LUMINOUS ORGANS 



747 



as we have already seen.^ that a central nervous control is made manifest 

 in many species by the presence of a diurnal rhythm, whereby the 24-hour 

 phase of luminescence persists even if the animal is kept in continuous 

 darkness for some time (the jellyfish. Pelagia — Heymans and Moore, 1924 ; 

 the fire-fly, Photinus — Buck, 1937 ; the balanoglossid, Ptychodera — 

 Crozier, 1920). 



The Chemical Mechanism of Bioluminescence 



Despite the expenditure of much study and speculation since the time 

 of Aristotle, the intimate chemical nature of bioluminescence is not yet 



N 

 7" C - , 



ECN -- 

 H - 





Fig. 900. — Cross-section of the Light Organ of .an Insect. 

 The light organ of the adult Photurus pennsylvanica. C, cuticle ; ECX, nucleus 

 of tracheal end-cell ; H. hypodermis ; X, nucleus of photogenic cell ; P, photogenic 

 layer ; R, reflector layer f T, trachea ; TC, tracheole (W. X. Hess, J. MorphoL). 



clear. The process is the reverse of a photochemical reaction wherein the 

 absorption of light induces chemical activity ; here the energy derived from 

 a chemical reaction is converted mto light. Such a chemical reaction is 

 oxidative in nature and converts a substance into an activated state in 

 which it can emit light as it lapses again into the non-activated state. The 

 occurrence of chemiluminescence in the inanimate world has long been 

 known ; it is shoA\ii. for example, by phosphorus - and a multitude of organic 



1 p. 21. 



2 PHOSPHORESCENCE, properly defined, is a delayed fluorescence, fluorescence occurs 

 when a substance, on radiation, emits light of a waye-length differing from the incident light. 

 The incident light is absorbed by molecules which are thereby changed into an actiyated form ; 

 these return to their original state giying off energy as they do so ; this energy, being absorbed 

 by other molecules capable of radiation, is emitted as fluorescent light. By delaying the energy 

 transfer, the emission of light occurs sometime after exposure as phosphorescence. The 

 commercial sulphides of Ca, Ba and Sr possess the property of phosphorescence and are used 

 in luminous paints. 



