Development of Cyathidium foresti (Echinodermata: Crinoidea, Cyrtocrinida) from early attached larvae to adult-like juveniles

To date, knowledge about ontogenetic development of crinoids has been exclusively based on comatulid species, since early stages of other crinoid taxa have not been available so far. The authors now present a scanning electron microscopical and light microscopical study on a developmental series of young sessile individuals of the cyrtocrinid Cyathidium foresti. This species displays a developmental type of its own. In some aspects, the early stages resemble the early attached larva of comatulids (e.g. vestibulum, enteric sac, somatocoelomic cavity) but differ clearly in other (e.g. far oral position of hydrocoelomic primordium, pattern of podia formation, early splitting of the roof, absence of chambered organ). Therefore a specific term is proposed for this new kind of larva: cyathidula. Older juveniles are quite similar to adults; the developmental course is direct. Consequently the group of holopodid crinoids to which Cyathidium and Holopus belong, can be concluded to originate phylogenetically from neotenic larvae.