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 
