68 THE NATURE OF ANIMAL LIGHT 



In general, we may divide luminous organisms into 

 two great classes according as the oxidizable material is 

 burned within the cell where it is formed or is secreted 

 to the exterior and is burned outside — intracellular and 

 extracellular luminescence. Many animals with intra- 

 cellular luminescence have quite complicated luminous 

 organs. It is an interesting fact that a great similarity 

 may be observed between the evolution of the complex 

 organs of vision and of these complicated organs. In the 

 simplest unicellular forms certain structures within the 

 cell serve as the photochemical detectors of light, while 

 in luminous protozoa, similarly, granules scattered 

 throughout the cell are oxidized with light production. In 

 the higher forms the eye contains groups of photosensitive 

 cells connected with aiferent nerves, lenses, and accessory 

 structures for properly adjusting the light, while lumi- 

 nous organs contain groups of photogenic cells in con- 

 nection with efferent nerves, lenses, and accessory struc- 

 tures for properly directing the light. It is interesting 

 to note that in the two groups where the eye has attained 

 its highest development, the cephalopods and vertebrates, 

 here also the luminous organ is found in greatest complex- 

 ity and perfection. In intermediate stages of evolution 

 the eye and luminous organ so closely approach each other 

 in structure that it is still a mooted question whether cer- 

 tain organs found in worms and Crustacea are intended 

 for receiving or producing light. 



We may also divide luminous forms into two groups 

 according as the oxidation of luminous material goes on 

 continuously, independently of any stimulation of the 

 organism ; or is intermittent, oxidation and luminescence 

 occurring only as a result of stimulation, using the word 



