Any of a phylum (Mesozoa) of small, multicellular, wormlike organisms characterized by a very simple, ciliated body with only a mouth, body cavity, and anus: it is a parasite in various marine invertebrates.
Webster's New World
Other Word Forms of Mesozoan
Origin of Mesozoan
From New Latin Mesozōaformer phylum namemeso– (because the animals were considered to be intermediate between the protozoans and the metazoans in structure) -zōapl. of-zōonanimal–zoon
American Heritage Dictionary of the English Language, 5th Edition