COMMUNITIES IN STANDING WATERS 331 



the borders of the littoral regions are occasionally characterized by an 

 abundant molluscan fauna and are then designated as sublittoral. A 

 truly abyssal stratum, comparable with that of the sea, into which no 

 light rays penetrate, is lacking in most inland waters, because of their 

 shallower depth; such a stratum is found only in Lake Baikal, Lake 

 Tanganyika, and the Caspian Sea. 



The deeper regions of lakes have other peculiarities besides the lack 

 of plant growth which contribute to the unique compositions of their 

 fauna. The amount of light which penetrates into the deeper waters is 

 much reduced. For this reason there are modifications of color in both 

 limnetic (see below) and bottom forms. The planarians which other- 



Fig. 97.— a, Limnaea stagnalis var. bodamica from moving water; b, L. 

 stagnalis from quiet water, normal to the arrow, beyond which the shell is 

 weighted with cement, producing growth similar to a (after Voigt) ; c, L. 

 stagnalis from a pond, the growth form produced by a strong growth of fila- 

 mentous algae on the shell, which hinders its motion. X 2/3. 



wise are black (Polycelis nigra, Planaria alpina) are represented by 

 pale yellow forms in the depths of the lakes in the Alpine foothills. 

 The temperature is very constant in the deeper pedon as compared 

 with that of the littoral region. In the Lake of Lucerne the tempera- 

 ture at depths of 30 to 200 m. is not less than 4.7° and not more than 

 10°; the variation at depths of 100-200 m. is only about 1°. 20 Steno- 

 thermal animals, adapted to the cold, with a few eurythermal forms, 

 find a favorable habitat in the greater depths. 



There is no marked movement of water at greater depths. Because 

 of this the floor is evenly covered with fine mud varying according 

 to its origin, which is usually organic. The dead bodies of plants and 

 animals which live in the open water, the excrements of fishes, and the 

 products of disintegration of shore plants sink to the bottom and form 

 a layer of detritus which becomes the food of innumerable mussels 

 (Pisidium) , slimeworms (Tubificidae) , and bloodworms (Chirono- 

 midae) . These continually engulf the mud and give off small mud-balls 



