10 THE BIOLOGY OF THE SEA-SHORE 



effect on the fauna and flora, the liability to incursions 

 by birds and animals during low water, and the free- 

 swimniing and floating life of the sea coming and going at 

 every tide, have all their influence upon the plants and animals 

 of the sea-shore. Finally, the shore area has strong claims 

 to be recognised as the birthplace of life and, says Thomson 

 (19 19), we may regard it as the great school wherein were 

 learnt many of the primary lessons of life. 



Shallow Sea. — This is the zone extending from the low- 

 water mark of spring tides to the edge of the continental 

 shelf, the term given to the ledge usually found surrounding 

 a continental land-mass. The extent of this life-area will 

 thus vary considerably, but, generally speaking, the point 

 at which the gentle slope of the continental shelf passes 

 abruptly into the steep gradient of the continental slope 

 may be said to be situated at a depth of about a hundred 

 fathoms. This depth, then, which marks approximately 

 the limit of wave action will also mark that of the so-called 

 shallow sea. 



As a haunt of life the shallow sea more or less resembles 

 the tidal area in regard to such factors as Hght, the presence 

 of a substratum with algal vegetation, wave shock, and 

 aeration, but the influence of the tides is only slightly felt. 

 In wealth and variety of forms it surpasses any other com- 

 parable area on the globe. 



Pelagic Realm. — The extent of the pelagic realm is best 

 conveyed by the term " open sea." More accurately, 

 it may be said to be a seaward extension of the shallow sea, 

 and to include the surface waters to a depth of a hundred 

 fathoms. It thus forms a roof to the abyssal realm. Its 

 most important features are the presence of abundant light 

 and the absence of a substratum. The animals frequenting 

 this area are either swimmers (Nekton), or drifting forms 

 (Plankton) sufficiently buoyant to remain at or near the 

 surface of the water (see Plate III). In view of the necessity 

 for buoyancy, pelagic animals tend to be of delicate structure. 

 They are also mostly transparent. Plant life is confined to 

 the unicelhdar algas which, however, exist in limitless 



