ECOLOGICAL 83 



besides some well-known birds and mammals. There are also hosts 

 of larval forms which are pelagic for a time. 



The fauna of the open sea is representative, but there are few of 

 the types which we can suppose to have lived there always. It may 

 be that forms like the minute water-fleas have been there almost 

 from the first, but most bear the impress of lessons which the open 

 sea could never have taught them. 



Pelagic animals tend to be delicate and translucent; many are 

 phosphorescent. The number of species, differing from one another 

 within a relatively narrow range, is often enormous; thus about 

 5,000 species of Radiolarians are known. The huge number of indi- 

 viduals, which frequently occur in great swarms, is equally character- 

 istic. Perhaps both facts indicate that the conditions of life are 

 relatively easy, as is also in^plied in the limitless food-supply afforded 



Dry Land^ 



Fig. 19. 



Schema of the Great Haunts of Life and their Possible Relations with one 



Another. 



by the unicellular Algae. The pelagic fauna is richest in the colder 

 seas. 



Abyssal. — Through the researches of the Challenger and similar 

 expeditions, we know that there is practically no depth-limit to the 

 distribution of animal life, though the population is denser at 

 moderate depths than in the deepest abysses, and though there is 

 probably a thinly peopled zone between the light-limit and the 

 greatest depths. We know, too, that there are abyssal representa- 

 tives of most types from Protozoa to Fishes, and that the distribu- 

 tion tends to be cosmopolitan, in correspondence with the uniformity 

 of the physical conditions. 



The abyssal fauna includes some Foraminifera and Radiolarians, 

 many flinty sponges, some corals, sea-anemones, and Alcyonarians, 

 a few medusae, annelids and other "worms" on the so-called red clay. 



