THE GREAT DEEPS 125 



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 luminous 

 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 luciferase, which 

 acts on its neighbour like a ferment. Some- 

 times the light is given out by a stuff manu- 

 factured 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 luminous 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 detect 

 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 



