ANIMAL LIFE IN THE SEA 157 
constituting the pelagic forms, which require no sub- 
stratum, but for the most part demand light, and usually 
display sensitiveness to it. Finally, at the bottom of 
the ocean, in the great depths, we have forms entirely 
independent of sunlight, 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 littoral 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 littoral 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- 
