Mesozoa
The
Mesozoa are enigmatic, minuscule, worm-like parasites of marine invertebrates. As of 2012 it was still unclear whether they are degenerate platyhelminthes or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm 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. In the 19th century, the Mesozoa were a wastebasket taxon for multicellular organisms which lacked the invaginating gastrula which was thought to define the Metazoa.