Mesozoan
| Mesozoa | |
|---|---|
 |
|
| Dicyema macrocephalum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa? |
| (unranked): | Bilateria? |
| (unranked): | Mesozoa |
| Phyla | |
The Mesozoa (singular: mesozoon) are enigmatic, minuscule, worm-like parasites of marine invertebrates. As of 2012[update] 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.
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.
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Wikipedia