FIELD BOOK OF PONDS AND STREAMS 



lighted bottom which mud dredgings show to be but sparsely 

 inhabited by a few small worms, mussels, and little red midge 

 larvae. 



In spite of their size large streams do not harbor so many 

 nor such interesting forms as the smaller ones; this is equally 

 true of large rivers as compared with small ones. Rivers 

 travel far, often through soft soil, and their waters gather 

 sand and silt only to drop them again along the way, smother- 

 ing fragile plants and animals beneath them. Only in sluggish 

 rivers or in the inlets of swift streams is there much shore 

 vegetation where the smaller clamberers can hide. With 

 all its uncertainties in drought and flood time, the small 

 stream still has greater advantages, for it is well lighted even 

 on the bottom, and its waters are generally free from mud. 



Fig. 22. — Diagram showing the distribution of ani- 

 mals across a slow stream. Plants — i, arrowhead; 2, 

 spatterdock. Animals — 3, dragonfly nymph; 4, tube 

 of midge larva; 5, tube of bloodworm; 6, mud-leech; 

 7, scud; 8, burrowing mayfly, Hexagenia; 9, white 

 clam; 10, whirligig beetle; 11, water boatman; 12, 

 water strider; 13, mayfly, Callibcetis; 14, caddis worm; 

 15, snail. 



■ Slow streams and marshes (Fig. 22). — A slow stream flows 

 gently and steadily over flat land; its bottom is of fine mud, 

 sometimes of sand; there are no rocks, it has no waterfalls or 

 rapids, and its waters are generally free from mud. Now and 

 then there are inlets crowded with cat-tails, reeds, pickerel 

 weed, and in some regions with Vv'ater-hyacinth (PI. II). 



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