COLORATION 



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Iridocytes 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 

 granules, or of rounded, polygo^, 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, 

 the result may be a chalky 

 white or a bright silvery 

 appearance. By interfer- 

 ence these colour elements 

 are also responsible for the Fig. 94 

 prismatic colours and 

 brilliant iridescence which 

 so many Fishes exhibit. 

 The optical properties of 

 guanin has led to its use in' 

 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 tlie 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- 



' Ahlette 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-hladder, gall-bladder, subcutaneous connective tissue, 

 muscle-fasciae, peritoneum, and the retinal epithelium and tapetum of the eye). 

 Fop references see Cunningham and MacMunn, op. cit. p. 781 et seq. 



■The coloration elements in the skin of 

 the upper side of a freshly -killtd normal 

 Flounder {Pleuronectes fiesus)y see^by trans- 

 mitted light. The stellate black bodies are 

 the black chromatophores ; the grey bodies 

 of similar shape represent the yellow chro- 

 matophores ; and the small grey plates the 

 iridocytes. (From Cunningham and Mac- 

 Munn.) 



