COMMUNITIES IN RUNNING WATERS 309 



cephala, Polycelis cornuta, and PL alpina, are distributed so that the 

 first lives in the warmer waters with stony bottoms (Fig. 83) , Polycelis 

 further up, while PL alpina lives in the upper streams and springs. For 

 great distances in the upper Alps and the highlands north of the Alps, 

 PL alpina is the sole occupant. In the middle region of the highlands 

 the regions of distribution of the three species are rather closely ap- 

 proximated (Fig. 84), and PL alpina is often limited entirely to the 

 source brooks or perhaps springs at the actual source. In the warmer 

 springs of the lower regions PL alpina and Polycelis are entirely 

 missing. PL alpina breeds in the winter, and its distribution indicates 

 that it is a relic of the glacial period and was found in all the streams 

 of middle Europe during that time; its optimum temperature is below 

 10°. Polycelis cornuta also breeds in the winter; but its optimum tem- 

 perature is several degrees higher. PL gonocephala, on the other hand, 

 lives in warmer waters, although it is not destroyed by low water 

 temperatures. P. cornuta migrated into the creeks after PL alpina, and 

 PL gonocephala followed still later. Their distribution in the brooks is 

 determined by their competition for nourishment correlated with 

 temperature, since each species is most efficient in its own optimum 

 temperature. 



The carp region. — In the lower courses of rivers, with substrata 

 of mud or fine sand and with slow currents, without coarse sediment in 

 suspension, and also in other river regions with similar conditions, the 

 inhabitants of the bottom are to a great extent detritus feeders, like 

 those of muddy floors of lakes and ponds. Tubificid annelids, which 

 have their anterior ends stuck in the mud which they eat as food, are 

 especially plentiful. Many insect larvae also feed on detritus, e.g., the 

 alderfly Sialis, the midge Chironomus, and several may flies; these 

 usually burrow in the mud in that they either creep along in the 

 surface layer {Sialis) or build mud tubes (Chironomus) or dig deeper 

 burrows. The banks of many slowly flowing streams are riddled by the 

 ramifying burrows of Ephemeridae (Fig. 85) ; they are found in such 

 numbers that when a mass of subimagoes emerge at the same time on a 

 sultry evening the air is filled with them as with a heavy snowstorm. 13 

 Mussels also occur, especially the small Sphaerium and Pisidiwn, and 

 the much larger Unio and Anodonta. In the Elbe below Hamburg, 

 Sphaerium rival the Tubificidae in numbers. The Bryozoa also belong 

 to the detritus feeders. These are plentiful in the Elbe and the Bille 

 near Hamburg, and occur in a number of beautiful forms;* they do 

 not, however, live on the mud, but need a firmer foundation for attach- 



* Pectinatella, Lophopus, Frcdericella, Paludicclla, Cristatella. 



