The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals (Metazoa). They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple, morphologically similar species. Although they were first discovered in 1883, a common name does not yet exist for the taxon; the scientific name literally means "flat animals".
Trichoplax is a small, flattened, animal around 1 millimetre across. Like an Amoeba, it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple epithelium enclosing a loose sheet of stellate cells resembling the mesenchyme of some more advanced animals. The epithelial cells bear flagella, which the animal uses to help it creep along the seafloor.
The lower surface engulfs small particles of organic detritus, on which the animal feeds. It reproduces asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme.
On the basis of their simple structure, the Placozoa were frequently viewed as a model organism for the transition from unicellular organisms to the multicellular animals (Metazoa) and are thus considered a sister taxon to all other metazoans: