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The Mesozoa are enigmatic, minuscule, worm-like parasites of marine invertebrates. As of 2012 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.[1]

As a result of these recent findings in molecular biology, the label mesozoan is now often applied informally, rather than as a formal taxon. 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.[2]

Mesozoa were once thought to be evolutionary intermediate forms between Protozoans and Metazoans, but now they are thought to be degenerate or simplified metazoa. Their ciliated larva are similar to the miracidium of trematodes, and their internal multiplication is similar to what happens in the sprocysts of trematodes. Mesozoan DNA has a low GC-content (40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group including annelids, planarians, and nemerteans.

The two main mesozoan groups are the Rhombozoa and the Orthonectida.

Rhombozoa, or dicyemid mesozoans, are found in the nephridia of cephalopods (squid and octopuses).[3] They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually into vermiform juveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva.

Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species. The best known of Orthonectida is the parasite of brittle stars. The multinucleate syncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are monoecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than the females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes its way to another star.

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