2l6 GENERAL PRINCIPLES OF ZOOLOGY. 



characteristic of the northern or arctic region, the penguins 

 of the antarctic. 



The Distribution of Aquatic Animals. To animal 

 geography further belongs the distribution of animals in 

 the sea, and in fresh water. Since most seas are con- 

 nected, the faunal regions cannot be distinguished so 

 sharply as in the case of the land faunas; conspicuous dif- 

 ferences are present only when two oceans are separated 

 by continents extending far to the north and south ; such, 

 for example, exist between the Red Sea and the geo- 

 graphically neighboring Mediterranean, between the east 

 and west coasts of North America, even where they are 

 separated only by the narrow isthmus of Panama. 



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 freely motile, 

 some fixed, which inhabit the plant-covered rocky or sandy 

 shore as far as a depth of IOO meters. 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 by its archaic character, for here very 

 often genera and entire groups of animals exist, like the 

 Hexactinellida, 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 which swim freely in the water, the "plankton"; here 

 belong many ccelenterates, medusa, and ctenophores, entire 

 groups of protozoa, like the radiolarians, many crabs and 

 crab larva ; of the mollusks the Heteropods and Ptcropods. 

 These animals live either at the surface of the sea itself 

 or freely suspended in greater or lesser depths, to 8000 

 meters or even more. Usually they are soft like jelly and 

 of glasslike transparency; this must be regarded as sym- 



