v THE COLOURS AND PIGMENTS OF CCELENTERA 91 
spectrum, they exhibit a lack of definiteness and purity 
of tone ; in dilute solutions chaetopterin is a delicate 
pure green, but when concentrated it becomes muddy 
and shows strong red fluorescence. It seems not un¬ 
reasonable to suppose that this peculiarity will affect 
the nature of the tint in the living organism. Thus 
if the same pigment occurs in very delicate trans¬ 
lucent tissues, and in denser and more opaque ones, 
it seems probable that the characteristic differential 
absorption will result in striking difference of tint in 
the two cases. May not this account for some of 
those marked differences in colour between old and 
young parts of a colony, between polypes and “ coral,” 
between tentacles and body, and so on, upon which 
we have already dwelt ? 
In the second place, the pigments of the chaeto¬ 
pterin group are remarkable for their extreme sus¬ 
ceptibility to the action of reagents, especially to 
acids and alkalies. Acids produce remarkable varia¬ 
tions in tint corresponding in part to new pigments ; 
it is probable that a considerable number of colouring- 
matters can be produced by the action of different 
reagents. Now Krukenberg noticed that the colour 
of sea-anemones varied according as they secreted a 
tryptic ferment (i.e. one which acts in an alkaline 
medium) or a peptic one (acting in an acid medium); 
it seems therefore probable that the colour in any 
given case will depend upon the reaction of the 
surrounding tissues. This may again account for 
some of the local variations in tint, while it is not im¬ 
probable that the vivid green colour so often seen may 
be due to a pigment derived from the pigment of the 
“ yellow cells.” Chaetopterin and enterochlorophyll 
