SEEN WITH THE EYE OF A FISH 29 



in making them inconspicuous, though even with these 

 fish reflection is an important factor. 



To appreciate how wonderfully the colour and mark- 

 ings on a fish conceal him, we must briefly consider the 

 colour cells themselves, how by their contraction and 

 relaxation a fish becomes light or dark, and how by an 

 alteration in the arrangement of these cells, fresh 

 patterns are made on the fish's body to hide him in 

 altered surroundings. 



Looking at the photograph of the skin of the stone 

 loach, we see black, orange and yellow pigment cells are 

 present. These cells have branching processes, and are 

 capable of rapid contraction and relaxation. The skin 

 and flesh of our bony fishes are colourless, and also 

 scattered between the pigment cells are light reflecting 

 spicules. Now look at the cells within the ring on the 

 illustration ; all are relaxed, and the space within the 

 ring appears dark. Should the cells in this ring contract 

 into little dots like those at point x, the colourless skin 

 would show through between the pigment cells, and the 

 space within the ring would become pale. 



We naturally ask ourselves what it is that makes 

 the pigment cells contract and relax ? Light is the 

 stimulus that causes the cells to contract ; in darkness 

 they again relax. Light, however, does not act directly 

 on the colour cells, but through the medium of the fish's 

 eye. To demonstrate this I divided a tank into two 

 chambers by means of a piece of linoleum, in which a 

 hole was cut large enough for a small pike to pass 



