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A small phylum of about 50 species of parasites of marine invertebrates with complex life cycles. Mesozoa are classified with the Metazoa, but they are regarded by some observers as intermediate between unicellular and multicellular animals; others consider them a degenerate group of flatworms. Mesozoa are divided into two distinct orders, the Orthonectida and Dicyemida; the latter are nephridial parasites of squids, octopods, and cuttlefish.
[meso- + G. zōon, animal]