THE GREAT DEEPS 125 



play of certain chemical processes in which 

 oxidation plays a central part. Incandescence 

 is light given off under the influence of great 

 heat, but animal luminescence is a " cold light " 

 with little or nothing in the way of heat rays. 

 In the cases which have been most studied, 

 the boring bivalve called Pholas, the lumi- 

 nous beetles called fire-flies, and the luminous 

 water-flea called Cypridina, there are always 

 two substances involved in the animal light. 

 There is a substance called luciferin, which is 

 oxidised, and there is a substance called luci- 

 f erase, which acts on its neighbour like a fer- 

 ment. Sometimes the light is given out by a 

 stuff manufactured in scattered or definitely 

 arranged glands, and then it may stream into 

 the water, or the whole clammy surface of the 

 animal may sparkle. In other cases, the light 

 is only seen inside special organs, the lumi- 

 nous organs, which are often very complex and 

 curiously like eyes. It is strange that organs 

 which produce light should sometimes show a 

 very striking resemblance to organs which de- 

 tect light, namely, eyes. If you say that it is 

 not so very strange, for the cat's eyes shine in 

 the dark, you are perhaps not altogether wrong, 

 for although the shining of the cat's eyes is 



