327 



gills, but also has a lung-cavity like that of the land snails, 

 into which air is carried by means of a long breathing- 

 siphon. 



Among Crustacea there arc several species of crabs (e. g. 3 

 Birgus latro, etc.) which live in dan)]) woods far from all 

 water, and whose respiration is carried on chiefly without 

 the intermediation of their normal medium. 



The advent of marine vegetation on the land has only 

 occurred up to the limit of the salt spray on exposed shores, 

 and here the number of species is small. But at or just be- 

 low high-tide limit, a number of alga? find a congenial abode, 

 and grow there in luxuriant masses. Chief of these in our 

 northern latitudes are the Fuci, with Fucu.s vesiculosus and 

 Ascophylhnn nodosum predominating. At low tide these 

 hang like a wet fringe over the exposed rocks and give shel- 

 ter to numerous species of the smaller littoral animals 

 beneath them.* 



The littoral districts of the marine and fluvial realms like- 

 wise interlock along the shores, where streams mouth into 

 the sea, or expand into broad estuaries. Here marine ani- 

 mals will venture up into the fresh-water littoral district; 

 while, similarly, fresh-water animals pass into the littoral 

 district of the marine realm. The common meeting-ground 

 of the two approaching faunas and floras is in the estuarine 

 or brackish water fades of the littoral districts. 



In an ideal cross-section of a shore supplied only with 

 terrigenous deposits we may distinguish a variety of fades 

 of the material composing it, which progressively decreases 

 in coarseness of texture seaward. These various fades may 

 be classed as follows : 



1. Rocky-cliff fades. 



2. Bouldery fades. 



3. Gravelly fades. 



4. Sandy fades. 



5. Muddy fades. 



* A similar interrelation between the littoral districts of the terrestrial and fluvial 

 realms occurs, with a passage of the fauna and flora of the one to the other. 



