Mesozoans were once thought to be intermediate between protozoa and metazoans, but are now considered degenerate metazoans or basal metazoans. The Mesozoa consists of three unrelated groups: Placozoa, Orthonectida, and Rhombozoa. Trichoplax adhaerens is the most basal living metazoan phylum known. Orthonectida have distinct males and females and swim by means of their cilia. Rhombozoans inhabit the kidneys of squid and are eutelic.
Mesozoans are tiny, simple, worm-like or blob-like parasites that were once thought to be intermediate between protozoa (motile single-celled organisms) and metazoans (multicellular animals). However, they are now generally thought of as degenerate metazoans, i.e. metazoans that are descended from more complex animals but have been secondarily simplified. However, there is still no strong consensus, and it could be that mesozoans are really just basal metazoans.
While the term “Mesozoa” was once intended to refer to a natural group consisting only of exclusive descendants of a common ancestor, the Mesozoa are now known to consist of three largely unrelated groups, all of which have been assigned their own phyla – the Placozoa (which consists of a single species, Trichoplax adhaerens), Orthonectida (small parasites of flatworms, molluscs and echinoderms) and Rhombozoa (kidney parasites of cephalopods such as squid). The Mesozoa are remarkable in their simplicity – although they have differentiated and organized tissues, this differentiation is quite simple – for example, cell types can be stratified.
Trichoplax adhaerens, the only species in the phylum Placozoa, is a soft-bodied animal 0.5 mm in diameter, resembling a giant amoeba. Also called “tablet animals,” Tricoplax is named for its ability to adhere strongly to surfaces, including microscope slides or glass pipettes. In 2006, Dellaporta et al reported the complete mitochondrial genome sequence of Tricoplax and showed that it is the most basal living metazoan phylum known, branching even earlier than cnidarians (jellyfish and corals). Cnidarians may have evolved from a simple organism like Tricoplax.
Another of the mesozoans, Orthonectida, consists of a wall of hair cells surrounding a nucleus of reproductive cells. Orthonectida are motile, swim by means of their cilia, have a larval stage and distinct males and females. The phylum includes only 20 species, Rhopalura ophiocomae being the best known. Orthonectids were once thought to be closely related to rhombozoans, another Mesozoic group, although they are now known to be unrelated.
Rhombozoans (also known as dicyemids), another parasitic Mesozoic phylum, range in size from 0.1 to 9.0 mm and inhabit the kidneys of squid. Like another microscopic animal, rotifers, dicemids are eutelic, which means that adult individuals all have a constant number of cells. Instead of adding new cells to grow, the size of each individual cell simply increases. Dicemids are found most frequently in temperate benthic environments, and more rarely in tropical areas.
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