{167}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, polygonal, or stellate bodies, or in flattened plates. Considered as an agent in the production of colour, the chief feature in the iridocytes is their opacity and great reflecting power; and according to the way in which light is reflected from them, the result may be a chalky white or a bright silvery appearance. By interference these colour elements are also responsible for the 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,"[128] 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.[129] [Illustration: FIG. 94.—The coloration elements in the skin of the upper side of a freshly-killed normal Flounder (_Pleuronectes flesus_), seen by transmitted light. The stellate black bodies are the black chromatophores; the grey bodies of similar shape represent the yellow chromatophores; and the small grey plates the iridocytes. (From Cunningham and MacMunn.)] 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 underlying muscles; and although the two kinds of coloration elements may be present in both layers, their relative {168}abundance varies in different Fishes, and in different parts of the surface of the same Fish. Where chromatophores are most abundant, usually on the back, the reflecting tissue is relatively scanty, and _vice versâ_. On the sides and belly of a Fish the place of the inner layer of the dorsal surface may be taken by the "argenteum." This layer is devoid of chromatophores, and consists of reflecting tissue in which the iridocytes form a continuous stratum, either in the form of granules, or as a close network of rod-like bodies or of polygonal plates in contact with one another, instead of being less numerous and more scattered as on the back. When iridescence is produced, it is due to the iridocytes of the outer layer of the skin; the dead whiteness and silvery lustre, on the other hand, have their origin in the different ways in which incident light is reflected from the inner layer or argenteum.