Scanning Em study of the living cyrtocrinid Holopus rangii (Echinodermata, Crinoidea) and implications for its functional morphology

Abstract

The unusual extant crinoid Holopus rangii d'Orbigny lacks a column, cements directly to hard substrates, has a conical to tubular dorsal cup in which plate sutures cannot be discerned, and has 10 tapered arms that are able to enroll swiftly into a “fist-like” configuration when disturbed. The first SEM study of H. rangii is presented herein. The gross morphology and skeletal microstructure of this species display many characteristics that bear an obvious relationship to the functional morphology of the living animal. Dorsal cups show a range of morphologies, reminiscent of those found in acorn barnacles and other tubular encrusters. During arm enrollment, the adjacent arms abut and form an impervious seal. Of the two specimens dissected, one had a regenerating arm.