206 THE ANIMALS OF LAKES AND RIVERS 



are at aU favourable. During the warm season the 

 larvae of two flies, a Tipula and a Chironomus, also 

 occur in the shore waters, and a water-boatman (Corixa) 

 is also found in the lake. Otherwise it is singularly 

 barren of animal Hfe. 



Cave Faunas. As caves usually contain a consider- 

 able amount of water, something may be said here of 

 the more characteristic animals found in them. Cave 

 animals hve in conditions which ia many respects 

 resemble those prevailing in the ocean abysses. Thus 

 there is often complete darkness, and the temperature, 

 though it may be low, is remarkably uniform. Green 

 plants, also, are necessarily absent. Some of the charac- 

 ters of abyssal forms therefore reappear here ; for 

 example, the eyes tend to disappear or become minute. 

 But as the Mght conditions are quite different from 

 those which obtain in the ocean depths, where as we 

 have seen (p. 172) certain rays penetrate deeply, we 

 find that cave animals tend to be white, instead of 

 showing the deep-red or black coloration found in 

 many animals which live ia the ocean near the Hght 

 limit. 



An interesting example of a cave animal is Proteus 

 anguinus, found in the caves of Camiola, Dalmatia, 

 and Carinthia. This animal is one of the permanently- 

 gilled amphibians already described, and reaches a 

 length of about a foot. The skin is without pigment, 

 and the eyes are below the skin. The animal remains 

 permanently within the water, though it rises to the 

 surface to breathe if the water is not quite pure. 

 Curiously enough, if the white body is exposed to hght 

 pigment develops in the skin, and perfectly black 

 specimens can be produced by gradual exposure. In 

 the caves of Texas there is an allied form {TypMomolge 



