AND OF LIGHT. 251 



one of the most interesting is Proteus anguineus, which 

 is found in the subterranean caves of the Karst Moun- 

 tains about Adelsberg. This amphibian is almost 

 white, but if kept for some time in the light, it gradu- 

 ally becomes pigmented. Pigment cells are, in fact, 

 still present in its skin,* and in all probability these are 

 directly stimulated to exert their function by the action 

 of the light. 



A similar effect of exposure to light has been 

 demonstrated by Cunningham f for the under sur- 

 face of the flounder (Pleuronectes flesus). This surface 

 is normally quite white, but by keeping young flounders 

 for nearly four months in a glass dish illuminated 

 from beneath by a mirror placed at a proper angle, 

 Cunningham found that 10 out of the 13 specimens ex- 

 perimented with developed black and yellow chromato- 

 phores. Three of the specimens showed well-developed 

 bands of pigment, similar to those of the upper side, 

 over the area occupied by the muscles of the longi- 

 tudinal fins. Subsequently, Cunningham and Mac- 

 Munn $ succeeded in keeping flounders alive under 

 these conditions of illumination for from 9J months to 

 nearly two years. They found that the amount of pig- 

 ment steadily increased with the duration of the ex- 

 posure, so that ultimately almost the whole of the lower 

 side might become pigmented. This colouration was 

 (histologically) of exactly the same kind as that of the 

 upper side in normal specimens, though it was never 

 by any means so marked. Its production is more re- 



* Vide Poulton's " Colours of Animals," p. 91. 

 f Zool. Anzeiger, xiv. p. 27, 1891. 

 \ Phil. Trans. 1893, B. p. 765. 



