LIFE IN GREAT DEPTHS 153 



These crumbs obviously must get scarcer and 

 scarcer the deeper they sink, since they are avidly con- 

 sumed by marine bacteria and by pelagic animals living 

 in great depths. Only over the bottom of shallow seas 

 is organic detritus from the uppermost water strata 

 profusely scattered, giving rise to an abundant fauna 

 of bottom-livino; organisms, the bottommost bein^ mud- 

 eaters which burrow into the upper sediment layers; 

 yet the presence in great depths of invertebrate animals 

 and fish proves that some means of subsistence actually 

 exists there. The invertebrates living on the abyssal 

 plain, especially crustaceans, worms, mussels, and 

 echinoderms, appear to be the main supporters of the 

 bottom-living fish. (See also page 163, below.) 



The absence of daylight naturally has a profound 

 influence on deep-sea fish, many of which have only 

 rudimentary eyes or even lack eyes altogether. Hence 

 it seems rather surprising that among bottom-living fish 

 bhndness is not the rule. Many of them have large, 

 well-developed eyes, proving that they are capable of 

 reacting to hght. 



Whence comes this light of the abyss? The answer is 

 simple: from the animals themselves, a high percentage 

 of which are provided with special light-emitting organs 

 or are capable of giving oft' secretions which emit a 

 diffuse light. The feeble light existing in great depths 

 is thus animal light. It is produced by a highly interest- 

 ing process of chemicoluminescence caused by the 

 oxidation of a substance called luciferine, a process 



