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 to 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, 92). 
With these are also the mussels (gt), 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 64c) and Pleuro- 
cera (Fig. 64d). There are a few plants that grow on the sandy bottom 
in such places, and among these one finds the snail (A mnicola limosa), 
