I9IO- CoLGAN. — The Adaptability of Littoal Mollusca. 133 



body whicl:, other things being equal, would result fmm this 

 exposure, is probabl}^ more than counterbalanced by access of 

 moisture from dew and mist and rain. And moisture of any 

 kind, whether fresh or saline, is probably in their case effective 

 in sustaining vitality. On the other hand, while the individuals 

 submitted to the artificial test of exposure to dry air in a warm 

 room take full advantage of the operculum as a retainer of 

 moisture, the}^ have throughout their exposure no access to 

 fresh supplies of moisture, whether fresh or saline. 



Again, the artificial fresh-water test ma^^or may not be more 

 severe than any similar test occurring under natural condi- 

 tions. In nature, it is more than probable that heavy rains 

 again and again saturate with fresh water the animals of such 

 individuals of Purpura and Littoiina riidis as dwell above 

 high-water mark, while in artificial immersion the operculum 

 undoubtedly serves as an efficient barrier against anj^ consider- 

 able access to the animal of this fresh water. It is, however, 

 by no means certain that the operculum acts, as Gosse believes 

 it to act, by perfect exclusion of the fresh water. The fact 

 that individuals of mau}^ littoral species exposed for many daj'S 

 to dry air or immersed for long periods in fresh or abnormally 

 saline water, return to activity almost immediately when 

 placed in sea water, is against this assumption. In all these 

 cases the operculum, just before resuscitation, is found to be 

 farwithdraw^n and closely fitting the shell-opening; and unless 

 we assume that the far-retired animal is sensitive to the very 

 slight variations of temperature which ma}^ occur on transfer 

 from one element to the other, we must conclude that the sea 

 water, which appears to exercise an almost instantaneous 

 reviving power, gains access in some wa}^ to the animal. It is 

 probably round the edge of the operculum rather than through 

 it that this access is had, for there is no reason to suppose that 

 its close-grained liorny substance is rapidly permeable by 

 water. 



Sandycove, Co. DubUn. 



1 A Naturahst's Rambles ou the Devonshire Coast, 1853, p. 60 



