312 ANIMALS IN INLAND WATERS 



rubble, the settling of living forms on the bottom is very limited; for 

 by tossing and nibbing, plants and animals are crushed and ground, 

 and living forms can find permanent attachment only on the firm 

 rocks and boulders which can resist the force of the current. Free- 

 swimming forms remain only in the quiet places in the secluded pools. 

 Perpendicular waterfalls of considerable height block the passage of 

 many animals, especially of fishes. Salmon overcome the barrier inter- 

 posed by falls by leaping and are thus able to range into the back- 

 waters of streams inaccessible to other fishes. Many mountain fishes in 

 the Andes and Himalayas have an adhesive organ by means of which 

 they can climb vertical rock walls. Most swimmers, however, are ex- 

 cluded from mountain streams. Other animals, also, whose relatives 

 swim readily in quiet waters, are stationary here. Of the fishes, Cottus, 

 Lota, and Nemachilus hide under stones ; the water mites have shorter 

 legs than those in quiet waters and do not have the swimming hairs 

 found on the latter, and the hairs on the swimmerets of the few ostra- 

 cods are also much reduced. It is a bottom fauna which is best adapted 

 to mountain streams, and many of these have developed means of 

 maintaining themselves in the current, e.g., sucking disks, claws, silk 

 attachment threads, expanded pectoral fins, which will be discussed 

 below. 



In comparison with similar conditions in the surf region of the 

 ocean, the number of permanent sessile forms is small. The fresh-water 

 sponge Ephydatia, and the bryozoan Plumatella, occasionally encrust 

 the stones on the bottom of the brooks. A few types of insects have 

 larvae which attach themselves in swift water. Otherwise only the eggs 

 or pupae, i.e., stages which do not need nourishment, are attached, such 

 as the egg cocoons of flatworms and leeches and the eggs of the water 

 mites. Too little nourishment is suspended in the water here for many 

 animals to forego the active search for food. The larvae of the black 

 fly, Simulium, depend on the microscopic food materials brought to 

 their fan-like feeding apparatus by the passing current; and the caddis 

 worm Hydropsyche spins a veritable plankton net in swift water. On 

 the other hand, the remaining inhabitants of mountain streams, aside 

 from the small number of those which settle among the sparse vegeta- 

 tion of quiet places, are dependent upon effective anchorage. Their 

 number is not large. In central Europe they are, of the worms, the 

 triclad flatworms (e.g., Planaria alpina) and leeches (Glossiphonia) ; 

 of the mollusks, a number of snails (Ancylus fluviatilis, several small 

 Limnaea, Neritella fluviatilis), and a few mussels (the brook pearl 

 mussel Unio margaritifer and species of Pisidium) ; of Crustacea, the 

 water flea (Gammarus pidex) and the river crayfish; a variety of 



