22 
COLOUR IN NATURE 
CHAP. 
which are placed near the ends of the fine tracheal 
capillaries. The substance of the luminous organ 
is made up of parenchyma cells, most easily perceived 
in the transparent layer. The result of his examina¬ 
tion convinced Emery that the luminous organ was 
a specialised portion of the fatty body, the arrange¬ 
ment of the tracheae being one of the most marked 
indications of specialisation. 
His explanation of the luminosity is as follows: 
the parenchyma cells (fatty cells) secrete the luminous 
substance, from them it is taken up by the tracheal 
end-cells, and, being here exposed to the action of 
the oxygen of the tracheal system, undergoes a pro¬ 
cess of oxidation resulting in the formation of light. 
This process can only occur in regions where the 
chitinous lining of the tracheae is very thin, as in the 
capillaries ; the free branching of the slender tracheal 
capillaries in the luminous organs especially provides 
for this. In a later paper Emery is inclined to lay 
more stress upon the parenchyma cells than upon the 
tracheal end-cells as the seat of luminosity. The 
luminosity is then due to the oxidation (or combus¬ 
tion) of a probably useless body stored in the cells of 
the organ (the fatty body). To this description we 
may perhaps add that recent investigations tend to 
emphasise the importance of the fatty body of insects 
as an organ connected with excretion. 
As to the nature of the light emitted by the 
different phosphorescent organisms, the observations 
are not sufficiently numerous to draw any conclusions. 
Moseley found that the blue and violet rays were 
absent in the light of three deep-sea Alcyonarians, 
while surface forms displayed light of various colours ; 
