mesozoan

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mes·o·zo·an

M5255500 (mĕz′ə-zō′ən, mĕs′-)n. Any of various small wormlike animals that are parasites of marine invertebrates and have an inner layer of one or more cells surrounded by an outer layer of ciliated cells.
[From New Latin Mesozōa, former phylum name : meso- (because the animals were considered to be intermediate between the protozoans and the metazoans in structure) + -zōa, pl. of -zōon, animal; see -zoon.]

mesozoan

(ˌmɛsəʊˈzəʊən) n (Animals) any of several wormlike parasites of the group Mesozoa that live in marine vertebrates