FISHES 211 



afforded by a black chromatophore. It seems probable, 

 although Mr. Holt does not say so, that these bean-shaped, 

 or, as he calls them, " prismatic bodies," are developed from 

 the network of granular matter present in the white bands 

 of the young. Each prismatic body consists of a number of 

 minute rod-like crystals. Probably the blue colour is due to 

 the interference of the light rays reflected from the prismatic 

 bodies, the red and yellow rays of light being extinguished. 

 It is impossible, however, with our present knowledge, to go 

 into this question. The important point is, that the blue 

 colour appears to be produced by a deposition of guanin in 

 particles of a certain shape. 



On the first dorsal fin in the adult male there are no blue 

 bands, but white ones corresponding to them. The white 

 appearance is due to a network of reflecting matter similar 

 to that of the blue in the second dorsal, but the prismatic 

 bodies are considerably smaller, and there are few black 

 chromatophores. This shows clearly that the blue colour is 

 due to the special state of aggregation of the guanin and the 

 presence of black chromatophores. Physiologically, there- 

 fore, the blue in the adult male is produced by the deposition 

 of guanin at the surface of the skin. 



The blue and yellow of the sides of the body are produced 

 in the same way as in the dorsal fins, but there are slight 

 differences. In the blue bands black chromatophores are 

 often abundant, and in the same plane, to some extent 

 superficial to the chromatophores, is a layer of prismatic 

 bodies, more abundant, and therefore less reticulate in 

 arrangement than in the second dorsal fin. In addition, 

 there is another layer of black chromatophores and prismatic 

 bodies at the inner surface of the skin near the flesh, the 

 latter overlying and between the former. The blue bands on 

 the body are capable of sudden intensification, and Mr. Holt 

 considers this is due to the expansion of the black chromato- 



