PRESERVING WATER ANIMALS 



ed several times. If plankton organisms are abundant they 

 will settle to the bottom of the dish as a brownish or greenish 

 sediment (Fig. 31, 4). 



The bottom. — On the pond bottom, in the mud or sand, are 

 mussels, crayfish (in limy regions), an occasional turtle, mud- 

 loving dragonfly nymphs, small mayflies like Ccenis and 

 Tricorythus, big burrowing ones — Ephemera, Hexagenia — and 

 a host of midge larvse and small worms such as Tubifex (Fig. 

 32). 



Fig. 32. — A netful of mud with its inhabitants: i, 

 Ccenis; 2, Tuhifex; 3, worm, Nais; 4, Tricorythzis; 5, 

 mud-tubes of midges; 6, nematode worm; 7, snail. 



Scoop up the mud and trash in small lots with a strong- 

 rimmed water-net. Let the load drain till the animals begin 

 to clamber; then pick them out and drop them in a white 

 dish for examination (p. 27). 



Slow streams. — The population of small coves and the 

 side waters of streams is similar to that near the pond shore 

 (PL II). Scoop and sweep with a water-net as in the ponds 

 but work upstream so the current may take away the muddy 

 water. 



Riffles and rapids. — Algas and water mosses on the stones 



33 



