Choanocyte-like cells with a collar of regularly arranged cylindrical microvilli around the base of the flagellum were observed in the ciliary bands of the Brachiolaria larva of Asterias rubens. The ambulacral ampullae and coelomic epithelia of adult Asterias and coelomic epithelia of Mesothuria contain similar cells with radial lamellae instead of cylindrical microvilli. Other similar but more modified types of cells, in which the inner edges of the radiating lamellae could be recognized as longitudinal ridges in the wall of a cylindrical flagellar pit, were found in ambulacral ampullae of Porania and in coelomic epithelia of Stichopus. Distinct indications of phagocytosis were seen in most of these cells. The present report together with previously published data lend support to the idea, that the choanocyte is a fundamental cell type in metazoans, probably derived phylogenetically from some flagellate ancestor.