DISTRIBUTION. 179 



fauna consisting of few species but numerous individuals, of which 

 the auks, polar bear, reindeer, and arctic foxes are characteristic of 

 the northern or arctic region, the penguins and the entire absence 

 of land mammals of the antarctic. 



The Distribution of Aquatic Animals. Since most seas are 

 connected, the faunal regions cannot be distinguished so sharply 

 as in the case of the land faunas; conspicuous differences are 

 present only when two oceans are separated by continents extend- 

 ing far to the north and south; such, for example, exist between 

 the Eed Sea and the geographically neighboring Mediterranean, 

 between the east and west coasts of North America, even where 

 they are separated only by the narrow isthmus of Panama. Then, 

 too, considerable differences may exist where currents of greatly 

 different temperatures meet. 



Changes in the Fauna Conditioned by Depth. Much more 

 remarkable in the marine fauna are certain differences brought 

 about by the changes of the conditions of life in the different 

 depths of the sea. A deep-sea fauna, a coast fauna, and a pelagic 

 fauna can be distinguished. The coast fauna embraces the 

 animals, some free, some fixed, whicli inhabit the plant-covered 

 rocky or sandy shore to a depth of a few hundred feet. The 

 deep-sea fauna swims, creeps, or is fixed at the bottom of the 

 ocean at depths of 1000 to almost 9000 meters; it is distinguished 

 from the coast fauna in part by its archaic character, for here very 

 often genera and entire groups of animals exist, like the Hexac- 

 tinellidae, crinoids, certain starfishes and sea-urchins, etc., which 

 for a long time were chiefly known through fossils from earlier 

 geological ages. 



The Plankton. The pelagic animal world comprises all forms 

 which swim freely in the water, the ' plankton'; here belong many 

 coelenterates, medusae, and ctenophores, entire groups of Protozoa, 

 like the radiolarians, many Crustacea and crustacean larvae; of the 

 molluscs the heteropods and pteropods. These animals live either 

 at the surface of the sea itself or floating at greater or lesser, 

 depths, to 8000 meters or even more. Usually they are gelatinous 

 and of glasslike transparency; this must be regarded as sympa-, 

 thetic coloring and adaptation to the transparency of the water. 



Distribution of Fresh-water Animals. In fresh water two 

 groups of animals must be distinguished, of which the one com- 

 prises mainly the more highly organized forms, the molluscs, 

 fishes, and Crustacea, the other the lower animal world. The 

 distribution of the former is mainly determined by the same factors 



