STRUCTURE OF LUMINOUS ORGANS 81 



also very complicated. The latter must have evolved 

 from the former, although it is not always possible to point 

 out the intermediate stages. It is not within the scope 

 of this book to discuss bioluminescence in its evolutionary 

 aspects. It may be worth while, however, to point out 



Fig. 29. — Sectional view of photogenic organ of a aguid, Abraliopsis (after Chun.) 

 refill refi^, reflectors; Zac, lacunar spaces; chr.t pigment screen of chromatophores; chr.^ 

 chromatophore; phot^ photogenic cells; I, lens; co., cuticle; v, blood vessel; fibr.t connective 

 tissue. 



briefly what is known concerning the use of the light to the 

 animal. There are four possibilities. 



(1) The light may be of no use whatever, purely for- 

 tuitous, an accompaniment of some necessary or even un- 

 necessary chemical reaction. 



This appears to be the case in the luminous bacteria 

 and fungi and perhaps the great majority of forms which 



