SELECTION OF A WATER 309 



fresh-water shrimp, and various mollusks, such as 

 snails, are most abundant. The ribbon-weed {Sparga- 

 mum ramosum) is generally covered with great 

 numbers of larvae and pupae of Simulium and other 

 smuts. 



Many other aquatic plants may be found, such as 

 the so-called American weed [Elodea canadense), the 

 various water-lilies, horn wort {Cera'tophyllum demer- 

 sum), which is commonly called squirrel's tail, but the 

 majority of these do not contain any great quantity of 

 the various forms of subaqueous animal life on which 

 the trout and grayling subsist. One of the water- 

 mosses {Fontinalis antipyreticd) establishes itself on 

 boulders in rapid parts of some chalk-streams. It 

 harbours a considerable number of fresh-water 

 shrimps and insect larvae, but trout generally do not 

 care to lie on it, so that it is often desirable, although 

 expensive, to remove it forcibly. 



Some quantity of mud will always be found in 



comparatively still and deep 

 Mud. reaches, but the foul, black 



foetid sort which is the ultimate 

 product of the decomposition of vegetable matter 

 (usually more or less polluted by sewage), beyond 

 holding a few water woodlice {Asellus), blood-worms, 

 which are the larvae of Ckironomus, water-bugs, such 

 as Corixcs, and alder larvae [Sialis lutaria), is not of 

 much use in respect to harbouring food for the fish. 

 The pale- coloured, sandy, gritty mud found in parts of 

 the chalk-streams is the home of the mayfly nymphs. 



