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