SPECIAL PROBLEMS OF SHORE LIFE 79 



summer (Davis, 191 3). For a similar reason, so Dr. Bruce 

 informs us, Arctic and Antarctic shores are entirely devoid 

 of invertebrate animals. 



To some extent, as we shall see in a subsequent chapter, 

 the extremes of temperature are avoided by means of 

 seasonal migrations when the fish of rock pools : blennies, 

 gunnels, gobies, and Crustaceans such as the edible crab 

 {Cancer pagiirus) and lobster {Homarus vulgaris) retire to 

 deeper water. 



Gosse (see Bate and Westwood, 1863) found shore- 

 hoppers {Talitrus locusta) burrowing some inches deep in 

 decomposing beds of algae, so hot that he could hardly bear 

 his hand among them. This occurs in summer when the 

 creatures are in need of moisture. On the other hand, such 

 beds of rotting algae afford warm shelter for these animals 

 during periods of frost and snow. 



The Effects of Varying Salinity. — Littoral waters are 

 continually receiving large quantities of fresh water from 

 the adjacent land by streams and rivers. The influence of 

 fresh water is most felt at certain seasons, and is most 

 marked in the neighbourhood of large estuaries, though there 

 are no figures to show how far this influence extends ; pro- 

 bably not to a great distance. On the other hand, there is 

 rarely a patch of rocky foreshore which does not receive its 

 quota of fresh water, a fact which is demonstrated by the 

 invariable presence of the green Enteromorpha. As we know 

 that fresh water is poisonous to the majority of marine 

 animals, it remains to be seen how shore forms are able to 

 resist a degree of dilution of the salt content of the water 

 which is often extreme. The question, which is an exceed- 

 ingly difficult one, depends on the degree of permeability 

 of living membranes by solutions of varying composition 

 and density. 



Several authors have established on different animals 

 (Dreser and Winter on mammals, Bottazzi on marine 

 invertebrates) that the glandular secretions have an osmotic 

 pressure equal to that of the blood of the same animals. The 

 experiments of Bottazzi (see Florentin, 1899) show, moreover, 



