26 MARVELS OF FISH LIFE 



Looking for an explanation as to how the body of 

 the dace forms so perfect a mirror, we find it in the 

 fact that in the deep layers of the skin are numerous 

 mother-o'-pearl-like spicules. These, except on the back, 

 are welded together to form a continuous reflecting sur- 

 face. On the sides of the fish, in addition to this deep 

 reflecting surface, spicules are also present outside the 

 scales, and these acting like so many prisms, catch and 

 break up the light reflected from below, and thus give 

 the sides of the dace their silvery appearance. 



The glorious iridescence and changing colours, seen 

 to perfection in the mackerel, are due to these prism-like 

 spicules being present above the colour cells in the skin. 

 Glitter and iridescence is another concealing scheme of 

 nature, found amongst surf ace -swimming fishes, for 

 light falling on waves and broken water cause a 

 dancing shimmering glare below the surface, and a 

 silvery iridescent fish in these surroundings is difficult 

 to detect. 



We have considered the reflecting power of the dace. 

 What of the dark back, and how does this assist in con- 

 cealing the fish ? 



If the dace reflected equally well all round his body, 

 the sides and under-surface would reflect the dark water, 

 but the back reflecting the light from above would show 

 a silvery gleam, and at once make the fish conspicuous ; 

 so in the skin of the back we find numerous dark-colour 

 cells, with light reflecting spicules scattered among them. 

 These colour-cells, by contracting and relaxing, regulate 



