Microscopical Anatomy of the Cyrtocrinid Cyathidium meteorensis (sive foresti ) (Echinodermata, Crinoidea)

Abstract Light microscopy and scanning electron microscopy of three specimens of Cyathidium meteorensis (order Cyrtocrinida) revealed some special morphological features. The brachial articulation is provided with a long tendon at the aboral side; the entire articulation surface, including the areas where ligaments attach is built up by labyrinthic stereom. The calycinal ossicle lacks any internal vestiges of a pentameric composition; vertical planes with changing stereom direction lie irrespectively of radial or interradial planes. Gut, ovary and testis are histologically inconspicuous, and the location of the gonads inside the calyx is quite unusual. Both sexes develop outer gonoducts which probably are functionally adapted madreporic canals. The coelomic system differs from that of other crinoids in that a chambered organ is completely lacking. Simultaneously, the aboral nervous subsystem has no aboral nerve centre and simply terminates aborally in the ring‐shaped commissure. A glandular axial organ is absent, as are typical sacculi. The data are in accordance with two proposals made previously for Holopus rangii , viz., that the animals can feed raptorially, and that cyrtocrinids probably have evolved by loss of aboral calycinal ossicles of ancestors. In part, our observations differ from those in Cyathidium foresti , so we have chosen to use the species name meteorensis which has been considered a synonym of foresti .