COMMUNITIES IN RUNNING WATERS 311 



water. In the tropics a number of selachians also enter fresh waters; 

 and many rays have entirely adapted themselves to life in the rivers. 



The abundance of nourishment which is offered by slow streams, 

 especially in the tropics, also attracts a number of higher vertebrates to 

 these regions. The Sirenia (Manatus and Halicore) come up the rivers 

 (Congo, Amazon, etc.) from the ocean and feed there on water plants. 

 Crocodiles and river turtles, otters and river dolphins {Platanista in 

 the rivers of India, and Inia in South America), feed on the plentiful 

 supply of fish. 



Increased velocity of current and prevailing gravelly and stony 

 bottom which is subject to motion and consequent friction, at least at 

 high water, conditions which are generally found in the middle courses 

 of streams, reduce the whole group of mud inhabitants. They occur 

 only in quiet places, in quiet bays or on the downstream side of gravel 

 banks; there we find tubificid worms and river mussels. Insect larvae 

 continue to be important, but the species are different from those found 

 on the muddy bottom of the lower courses. Instead of midge larvae 

 and the burrowing Ephemeridae, more and more of those occur which 

 take shelter under stones; other species of Ephemeridae, the larvae of 

 stone flies (Perlidae) which are completely missing in the mud, a 

 number of libellulid dragonfly nymphs, and above all a great number 

 of caddis worms with their stone cases, are to be found in such places, 

 as are also some small water beetles. The few fresh-water, air-breathing 

 snails which occur here are also found clinging to stones: Limnaea, 

 Physa, and Ancylus fluviatilis ; and of the gill-breaking snails there are 

 Goniobasis and Vivipara. Some of the fishes of this part of the river 

 belong to the barbel region and some to the grayling region. The 

 sparse, rapidly moving plankton has already been mentioned. 



The trout region. — The influence of increased velocity of current, 

 however, becomes much more noticeable in the upper streams, the 

 mountain rivers, or brooks. 15 There are, of course, various gradations 

 even here; the brooks of the intermediate mountains can be divided 

 into salmon brooks in their lower reaches where the fall is not so 

 marked, and trout brooks in their upper regions where the fall is con- 

 siderably greater. The mechanical erosion of the current with its wild 

 cataracts and rapids becomes most evident, however, in the upper 

 mountain streams. 



The whole bottom is stony, usually undermining the bordering 

 cliffs, and is covered with loose rocks and stones which are being sifted 

 by the force of the current; the coarseness of the rubble thus varies 

 with the elevation, and even in the trout brooks, stones the size of a 

 fist or a head are easily carried along. Because of this movement of 



