FUNCTION OF FATTY PIGMENTS 163 



have, however, suggested that they possess a definite 

 nutritive meaning. 



In these animals the colour of the skin is due, 

 as we have seen, to minute stellate masses of such 

 substances. These mobile pigment-stars form the 

 ' chromatophores.' But not only is the skin thus inter- 

 spersed, as it were, with pores, from which the pig- 

 ments overflow into channels or into which they retire, 

 leaving the channels clear and colourless, but a similar 

 system of pigmented holes and crannies traverses the 

 muscles, the digestive system, the nerves, and the 

 eyes. The whole prawn, in fact, is bathed within by 

 pigment veinings. Along the courses of these channels 

 lies a second series of tubes that contain, in green 

 specimens, minute grains of colourless fat, and if the 

 prawn be starved and kept in the light the fat does 

 not disappear. 



If, on the other hand, both food and light are 

 excluded, the prawns, having exhausted what food 

 they had to start with, absorb pigment and fat also ; and 

 if such lean specimens are taken out again into the 

 light at the end of a fortnight, they will in the course 

 of a single day show not only a fatty skin, but a far 

 denser accumulation of fat than is to be seen in a 

 fresh-caught prawn. This result points very strongly 

 to the conclusion that the red and yellow colouring 

 matters, even of such highly organised animals as 

 prawns, are, as in some plants, able to form fat out of 

 its simple elements, and are, therefore, no mere deco- 

 ration, but factories, under the shade of which the 



M 2 



