LUMINESCENCE IN THE SEA 229 



is the product of a special gland or of definite organs which 

 may have a complicated structure comparable mechanically 

 to an eye or a bull's-eye lantern, and where the emission 

 of light is a direct response to special stimulation (as in 

 higher Crustacea and fishes) utility must be assumed ; and 

 the different colours and intensities of the light produced, 

 the different forms and situations of the glands or photo- 

 spheres and the different light schemes or patterns all suggest 

 that the use is not one and the same in all cases, but may 

 differ widely in the different luminous organisms. 



In stating these uses we are on somewhat uncertain 

 ground. Much experimental evidence is necessary, such as 

 can only be obtained on oceanographic expeditions and by 

 observations on the living organisms at biological stations. 

 But it seems probable (1) that luminous lures such as are 

 seen on some fishes may serve as an attraction or bait for 

 prey ; (2) that some photospheres may be recognition marks 

 for the attraction of other individuals of the same species 

 for mating or other purposes ; (3) that the sudden flashing 

 of light may be a protection of an alarming or warning 

 nature to enemies, like the brilliant colours and threatening 

 attitudes of some land animals (possibly the warning may 

 be an indication of a distasteful animal to be avoided as 

 food) ; (4) that the luminous clouds of secretion sometimes 

 emitted may distract an enemy and allow an active Copepod 

 to escape ; (5) that a detached luminous fragment cast off 

 from the body may be a " sacrifice lure " to deceive the 

 enemy ; (6) that in the case of some stationary animals 

 where the nutrition depends upon ciHary currents or upon 

 waving tentacles, the light may attract swarms of minute 

 organisms which can then be captured as food ; and (7) 

 that in the case of predaceous animals prowling about the 

 dark sea-bottom, lights on the head, near the eyes or on the 

 lower surface of the body may be of use for general illumina- 

 tion of the abysses in the constant search for food. 



The various cells, tissues and organs that give rise to 



