THE WONDER OF LIFE 489 



account and made vitally impoitant, so it may be with 

 the production of light. 



The facts in regard to luminescence in organisms raise 

 many unanswered questions and demand further investiga- 

 tion, but there is no particular difficulty about the bare 

 fact that hght is produced in laboratories which give off 

 heat, as we aU experience, and generate electricity, as we 

 may experience on close acquaintance with the Electric 

 Eel or the Torpedo. 



A noteworthy fact in regard to luminescence is its wide 

 distribution in the sea. When the oars drip sparks on a 

 summer night we see the luminescence of Noctiluca ; and 

 there are many pelagic animals — some Radiolarians, some 

 Medusae, most Ctenophores, some ' worms ', many crus- 

 taceans, a few molluscs, Tunicates hke the splendid Pyro- 

 soma, and various surface fishes — ^which are luminescent. 

 In the shore area there are luminescent Echinoderms, 

 especially Brittle-stars ; the boring Tholads — ^with their 

 miners' lamps ; and various members of the great alhance 

 of Stinging Animals or Ccelenterates. In the great abysses, 

 as we have noticed, luminescence is common, e.g. among 

 Alcyonarians, Medusae, Echinoderms, Crustaceans, Cuttle- 

 fishes, and true fishes. On land we know it best in glow- 

 worms, fiie-flies, and other insects, but it also occurs in 

 some Myriopods (e.g. Geofhilus electricus), and in some 

 earthworms (e.g. Photodrilus). It is usually said to be 

 quite absent in freshwater animals, but we have some 

 suspicions as to the accuracy of this generahzation, bearing 

 in mind Nelson Annandale's freshwater Lampyrid and 

 allegations of luminescent Chironomid larvae. That there 

 are phosphorescent Bacteria is well known — every one can 

 see them on fish hung up to dry — ^and it is probable that 



