182 



THE LIFE OF CRUSTACEA 



less and less salt, by the inflow of rivers, certain 

 species which were able to accommodate themselves 

 to the altered conditions would survive. Some of 

 the species living in the Swedish lakes have since 

 been found to have a wider distribution. Thus, 

 Mysis relicta, which should perhaps be reckoned as 

 only a variety of the Mysis oculata of Arctic seas, has 

 been found in lakes in Russia, North Germany, and 

 North America (Lake Superior and others), and has 

 lately been discovered in Lough Neagh and some 

 other lakes in Ireland. 



The brackish waters of the Caspian Sea contain a 

 very remarkable assemblage of animals, including 

 many Crustacea, which, although now quite isolated 

 from the oceans, are certainly of marine, and in part 

 of Arctic, origin. Among these are some species 

 closely allied to or identical with those of the Swedish 

 lakes already mentioned, together with a great variety 

 of species of Mysidacea, Cumacea, and Amphipoda, 

 which appear to have been evolved from marine 

 forms since the Caspian was cut off from communica- 

 tion with the Arctic Ocean. 



To such assemblages of animals derived from 

 marine species and isolated in inland lakes the name 

 of " relict " faunas has been given. It is necessary to 

 use caution, however, in extending this explanation 

 of their origin to every case of peculiar lake faunas. 

 For example, there are difficulties in the way of 

 supposing that Lake Baikal was ever in open and 



