C. THE DISTRIBUTION OF ANIMALS IN INLAND 

 WATERS, A PHASE OF LIMNOLOGY 



CHAPTER XVI 



THE ENVIRONMENTAL FACTORS IN INLAND WATERS 



The fauna of inland waters differs in many important respects from 

 that of the ocean. Whole groups of animals, such as echinoderms, 

 brachiopods, gephyreans, squids, and tunicates, which are plentiful in 

 the ocean, are wholly absent in inland waters, and many others have 

 fewer species which are often less rich in individuals: for instance, the 

 Coelenterata, the Nemertinea, the annelids, of which only a few species 

 are found, and the Bryozoa. Only a few groups are more plentiful 

 both in number of individuals and of species in inland waters than in 

 the ocean, e.g., ciliates and rotifiers; or are entirely restricted to lakes 

 and rivers as are the Gastrotricha and the amphibians. As has been 

 stated before (p. 35), the reason for these differences, especially the 

 difference in number of individuals and species, is that the living con- 

 ditions in inland waters vary so much from the optimum that they 

 demand certain definite adaptive adjustments in the animals which 

 relatively few species have been able to acquire. 



The greatest differences in the environments of oceanic and inland 

 waters are those relating to space and time. The ocean covers immense 

 portions of the earth's surface and is one continuous body with only 

 partially separated arms or seas, while the fresh-water lakes, rivers, 

 and seas form innumerable bodies of water of every size, spread over 

 the continents as islands are in the sea, each one more or less isolated 

 from the rest. The ocean has probably been in existence as one body 

 of water since the beginning of geological evolution, or at most sepa- 

 rated into a few large divisions for comparatively short periods of 

 time. Inland waters, on the other hand, as a rule are of comparatively 

 short duration and appear and disappear in rather rapid succession. 



The restriction of space becomes evident in the extraordinary 

 variation in size of inland bodies of water. They are usually of no 

 great extent or depth. With the exception of the Caspian Sea, which 

 has an area of 438,000 sq. km., no lake or inland sea has an area of 

 more than 100,000 sq. km. Only 17 lakes and seas have a surface area 



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