Early stalked stages in ontogeny of the living isocrinid sea lily Metacrinus rotundus

The early stalked stages of an isocrinid sea lily, Metacrinus rotundus, were examined up to the early pentacrinoid stage. Larvae induced to settle on bivalve shells and cultured in the laboratory developed into late cystideans. Three‐dimensional (3D) images reconstructed from very early to middle cystideans indicated that 15 radial podia composed of five triplets form synchronously from the crescent‐shaped hydrocoel. The orientation of the hydrocoel indicated that the settled postlarvae lean posteriorly. In very early cystideans, the orals, radials, basals and infrabasals, with five plates each in the crown, about five columnals in the stalk, and five terminal stem plates in the attachment disc, had already formed. In mid‐cystideans, an anal plate appeared in the crown. Late cystideans cultured in the field developed into pentacrinoids about 5 months after settlement. These pentacrinoids shared many crown structures with adult sea lilies. On the other hand, many features of the stalk differed from those in adult isocrinids, while sharing many characteristics with the stalk of feather star pentacrinoids, including disc‐like proximal columnals, high and slender median columnals, synarthrial articulations developmentally derived from the symplexial articulations, limited formation of cirri only in the proximal columnal(s), and an attachment disc. On the basis of these findings, phylogenetic relationships among extant crinoid orders are discussed.