II THE PIGMENTS OF ORGANISMS 45 



therefore only in rare cases of direct importance in 

 coloration. Of this group, " enterochlorophyll," 

 bonellin, and chEtopterin are good examples ; the 

 two latter give rise to external coloration in the 

 forms in which they occur. We shall make frequent 

 reference to these pigments in the course of the 

 following chapters. 



The occurrence of green colour in organisms from 

 which yellow lipochromes only can be extracted, is 

 an exceedingly common phenomenon, which has been 

 observed in many different animals. The green 

 colour is doubtless due in many cases to the structure 

 of the coloured parts ; in others it may be due to a 

 combination of the lipochrome with a base ; there is 

 little doubt that the green colour of many Crustacea 

 is in part due to this cause. Apart from chlorophyll 

 and lipochrome combinations, green pigments occur 

 freely in many Ccjelentera, in many worms, and in 

 some Mollusca; in these cases they are usually soluble 

 in alcohol and often give banded spectra, but most 

 have been very imperfectly investigated. In Verte- 

 brates green pigments are almost absent ; as in the 

 case of blue pigments, the most marked exception is 

 probably the green pigment which colours birds' eggs, 

 though according to Sorby the green colour is here 

 produced by a mixture of blue and yellow. As a 

 colour, green in animals is most abundant among 

 those inhabiting trees or herbage, but is here very 

 frequently structural. In marine animals it seems to 

 occur in large masses chiefly in coral-reefs, where the 

 corals are often largely green. In these situations 

 green plants are relatively rare, and accordingly the 

 green colouring-matter has been supposed by Dr. 



