192 COLOUR IN NATURE chap. 



digestive gland and its secretion, intermingled with 

 the contents of the gut, and finally in unaltered form 

 in the faeces. It thus acts like a true bile pigment. 

 We have already frequently spoken of the peculiar 

 group of pigments to which enterochlorophyll belongs, 

 and of the facility with which they can be made to 

 yield bright -coloured derivatives. It is evidently 

 here a useless substance, and it is very probable that 

 in some cases, instead of being eliminated unaltered 

 in the faeces, it may undergo modification into other 

 pigments which may be deposited in the mantle or 

 shell. We have already noticed such a suggestion 

 in the case of Acmaa, and it is possible that many 

 of the bright pigments of Mollusca arise in this 

 way. 



The yellow, orange, or black pigments of naked 

 forms such as the slugs, Doris, etc., are probably 

 due either to lipochromes or the dark - coloured 

 " melaines." 



It seems probable that the pigments of Cephalo- 

 poda are chiefly of the dark - coloured nitrogenous 

 type, though they have not been fully investigated. 

 The beautiful changing colours are in large part due 

 to the movements of the chromatophores, which as 

 they expand or contract alter the whole appearance 

 of the animal. So long ago as 1852 Brucke watched 

 the colour-changes of Sepiola rondeletii, and noting 

 how the tints varied in the order pf the spectrum 

 from blue to green, yellow, and red, came to the 

 conclusion that the colours were optical. He thought 

 that they were due to the colours of thin plates, but 

 according to Krukenberg they are due to fine ridges 

 in the surface of the cells. 



