VI COLORATION G7, 
Tridocytes consist of guanin, which, in its chemical reactions, 
closely resembles the guanin obtained from guano, and therefore 
is to be regarded as a further illustration of the utilisation of 
waste excretion products for the production of colour in animals. 
In forming iridocytes the guanin is deposited in the shape of 
eranules, or of rounded, polygonal, or stellate bodies, or in 
flattened plates. Considered 
as an agent in the produc- 
tion of colour, the chief 
feature in the iridocytes is 
their opacity and great re- 
flecting power ; and accord- 
ing to the way in which 
light is reflected from them, “s* 
the result may be a chalky 
white or a bright silvery 
appearance. By  interfer- 
ence these colour elements 
are also responsible for the Fic. 94.—The coloration elements in the skin of 
: . the upper side of a freshly-killed normal 
p rismatic colours and Flounder (Plewronectes flesus), seen by trans- 
brilliant iridescenece which mitted light. The stellate black bodies are 
; aes the black chromatophores ; the grey bodies 
so many Fishes exhibit. of similar shape represent the yellow chro- 
The optical properties of matophores ; and the small grey plates the 
; : z iridocytes. (From Cunningham and Mac- 
guanin has led to its use in ~— Munn.) 
the manufacture of artificial 
pearls. “Essence d’orient,” or “blanc d’ablette,”’ from which 
these pearls are made, principally in Paris, is obtained from the 
scales of the Bleak (Alburnus lucidus), and is really the guanin 
of which the iridocytes of this Cyprinoid are composed. It is 
also to the presence of crystals of guanin that the beautiful 
metallic lustre of the iris in many Fishes is due.” 
The chromatophores and iridocytes are chiefly disposed in two 
layers in the skin, one outside the scales and the other on the 
inner surface of the scales, between the latter and the under- 
lying muscles; and although the two kinds of coloration 
elements may be present in both layers, their relative abund- 
<a) 
Qh o> 
a) v 
es 
1 Ablette is the French name for the Bleak. 
? Either singly or in combination with lime (Guaninkalk), guanin is often present 
in the tissues of Fishes (air-bladder, gall-bladder, subcutaneous connective tissue, 
muscle-fasciae, peritoneum, and the retinal epithelium and tapetum of the eye). 
For references see Cunningham and MacMunn, op, cit. p. 781 et seq. 
