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Mesozoa are tiny, simple, worm or blob-like parasites that were once thought of as intermediates between protozoa (motile single-celled organisms) and metazoa (multicellular animals). However, now they are generally thought of as degenerate metazoa, that is, metazoans that descended from more complex animals but were secondarily simplified. Still, there is no strong consensus as of yet, and it may be that mesozoa are truly just basal metazoans.
While the term "Mesozoa" was once meant to refer to a natural group consisting only of exclusive descendants of a common ancestor, it is now known that mesozoa consist of three largely unrelated groups, all of which have been given their own phyla — the Placozoa (which consists of a single species, Trichoplax adhaerens), Orthonectida (tiny parasites of flatworms, mollusks, and echinoderms), and Rhombozoa (kidney parasites of cephalopods like squid). The Mesozoa are remarkable in their simplicity — though they have differentiated and organized tissues, this differentiation is quite simple — for instance, cell types may be layered.
Trichoplax adhaerens, the only species of phylum Placozoa, is a soft-bodied animal 0.5 mm across, reminiscent of a giant amoeba. Also called "tablet animals," Tricoplax is named for its ability to strongly adhere to surfaces, including such as microscope slides or glass pipettes. In 2006, Dellaporta et al reported the complete mitochondrial genome sequence of Tricoplax and demonstrated that it is the most basal known living metazoan phylum, branching off even earlier than cnidarians (jellyfish and corals). Cnidarians may have evolved from an organism as simple as Tricoplax.
Another of the mesozoa, Orthonectida, consist of a wall of ciliated cells surrounding a core of reproductive cells. The Orthonectida are mobile, swimming by means of their cilia, and have a larval stage as well as distinct males and females. The phylum only includes 20 species, Rhopalura ophiocomae being the best-known. The Orthonectida were once thought to be closely related to Rhombozoa, another mesozoan group, though now they are known to be unrelated.
The Rhombozoa (also known as dicyemids), another parasite mesozoan phylum, range in size from 0.1 - 9.0 mm and inhabit squid kidneys. Like another microscopic animal, rotifers, dicyemidsare eutelic, meaning adult individuals all have a constant number of cells. Instead of adding new cells to grow, the size of each individual cell simply increases. Dicyemids are found most frequently in temperate benthic environments, and more rarely in tropical areas.