174 THE OCEAN 



the tropical regions, especially in the neigh- 

 bourhood of continental land and of coral 

 reefs, but though the aggregate number of 

 species may be less in temperate and polar 

 coast waters than in tropical waters, this is 

 counterbalanced by the extraordinary abund- 

 ance of individuals in the cold waters towards 

 the poles. 



Littoral and Shallow-Water Benthos. — ^In con- 

 sidering the marine fauna of coastal regions 

 it is usual to subdivide the area into zones 

 according to the physical conditions and the 

 materials covering the bottom. Thus in 

 northern waters we may recognise a littoral 

 zone extending from high-water mark down 

 to about 20 fathoms, and a sub-littoral or 

 shallow-water zone from about 20 fathoms 

 down to about 100 fathoms. Beyond 100 

 fathoms we approach deep-sea conditions, 

 the temperature and salinity becoming more 

 and more uniform and the currents less marked, 

 while the fauna at the same time becomes 

 more and more uniform and more widely 

 distributed both horizontally and vertically. 



The littoral zone is subdivided, according 

 to the bottom conditions, into areas and belts 

 in different localities, such as the low-tide 

 area, barnacle belt, fucoid belt, laminarian 

 belt, zosteran belt, hard bottom, sandy bot- 

 tom, muddy bottom, each characterised by its 



