its original intertidal posi- 

 tion. 



Throughout all the experiments, the extreme variability 

 of the factors under which the "dry" group was tested is, I 

 think, quite suificient explanation for its less consistent results 

 than those for the "immersed" group. The "dry" jars were 



