286 MARINE ANIMALS 



number of relicts, from its connection with the White Sea by means of 

 the Ladoga Sea and Onega Sea, e.g., the starworm, Halicryptus spi- 

 nalosus, which likewise occurs at Spitzbergen and the White Sea, but 

 is lacking in the North Sea, the Skagerrak, and the Kattegat; 64 so also 

 Mysis oculata, which is represented by M. relicta in the neighboring 

 fresh-water lakes, is found in the northern seas, but is absent in the 

 North Sea. It is because of this geological youth of the Baltic that the 

 number of endemic species is small. Apart from a few races of fishes, 

 the copepods Pseudocalanus elongatus and Temora longicornis may 

 be mentioned as endemic. 



The peculiar development of the animal life in such seas shows how 

 obviously the composition of the animal communities depends upon 

 environmental conditions, while the study of the effects of the freshen- 

 ing of the waters of the eastern Baltic Sea makes a suitable transition 

 between the consideration of marine animal communities and those of 

 the fresh waters, which are treated in the following section. 



BIBLIOGRAPHY 



1) Ortmann, 1896, Grundziige der marinen Tiergeographie, p. 47 ff.; Meisen- 

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 4) Ortmann, 1893, Erg. Plankton Exped., 2, Heft Gb., p. 1139.— 5) Murray & 

 Hjort, 1912, Depths of the Ocean, p. 526.-6) Murray & Hjort, op. cit., p. 157.— 

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 1900, Fauna Arct., 1, p. 309.— 17) Lohmann, 1907, Erg. D. siidpol. Exped., 9, 

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 Biol., 5, p. 55.-24) Ekman, 1935, Tiergeographie des Meeres, p. 327.-25) Gies- 

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