332 ANIMALS IN INLAND WATERS 



from which the largest part of the organic material has been extracted; 

 the rest is finally disintegrated by the action of bacteria. 



When more organic material sinks to the bottom than can be 

 consumed by the deep-water fauna and the bacteria, black vile- 

 smelling muck is produced which is rich in sulphuretted hydrogen. 

 Peat develops where these organic elements are disinfected by the 

 prevalence of humic acid, and are thus kept from disintegrating. 

 Bottom mud in all deep lakes comes primarily from decaying plankton ; 

 accordingly the mud of the lake bottom varies with the various com- 

 positions of the plankton, and in especially pronounced deposits one 

 can identify different sorts such as diatom mud, cyanophyceous mud, 

 and chitinous mud. Where, however, the rapidly flowing streams of 

 mountains or foothills contribute masses of mineral sediment, a sterile, 

 finely grained, more or less solid, terrigenous mud becomes deposited 

 on the bottom, which is inhabited by an exceedingly sparse fauna. 

 The sediments which are thus contributed, sometimes in excessive 

 quantities, carry many small plankton animals with them into the 

 deeps and bury them in the mud. 



No influence of increased water pressure on the animals living on 

 or near the bottoms of lakes has so far been demonstrated, although 

 the evidence is not yet fully collected from Lake Baikal and Tangan- 

 yika. Great uniformity of environment prevails at the bottom of deep 

 lakes; there is no change of temperature, scarcely any change in the 

 amount of light, no movement of the water, and no variation of sub- 

 stratum. For this reason many characteristics of periodicity which are 

 common in the biota of shallow waters are not evident here; a resting 

 period during the winter does not occur; the pea mussel, Pisidhim, 

 in the deeper regions, shows no clearly marked rings of annual growth, 

 but has a uniformly developed shell. 21 



Animal life of deep water.- — The deep fauna of lakes consists of 

 ubiquitous eurythermal forms and stenothermal cold-water forms, in 

 accordance with the conditions of temperature. The largest part of 

 the fauna is composed of rhizopods, Tubificidae, Corethra, chironomid 

 larvae, the larvae of biting gnats, and the small bivalve Pisidium. 

 Rhizopods occur in a rather large number of species, often in sur- 

 prisingly large numbers of individuals, widely distributed, especially 

 in the deep lakes ; these consist partly of ubiquitous forms, partly also 

 of species which are limited to the deeper parts of the lakes. A common 

 characteristic of all these deep-water rhizopods is that they are larger 

 than their relatives in shallow water. This is especially noticeable in 

 Cyphoderia ampulla (measuring 200/i, in deep water, 110/a in shallow 

 water) , a condition which is similar to that in many deep-sea forms 



