Myzostomida

Article By:

Eeckhaut, Igor Laboratory of Marine Biology, University of Mons-Hainaut, Mons, Belgium.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.443700

Small, soft-bodied marine worms associated with echinoderms, mainly crinoids. They are found in all oceans from subtidal to over 3000 m (9840 ft) depth. These worms are often considered to be an order of Polychaeta, although the first molecular analyses based on DNA comparison estimated that they would be a protostome group nested outside the Annelida. The Myzostomida include about 180 species within eight families (see table). Most of them are ectocommensals of crinoids (that is, they live on their outer body surface without affecting them), but some are parasites of crinoids, asteroids, or ophiuroids that infest the gonads, coelom, integument, or digestive system. The association between myzostomids and echinoderms is very old: signs of parasitic activities, similar to those induced by extant myzostomids, are found on fossilized crinoid skeletons dating back to the Carboniferous. The body plan of most myzostomids is singular and differs from the regular body plan of polychaetes as they are incompletely segmented, parenchymous, acoelomate organisms with chaetae. See also: Annelida; Polychaeta

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