ANIMAL LIFE IN THE SEA 157 



constituting the pelagic forms, which require no sub- 

 stratum, but for the most part demand hght, and usually 

 display sensitiveness to it. Finally, at the bottom of 

 the ocean, in the great depths, we have forms entirely 

 independent of sunhght, but directly or indirectly 

 dependent upon the presence of a substratum. These 

 are the abyssal animals, whose existence was first fully 

 demonstrated by the Challenger expedition. 



We can thus divide the inhabitants of the ocean into 

 three well-defined classes, each class showing adapta- 

 tions to a natural region. These three groups are 

 (1) the littoral, (2) the pelagic, and (3) the abyssal, 

 each of which we must consider separately. As we 

 shall see, the members of the different groups do not 

 necessarily restrict themselves throughout their whole 

 life history to one of the natural regions. It is, for 

 example, very common to find that Httoral forms have 

 pelagic larvae, or pelagic stages in their life-history, 

 able to take advantage, for purposes of distribution, 

 of the many currents which influence the surface waters 

 of the sea. 



(1) The littoral zone extends outwards from the 

 margin of the land to the edge of the continental shelf, 

 or roughly to the one hundred fathom line. Beyond 

 this line the sea-bottom usually slopes rapidly in the 

 Continental Slope to the great depths, where the 

 littoral animals are replaced by the abyssal ones. 

 Shallow seas, such as the North Sea, contain only 

 littoral and pelagic animals. Throughout its exten- 

 sion the Httoral area is characterized by its great 

 wealth of food, and its great variations in the physical 

 conditions. The basal sources of food are here three : 

 (1) the waste of the land, (2) the fixed algae of the 

 shore rocks, (3) the minute floating algae or phyto- 



