The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals (Animalia). 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 by the German zoologist, Franz Eilhard Schulze (1840-1921) and since the 1970s more systematically analyzed by the German protozoologist, Karl Gottlieb Grell (1912-1994), a common name does not yet exist for the taxon; the scientific name literally means "flat animals".