TWO YOUNG NATURALISTS. 289 



by creating a vacuum beneath them ; acorn shells, 

 that appear on the rock as little white projections, 

 very hard and sharp, all these being species that 

 have nothing to fear from the shocks of the waves. 

 In the crevices, and in the minute hollows where 

 protected from the surf, periwinkles and dog peri- 

 winkles (Purpurae), deposit their eggs ; while still 

 more in the interior, some in the very heart of the 

 rock, the numerous group of the corroders of the 

 stone, the piddock, the Saxicavse, the Venerupis, carry 

 on without any relaxation their incessant though 

 unseen task of destruction. 



If the ear be applied to the rock, a noise arising 

 from their unceasing action may be heard, a strange 

 sound, almost defying description, caused by the 

 energy of vital action within the interior of the life- 

 less rock. 



Farther off, at the limit of low water, commences a 

 zone in which the action of the sun's rays is less 

 powerful, and here the agreeable and lively green 

 tints are replaced by more sombre shades of bistre- 

 brown and olive. 



Between the great brown stalks and the interlaced 

 leaves of Laminaria, there is also a considerable popu- 

 lation, of a quite different character. This is a lurk- 

 ing-place for certain molluscs without shells, the 

 species of Doris and Tritonia, designated by the fishers 

 under the common name of sea-slugs. As well as 



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