STRUCTURE OF LUMINOUS ORGANS 83 



fungi but Noctiluca and other animals are not known in 

 a non-luminous condition, although we can see no definite 

 value to the organism of this poY\/"er of luminescence. 



In the case of sea pens, however, we might suppose 

 that the light acts as an attraction to small organisms on 

 which the sea pen feeds, although these creatures only 

 luminesce when stimulated in some way, which rather de- 

 tracts from the above suggestion. 



(2) The light may act as a warning to scare away 

 predacious animals which would otherwise feed on the 

 luminous organism. Perhaps this is the case in the sea 

 pens, although these forms possess nematocysts which 

 should serve as adequate protection. The marine worm, 

 Chcetopterus, is brightly luminous and lives its whole life 

 in an opaque parchment tube. If this tube were torn open 

 by a predacious form we might conceive that the attacking 

 animal would be alarmed by the light and refrain from 

 destroying the worm. The ClicEtopterus, however, could 

 not rebuild another tube and its light would only protect 

 it in the night time. These cases mil suffice to indicate 

 the difficulties and perplexities of the' problem. Perhaps 

 we may add one more guess and suppose that the light of 

 certain fishes is actually for blinding or distracting their 

 enemies or blinding the forms on which they feed. Until 

 this use of luminous organs has actually been observed, we 

 can give little credence to it. 



(3) The light may serve as a means of recognition 

 or a sex signal to bring the sexes together for mating. It 

 would seem from the work of Mast and of McDermott that 

 this is the case in the common fireflies and it may be the 

 case in the toad-fish, Poricthys, which is only luminous 

 in the spawning season and in the worm, Odontosyllis, of 



