LUMINOUS GLANDS 291 



usually answered by pointing to the liability of flat fish lying 

 on the bottom of the sea to become resting-places of para- 

 sites, corallines, and other fixed growths. Very mild shocks 

 would suffice to disturb the peace of would-be settlers In 

 the same way, the electric organs of fresh-water fish may, 

 when rudimentary, have protected the skin from invasion by 

 moulds. 



LUMINOUS GLANDS. 



If it be difficult, when considering the dispersal of energy as 

 mechanical work, heat, or electricity, by living tissues, to 

 bring the phenomena into line with those of which physics 

 takes experimental cognizance, how are we to approach the 

 problems involved in the generation of light ? Yet the 

 photogenic property of protoplasm is widely distributed. 

 Protozoans and various other invertebrate animals cause the 

 so-called phosphorescence of the sea. The abysmal depths of 

 ocean are lighted by forests of luminous polyps, and traversed 

 by fishes whose heads are furnished with lamps. By her own 

 light the female glow-worm enables her winged mate to keep 

 his tryst. Fireflies (Lampyrus) flash amongst the orange- 

 trees of Italy, and blaze (Pyrophorus) beneath the mangoes 

 of Ceylon. 



Luminous organs vary too widely in structure to allow 

 us to pick out, as in the case of electric organs, the features 

 which are common to them all. In Pyrophorus the organ is 

 a double mass of cylindrical cells near the tip of the abdomen. 

 The cells are set vertically to the surface, and are supported 

 by a tubular membrane. Their substance contains a kind of 

 fat. Beneath them there is a layer of cells, not luminous, but 

 evidently a part of the photogenic apparatus, containing 

 chalky granules. The organ is well supplied with nerves 

 and with respiratory tubes (tracheae). 



More interesting than its structure is the study of the peculiar 

 character of the light which the organ emits. It gives a 

 spectrum which extends from the red (beyond Fraunhofer's 

 line B of the solar spectrum) to the first blue rays (F). It 

 shows no lines. Green rays appear only when the light is 

 bright, and then they are the brightest of all the rays. The 

 light is practically destitute of actinic or chemical rays. A 



192 



