1 82 THE SEAS 



them by this cloak of invisibility is very great, and as they 

 are not very active animals and enemies are numerous 

 in the shore waters where they live, they possibly owe 

 their survival to the fact that they are able to escape any 

 but the most careful scrutiny. 



Anyone who has ever examined a living squid, or one 

 that has only very recently died, cannot fail to have been 

 impressed by the wonderful play of colour which sweeps 

 over the torpedo-shaped body which blushes with delicate 

 colour, and then as quickly blanches. In this case also the 

 colour is controlled by tiny pigment cells, or " chromato- 

 phores," in the skin, but these are of quite a different nature 

 from those of the .ZEsop prawn. Each one contains one colour 

 only and this is not spread into branches or withdrawn 

 according to whether or no it is needed, but instead the 

 whole pigment cell is expanded or contracted by tiny 

 muscles attached to the corners and in this way the area 

 displaying that particular colour is increased or decreased 

 (Plate 74). In this case the value of the colour change is far 

 from obvious as the squids are swiftly moving animals which 

 live near the surface of the sea with no darker background 

 with which they might match their bodies. As in the 

 allied cuttle-fish and octopus, changes of colour appear to 

 be, in a sense, emotional, for the animals become suffused 

 with colour at the sight of food. 



Many fish have the power of colour change. Especially 

 is this the case in the flat-fish, such as the sole, turbot, 

 plaice or flounder, which live on the sea bottom and are 

 liable to be attacked from above. If they are examined 

 it will be seen that, whereas the under side of the body which 

 lies on the bottom is unpigmented, the upper surface is 

 mottled grey and brown so as to provide an almost perfect 

 match with the background of mud, sand, or gravel (Plate 

 67). Here again pigment cells in the skin are responsible 



