PIGMENTS IN THE COELENTERATA

Summary (i) The nature, variety and significance of coelenterate pigments is discussed. They may be grouped under: miscellaneous pigments, the pyrrolic series, purines, melanins, carotenoids. (2) The nature of several well‐known pigmentations is not fully understood. Chief of these are the deep blues and browns of pelagic coelen‐terates, possibly due to carotenoid‐protein compounds; and the remarkable stable colours of the spicules of Alcyonaria. The latter appear to be combined with calcium and iron. (3) Of the pyrrolic series haematins are common. They play little part in coloration and are probably simply part of the intracellular respiratory system. Bile pigments also occur and attention is drawn to the interesting pigment of this kind present in Calliactis parasitica. (4) Purines only contribute white to the colour pattern of some anemones. They occur as uric acid and possibly as other purines. (5) Black and brown melanins occur in Metridium senile and probably in other anemones. Even white anemones may possess a complete tyrosinase system. (6) It seems that coelenterate colours are due chiefly to carotenoids. These are found in species from all parts of the phylum. Though the evidence is incomplete, the carotenoids are in some cases known to be derived from the food ( Hydra , actinians). Sometimes at least, however, they are altered to ‘animal’; carotenoids, as in the ‘metridene’ from Metridium. Different colour varieties may form chemically different carotenoids. (7) Detailed study of actinians shows a surprisingly complex pigment situation. An individual may possess several distinct carotenoids, and different varieties of the same species are characterized by different series of pigments. (8) Little is known of the biochemical significance of the pigments. There are indications that the presence of carotenoids is in some cases related to light intensity. There is also evidence that these pigments are of importance in gamete formation and in regeneration. The biochemical significance of the varietal colour differences is not known. Attention is drawn to Stephenson's observations on the importance of range of colour and of colour pattern in relation to morphological axes of differentiation. (9) The genetic significance of the existence of colour varieties is discussed. True polymorphism with respect to colour is common in coelenterates. No genetic explanation can be offered without further experiment. The resemblance of the colour systems of Metridium to the blood group systems in man is commented on. Attention is drawn to the widespread character of colour polymorphism in the lower invertebrates.