COLORATION 225 



phores in certain regions and their comparative absence else- 

 where, or to their expansion in particular areas as contrasted 

 with their contraction in other parts of the body. 



But the colours are not all due to pigment, and the presence 

 of a peculiar reflecting tissue composed of structures known as 

 iridocytes also plays an important part. These are made up of 

 opaque crystals of a substance called guanin, owing its origin 

 to a waste product given off from the blood, whose chief 

 feature is the power of reflecting light. Both the chalky white 







ff$J3iaeA, cAfoyrta.tof>hore ^^0(jD 



^m^ JLeilou ckroma,tophore 

 G> Iridoei/te 



Fig. 90. 



The coloration elements in the skin of the upper side of a freshly killed Flounder 

 (Flesus), seen by transmitted light. 



and the bright silvery appearance of fishes are due to the manner 

 in which light is reflected from them, and by interference these 

 same colour elements are also responsible for the prismatic 

 hues and brilliant iridescence characteristic of so many fishes. 

 The relative abundance of either type of colour agent varies 

 greatly, not only in diflferent species, but in different parts of 

 the same individual, chromatophores being most abundant in 

 the dark back region, whereas, in the pale belly the iridocytes 

 play the chief part. 



Thus, the relative abundance of chromatophores and the 



