The Alcyonaria exhibit a great variety of COLOUR. Very little is known at present of the chemistry of the various pigments found in the group, but they may conveniently be arranged in two sections, the soluble pigments and the insoluble pigments. To the former section belong various green and brown pigments found in the anthocodiae and superficial coenenchym of many genera. These are related to chlorophyll, and may be very largely the product, not of the Alcyonarians themselves, but of the {338}symbiotic "Algae" (cf. p. 261) they carry. A diffuse salmon-pink colour soluble in spirit occurs in the living _Primnoa lepadifera_ of the Norwegian fjords, and a similar but paler pink colour occurs in some varieties of the common _Alcyonium digitatum_. Gilchrist[366] states that when he was preserving specimens of _Alcyonium purpureum_ from Cape waters a considerable quantity of a soluble purple pigment escaped. But the predominant colour of Alcyonarians is usually due to the insoluble pigments of the calcareous spicules. These may be of varying shades of purple, red, orange, and yellow. The colours may be constant for a species or genus, or they may vary in different specimens of one species, or even in different parts of a single colony. Thus the skeletons of _Tubipora musica_ from all parts of the world have a red colour, the species of the genus _Anthomastus_ have always red spicules. On the other hand, we find in _Melitodes dichotoma_ red and yellow varieties in the same locality, and in _M. chamaeleon_ some of the branches of a colony are red and others yellow. In _Chironephthya variabilis_ the colour of the spicules in any one specimen varies considerably, but in a collection of several specimens from a single locality a kaleidoscopic play of colours may be seen, no two specimens being exactly the same in the arrangement of their colour pattern. The influences that determine the colour of the spicules is at present quite unknown, and in view of the great variability that occurs in this respect, colour must be regarded as a most uncertain guide for the determination of species. The blue colour of the genus _Heliopora_ is due to a peculiar pigment which shows characteristic bands in the spectrum.[367]