COMMUNITIES IN STANDING WATERS 345 



characteristic which enables them, as well as the Tubificidae, to 

 utilize even the smallest quantities of oxygen; they are consequently 

 without competition in such regions. The Corethra larvae rise at night 

 to the upper water strata which are rich in oxygen, and there renew 

 the oxygen in their tracheal bladders, so that they can then draw on 

 this supply. Many fishes (carp, eel, pike) are adapted to temporary 

 life in water which is poor in oxygen by the fact that their haemoglo- 

 bin reaches the same oxygen saturation at an oxygen pressure of 2-3 

 mm. which that of the trout reaches at a pressure of 18 mm. 49 Such 

 fishes also draw upon the oxygen stored in the swim bladder and 

 further have adapted their feeding habits so that they oscillate be- 

 tween the stagnant water of the hypolimnion and the oxygenated 

 water above. The pedonic fauna may be numerous in individuals 

 even if it is poor in number of species. 



Eutrophic lakes may represent a period of old age in oligotrophy 

 lakes; the transformation is produced by a filling up of the lake 

 which makes it more shallow ; and, in consequence of a change of the 

 surrounding region into fertilized cultivated ground, it receives a larger 

 mass of nutritive salts. Lake Zurich, for instance, is in such a transi- 

 tional stage. 



Ancient fresh-water lakes. — As has been mentioned in Chapter 

 VI, a few of the larger lakes of the world have escaped the common 

 fate of transitoriness, and have presented an opportunity for long- 

 continued evolution of their faunae. 



The fauna of Lake Baikal is extraordinary for its wealth of species 

 representing relatively few generic types, which are for the most part 

 endemic. The Baikal seal also occurs in the small Lake Orion in the 

 mountains near by. Of the fishes, 1 family, 7 genera, and half of the 

 34 species known up to 1934 are endemic. 



There are 300 species of gammarid crustaceans, all endemic ex- 

 cept the common Gammarus pulex. The 12 species of Branchiopoda 

 are not endemic, and only a few of the recorded copepods are limited 

 to the lake. These and other small crustaceans are not well known, 

 and new species are being discovered constantly. Of the 75 known 

 snails, 68 are endemic, together with several genera. One family, the 

 Baicaliidae, until recently supposed to be confined to Lake Baikal, 

 is now found to be represented also in Lake Ochrida in the Balkan 

 Peninsula, and is known also from Pliocene fresh-water deposits in 

 Hungary. Of the 15 recorded bivalve mollusks, 13 are endemic. 



More than 100 species of planarian worms have been found in the 

 lake, all endemic. In contrast, only about 50 species of planarians are 

 recorded from the whole of Europe. The 30 or so oligochaete worms 



