VI COLOURS OF CRUSTACEA AND ECHINODERMA 133 



5. Holothuria. — Of the external colouring of the 

 sea-cucumbers there is not much to say ; as a rule 

 they tend to be dull and dark in tint. Black or 

 brown colours are not uncommon, while greenish- 

 violet or gray also occur. The relative dulness {of 

 the tints is curious in view of the fact that the 

 pigments are numerous and often brilliant. 



The Pigments of Echinoderma 



I. The pigments of the star-fishes, as their colour 

 and their distribution indicate, are largely lipochromes, 

 and are probably very similar in nature to those 

 of the Crustacea. As in the Crustacea, the pig- 

 ments are not confined to the skin but occur also 

 in the internal organs and especially in the ovary. 

 M'Munn (1883) describes hsematoporphyrin, on 

 spectroscopic grounds only, in the integument of 

 Asterias glacialis. He has also shown that entero- 

 chlorophyll occurs in considerable amount in the 

 digestive caeca. As to the nature of the blue and 

 violet pigments of star-fishes, there is much more 

 difficulty. Krukenberg describes a blue pigment in 

 Astropecten auranticus which is soluble in water, and 

 is readily turned red by the action of heat, alcohol, 

 and other reagents. The blue pigment does not 

 apparently affect the colour of the organism during 

 life, and the blue colour is only apparent when the 

 superficial lipochrome has been removed by alcohol. 

 Krukenberg regarded this blue pigment as being in 

 all probability identical with cyanein, the blue pig- 

 ment of jelly-fish. It does not seem to be the same 

 as the blue pigments of Crustaceans. 



