164 GENERAL PRINCirLES OE ZOOLOGY 



Then, too, consi(lcr;il:ilc dilTercnccs may exist where currents of greatly dilTerent 

 temperatures meet. 



Much more reniarkalile in llie marine launa are dilTerences caused by the 

 coneiitions of life in the different depths of the sea. A dci'p-sca fauna, a coast 

 fauna, anil a pila-^ic fauna can be distinj,'uished. The coast fauna embraces the 

 animals which inhabit the plant-co\ered rocky or sandy shore to a depth of a 

 few hundred feet. The deep-sea fauna swims, creeps, or is li.\ed at the bottom 

 of the ocean at depths ol looo to the greatest ileplh vet known, 9430 meters, 

 t;i56 fathoms; it is distinL;uished from the coast fauna in part by its archaic 

 character, for here very often genera and entire groups of animals exist, like the 

 Hexactinellid;e, crinoids, etc., which long were chielly known through fossils 

 from earlier geological ages. 



The Plankton. — The ]ielagic fauna comprises all forms which swim freely 

 in the water, the plankUni: here belong many cielentcralcs, medusa?, and ctcno- 

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

 heteropods and jjteropods. These animals live cither at the surface of the sea 

 itself or lloating 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 adaptatioit to the transparency of the water. The 

 plankton of the deejj seas, extending up to about Soo meters, forms a special 

 fauna characterized by the bro"wiiish-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, lishes, and Crustacea, the other the lowe^r 

 animal world. The distribution of the former is mainly determined by the 

 same factors which inlluence terrestrial forms; they are therefore of great impor- 

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

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

 etc.) and on tlie 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, howe\er, is cosmo- 

 politan. The same infusorians and rhizopods, copejiods, fresh-«ater 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 liy the \\ind. or may 

 be carried \\'ith the mud by aquatic liirds, and upon again reaching the water 

 resume its active state. 



VL DISTRIBUTION OF ANIMALS IN TIME. 



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

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

 jialeontological^ facts to understand the li\ing forms, and espexially the \erte- 

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



be gi\'en here. 



I. Azoic uk Arciii-.ax Era. 



No organisms are certainly known from Ihis age. 'T"he animal naliu-e of 

 Eitzoou canadciisc of the Eaiirentian beds, once referred to the loraminifi'ra, is 

 more than doubtful. 



