Eighteen species of myzostomes were found in association with crinoids collected in Papua New Guinea. Thirteen of the former are described by means of in vivo, light microscopical, and SEM-based observations. Five are new to science: Myzostoma cuniculus, M. laingense, M. nigromaculatum,
and M. longitergum are ectocommensals; Contramyzostoma sphaera is a parasite living in a soft cyst induced upon its host. The eight previously described species, six of which are redescribed in detail, include Myzostoma ambiguum Graff, M. capitocutis Eeckhaut, VandenSpiegel
and Grygier, M. fissum Graff, M. mortenseni (Jägersten), M. polycyclus Atkins, and M. stochoeides Atkins, as well as Hypomyzostoma crosslandi (Boulenger) and Notopharyngoides aruensis (Remscheid). Most are ectocommensals but the last is an intradigestive
symbiont. New Indo-West Pacific distribution and host records for all eight are listed, based on surveys of museum collections, and their entire range are mapped. Hypomyzostoma Perrier, 1897 (type species Myzostoma folium Graff) is resurrected for one of the species-groups previously
recognized within Myzostoma, and a lectotype is selected for H. crosslandi. Specimens from Singapore previously identified as M. elegans Graff are reassigned to M. capitocutis.
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