170 THE OCEAN 



lates, coelenterates, crustaceans, cephalopods, 

 tunicates and fishes. The fact that some 

 creatures have this power and others have 

 not, and the distribution of phosphorescent 

 organisms as regards depth, are among those 

 puzzHng phenomena with which the marine 

 biologist has to deal. The development of 

 phosphorescent organs appears to be greatest 

 in warm water, but it is also found in cold 

 waters, as in the case of Peridinium and 

 Nyctiphanes. 



Sometimes the light is due to the secretion 

 of a slimy luminous substance, which may 

 cover the entire body ; sometimes it proceeds 

 from a nucleus ; sometimes it is produced by 

 special light-organs more or less complicated 

 in structure, from a simple luminous cell in 

 the epithelium to complex glandular bodies 

 having a lens and reflector, functioning some- 

 what after the manner of a " bull's eye 

 lantern." 



Development of Light-Organs. — Light-organs 

 occur mainly in pelagic animals and, as regards 

 the fishes, are characteristic of those forms 

 living in depths down to 300 fathoms. The 

 size of the light-organs seems to correspond 

 with different depths, the larger organs being 

 found in fishes living in the sub-surface waters. 

 Thus of the six species of Cyclothone five live 

 in deep water, and these are black and have 



