The Collaborative International Dictionary
Mesozoa \Mes`o*zo"a\, n. pl. [NL. See Mesozoic.] (Zo["o]l.) A group of very lowly organized, wormlike parasites, including the Dicyemata. They are found in cephalopods. See Dicyemata.
The Mesozoa (singular: mesozoon) are enigmatic, minuscule, worm-like parasites of marine invertebrates. it was still unclear whether they are degenerate platyhelminthes ( flatworms) or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. Decades ago, Mesozoa were classified as a phylum. Molecular phylogeny studies, however, have shown that the mysterious mesozoans are polyphyletic. That is, they consist of at least two unrelated groups.
As a result of these recent findings in molecular biology, the label mesozoan is now often applied informally, rather than as a formal taxon. Some workers previously classified Mesozoa as the sole phylum of the lonely subkingdom Agnotozoa. Cavalier-Smith argued that at least some of the mesozoans are in fact protistans, not animals.