316 ANIMALS IN INLAND WATERS 



may be employed in running waters. A mussel of the mountain streams 

 of southern South America, Byssanodonta, attaches itself to the sub- 

 stratum by means of threads from its byssus gland, and, as in many 

 attached mussels in the ocean, the foot is decreased in size through 

 retrogressive development. When the spring snail, Bythinella dunckeri, 

 retreats into its shell it attaches a thread to any fixed object and thus 

 is protected against being washed away. Besides these a few insect 

 larvae spin threads for anchorage ; the larvae of caddis flies can attach 

 themselves by means of a few threads, and the larvae of the black flies, 

 which demand a minimum current of 0.3 m. a second, often living on 

 the brink of falls, attach several threads to the substratum in order 

 to hold fast while they change their location. Their threads are so 

 tough that the larvae can float out into the water without being car- 

 ried away. 



Mussels are buried in the quiet parts of the river bottom which is 

 covered with finer gravel and sand, and they are thus protected from 

 being washed downstream. There are only a few of them in the fast- 

 flowing waters of central Europe. The brook pearl mussel, Unio mar- 

 garitifer, lives in the brooks of north and middle Europe (Scandinavia, 

 British Islands, Germany, France) which are poor in lime because they 

 flow over the granitoid rocks. In thickened old specimens, the umbo of 

 these mussels is often much corroded by free carbonic acid in the water. 

 The small ubiquitous Pisidimn is also widely distributed in mountain 

 brooks. 



Since the fauna of mountain brooks mainly consists of larvae of 

 insects (Ephemeridae, Perlidae, Trichoptera, etc.) which hatch in the 

 spring in the lower stretches, and in early summer in the higher regions, 

 the peculiar facts that these brooks are most densely inhabited during 

 the fall and winter months, are reduced in fauna in the warmer sea- 

 son, and do not contain developing larvae again until fall, are easily 

 explained. 



The zonation of the highlands, which sorts terrestrial animals and 

 plants into different belts according to elevation, does not become so 

 evident in running waters, except by progressive reduction of the 

 fauna. Even so, in his examination of the distribution of Entomostraca 

 of the Colorado mountains, Dodds 21 found that the species in higher 

 altitudes tend to be found in higher latitudes in the lower-lying plains 

 to the east. In the regions of perpetual snow and the adjacent areas of 

 treeless meadows little plant material reaches the brooks as food, and 

 the algae are rather sparse at such a height. Thus the amount of animal 

 life decreases rapidly with an increase in elevation; the number of 

 species is reduced, and the individuals are smaller. That this fact is 



