The eumelanin isolated from ink sacs of the cuttlefish, Sepia officinalis L., Cephalopoda. Present in the raw ink as a suspension of small, dark granules in a colorless plasma. Structure is a macromolecule or probably a mixture of polyacid polymers in which the predominant chemical unit is of the indole type. Structure studies: Piattelli, Nicolaus, Tetrahedron 15, 66 (1961); Piattelli et al., ibid. 19, 2061 (1963). Review: R. A. Nicolaus, Melanins (Hermann, Paris, 1968) pp 68-91.