n THE NATURE OF ANIMAL LIGHT 



no development of luminosity along direct evolutionary 

 lines, although a more or less definite series of gradations 

 with increasing structural complexity may be traced out 

 among the forms with highly developed luminous organs. 



While the accompanying list of luminous genera aims 

 to be fairly complete, there are no doubt omissions and 

 some inaccuracies in it. Anyone who has ever tried to 

 determine what animal is responsible for the occasional 

 flashes of light observed on agitating almost any sample 

 of sea water will realize how difficult it is to discover 

 the luminous form among a host of non-luminous ones, 

 especially if the animal is microscopic in size. It is not 

 surprising, then, to find many false reports of luminous 

 animals in the literature of the subject and we cannot 

 be too careful in accepting as luminous a reported case. 

 The difficulty lies chiefly in the fact that all luminous 

 organisms with the exception of bacteria, fungi, and a 

 few fish, flash only on stimulation, and, while it is easy 

 enough to see the flash, the animal is lost between the 

 flashes. The only safe way to detect luminous organisms 

 is to add a little ammonia to the sea water. This slowly 

 kills the organisms and causes any luminous forms to 

 glow with a steady, continuous light for some tune, a 

 condition accompanying the death of the animal. Not 

 all observers, however, have followed this method. One 

 must always be on guard against confusing the light from 

 a supposed luminous form with the light from truly 

 luminous organisms living ujDon it. The reported cases of 

 luminosity among marine alg^ are now known to be due 

 to hydroids or unicellular organisms living on the alga. 



We know also that many non-luminous forms may 

 become infected with luminous bacteria, not only after 



