170 THE NEGATIVE GEOTROPISM OF THE PERIWINKLE 



The wide variation in numbers over these areas at any 

 given leading is capable of explanation by the prevailing wea- 

 ther conditions, coupled with the nature of the substratum of 

 the areas. The first reading, that of November 28, reveals 

 a reasonable uniformity of distribution over all the areas, a 

 condition which agrees with the expectations, as there is no 

 marked difference in the substratum of the areas such as mud 

 f^ats, extensive growths of rockweed, etc. Up to the time of 

 the start of the observations there had been no marked change 

 to cold weather. The Dec. 12 records show a marked diminu- 

 tion of numbers of animals on areas "D" and "E" and a de- 

 crease also, though not so marked, in area "C". An explanation 

 may be sought in the surface characters of the areas. Areas 

 "A" and "B" provided sheltered crannies, while large fiat rocks 

 and gravel covered the other areas. The periwinkles were 

 found in the most sheltered places. The season at this time 

 was one of neap tides so that these areas were immersed almost 

 constantly. Thus the animals in damp crannies and tiny tide 

 pools would be less subject to the effects of cold thgn those in 

 exposed places. The readings on December 27, taken at a 

 period of spring tides following a period of severe cold, show a 

 marked increase in the number of periwinkles which have 

 come from the upper- littoral zone. 



The areas were at this time supporting a maximum of 

 Littorina, every cranny being filled to capacity. The March 

 11 figure shows that an upward migration has begun. 



The problem involved by these observations was whether 

 it was a reversal of negative geotropism or loss of activity that 

 caused the periwinkles to travel from the upper to the lower 

 zones. It seemed a reasonable hypothesis that they might 

 have been washed down, and such indeed proved to be the case. 

 Observations showed an almost total lack of activity in cold 

 water and there was no evidence of any measurable negative 

 geotropism in individuals tested, nor of any power to cling to 

 the substratum. Tested in water of a temperature near the 

 freezing point the only evidence of activity in the partly extended 

 animal was a feeble waving of tentacles, even the stimulus of a 

 sharp instrument pricking the foot bringing no marked response. 



