SWIFT STREAMS 99 



Here we find the caddis- worm (Helicopsyche) (Fig. 41, p. 96), which has a 

 spiral case made of sand grains. These are most abundant where some 

 sand and swift current are both found. There is from time to time some 

 vegetation in such situations and on it we find the brook damsel-fly 

 nymph (Calopteryx maculata), the adult of which is the black- winged 

 damsel-fly. 



Characters of the formation: The swift-stream formation has a 

 striking behavior character, namely, strong positive rheotaxis. Other 

 physiological characters, such as the toleration of only low temperatures 

 and high oxygen content, and the necessity for current for the successful 

 carrying-on of their building operations, are probably common to the 

 animals. So far as the fishes of the rapids are known, they breed on 

 coarse gravel bottom or under stones. The mores of the formation are, 

 then, current resisting and current requiring, dependent upon large 

 stones or rock bottom for holdfast and building materials. 



c) Sandy and gravelly bottom formation {pools) (Stations 15-22; 

 Tables XVII-XXV). — The pools of streams with characteristic forma- 

 tions are usually 2 or 3 to 10 feet deep, depending upon the size of the 

 stream. The bottom is sand or coarse gravel. In these we find condi- 

 tions very different from those in the rapids. The pools are the home 

 of the rock bass (Ambloplites rupestris), the small-mouthed black bass 

 (Micropterus dolomieu), the sunfishes (Lepomis pallidus and megalotis), 

 and the perch (Perca flavescens), together with a number of interesting 

 small fishes whose distribution is shown in Tables XXI and XXII 



(79> 9 2 )- 



With these are also the mussels (91), frequently as many as nine or 

 ten species, among which are Lampsilis luteola, ventricosa, and liga- 

 mentina, the little Alasmidonta calceola (Figs. 57, 58), and Anodontoides 

 ferussacianus (Figs. 59, 60), the last-named being perhaps the most 

 characteristic of them all. They are often found beneath the roots of 

 willows along the sides of the pools. Mr. Isely found that mussels 

 migrate to shallow water during flood time. Mussels are dependent 

 upon fish for a part of their lives. The young are carried by the adult 

 until ready to attach to the body of the fish (99). When they leave the 

 fish they are able to take care of themselves. Burrowing in the gravel 

 are bloodworms (Chironomus sp.) (95, 98), the burrowing dragon-fly 

 nymph (Gomphus exilis), a burrowing May-fly (Fig. 64a, p. 107), a caddis- 

 worm, and occasionally snails, Campeloma (Fig. 61 or 64^) and Pleuro- 

 cera (Fig. 64^). There are a few plants that grow on the sandy bottom 

 in such places, and among these one finds the snail {Amnicola limosa), 



