The Mesozoa are parasitic worm-like animals. It is unclear as to whether they are degenerate platyhelminthes or truly primitive metazoans. Generally, they consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. At one time they were classified as a phylum, but are now generally believed to be unrelated to one another, so the term mesozoan is applied descriptively rather than taxonomically.

The two main mesozoan groups are the Rhombozoa and Orthonectida.

Rhombozoa

Rhombozoa, or dicyemid mesozoans, are found in the kidneys of cephalopods. They range from a few milimeters 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

Orthonectida are found in the body spaces of various marine invertebrates.

Other groups included with the mesozoa are the Placozoa and the Monoblastozoa.

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