164 GENERAL PRINCIPLES OF ZOOLOGY 



Then, too, considerable differences may exist where currents of greatly different 

 temperatures meet. 



Much more remarkable in the marine fauna are differences caused by the 

 conditions of life in the different depths of the sea. A deep-sea fauna, a coast 

 ''niiia, and a pelagic fauna can be distinguished. The coast fauna embraces the 

 animals which 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 the greatest depth yet known, 9430 meters, 

 5156 fathoms; 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 

 Hexactinellidas, crinoids, etc., which long were chiefly known through fossils 

 from earlier geological ages. 



The Plankton. The pelagic fauna comprises all forms which swim freely 

 in the water, the plankton; here belong many ccelenterates, medusae, and cteno- 

 phores, entire groups of Protozoa, like the radiolarians, many Crustacea, 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 

 sympathetic coloring and adaptation to the transparency of the water. The 

 plankton of the deep seas, extending up to about 800 meters, forms a special 

 fauna characterized by the brownish-red color, which is also found in the bottom 

 animals. 



Distribution of Fresh-water Animals. In fresh water two groups of 

 animals must be distinguished, of which the one comprises 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 which influence terrestrial forms; they are therefore of great impor- 

 tance in matters of geographical distribution, yet it must be remarked that many 

 fish at the breeding season ascend from the seas to the rivers (salmon, alewives, 

 etc.) and on the other hand, others like the eels go from the rivers to the seas, so 

 that the sea is not that sharp boundary for these animals that it is for land ani- 

 mals. The distribution of the lower fresh-water animals, however, is cosmo- 

 politan. The same infusorians and rhizopods, copepods, fresh-water sponges 

 and polyps which occur in America seem to be distributed over nearly the entire 

 earth. This is connected with the fact that all these animals have resting 

 stages in which they endure desiccation. The resting stage, be it as a hard- 

 shelled egg or as an encysted animal, may be borne about by the wind, or may 

 be carried with the mud by aquatic birds, and upon again reaching the water 

 resume its active state. 



VI. DISTRIBUTION OF ANIMALS IN TIME. 



It is the province of paleontology or paleozoology, to treat of animals in the 

 earlier periods of the earth's history, but since it is necessary to draw upon 

 paleontological facts to understand the living forms, and especially the verte- 

 brates, an outline of the geological periods with the characteristic animals may 

 be given here. 



I. Azoic OR ARCHEAN ERA. 



No organisms are certainly known from this age. The animal nature of 

 Eozoon canadense of the Laurentian beds, once referred to the Foraminifera, is 

 more than doubtful. 



