738 THE EYE IN EVOLUTION 



of animals which is excited only on stimulation, as a rule a bacterial or fungal 

 glow is continuous both by night and day so long as a supply of oxygen is 

 available ; but in Photoblepharoii, a littoral fish from the Banda Sea, the 

 luminous organ can be covered at will with an opaque shield, while in 

 another East Indian fish, Anomalojys, it can be everted or withdrawn into a 

 pouch beneath the eye where it is hidden from view so that the illusion of 

 intermittency is given (Figs. 884 and 885) (Hein, 1913 ; Harvey, 1940) ; as 

 these fish swim in large shoals they flash their lights at rhythmic intervals, 

 using them probably as a social signal. Again, infection of the Amphipod, 

 Talitrus, sand-fleas, squids and other organisms, with luminous bacteria 



Fig. 886. — Quatrefages's famous Figure of Noctiluca. 



Showing the u'l-egular distribution of luminescence and the points of light coming 

 from granules in the protoplasm (E. N. Harvey's Bioluminescence, Academic Press). 



makes their bodies glow ; while the pale luminescence of decaying fish or 

 meat is due to harmless organisms such as Microspira photogenica, Pseudo- 

 monas lucifera, or Micrococcus phosphoreus . It is this which causes the pale 

 glow of meat hanging in refrigerators or sometimes of dead bodies in the 

 dissecting room at night ; such a glow used to be a welcome sign in a pre- 

 Listerian surgical ward for these organisms were non-suppurative. 



Protozoa, however, are the most abundant source of this form of light, 

 for to them is largely due the " phosphorescence " of the sea. Much of this 

 is derived from the vast blankets of Radiolarians and Dinoflagellates, and 

 particularly the dinoflagellate, Noctiluca yniliaris} which make up a large 

 proportion of the planktonic fauna, particularly as they swarm in early 

 summer and multiply prodigiously in the autumn. These marine organisms 

 do not emit light unless at night and until the water in which they float is 

 disturbed, but in the darkness a broken surface glows with sheets of cold 

 fire and every wave -crest is aflame, while the tracks of the schools of fish 

 become streaks of molten metal (Fig. 886). "It is impossible to behold 

 this . . . wonderful and most beautiful appearance ... as if [the waters] 



1 The luminescence of Noctiluca formed the subject of the early classical paper by 

 Quatrefages (1850) and was extensively studied by Pratje (1921). See sketch, p. 179. 



