744 



THE EYE IN EVOLUTION 



The Biological Mechanism of Bioluminescence 



We have already noted the exploitation of the adventitious hght 

 produced by luminous bacteria which occurs in certain molluscs, crustaceans 

 and fishes ; these may be either symbiotic or parasitic in habit.^ Apart 



Fig. 896. — Scale-worm Attacked by a Crab. 



The rear half, used as a sacrificial lure, is brightly luminescent to attract the 

 attention of the crab, while the front portion ceases to luminesce and crawls away 

 in the shadow to reproduce a new tail (reproduced from Dahlgren, from a drawing by 

 Bruce Horsfall ; E. N. Harvey's Living Light). 



from these, animals produce biohuninescence in one of two ways — 

 either extracellularly or intracellularly. In unicellular organisms light- 

 producing granules are scattered throughout the cytoplasm, particularly 

 near the periphery, and on stimulation a glow passes like a wave through- 

 out the cell (Quatrefages, 1850 ; Pratje, 1921). In multicellular animals, 



UA^^^V^UJti^^^j^4^iiiiii 



Fig. 897. — Section of the Aboral Umbrella Surface of Pelauia .\oviiluca. 



Showing luminous cells, I, mucous cells, m, and cells with contents discharged, d 

 (modified from Dahlgren) 



1 p. 737. 



