Placozoa  The Placozoa are a basal form of free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical "Trichoplax adhaerens", "Hoilungia hongkongensis", and "Polyplacotoma mediterranea", where the last appears most basal. The last two have been found only since 2017. Although the Placozoa 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 means "flat animals". "Trichoplax" is a small, flattened, animal around 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 complex animals. The epithelial cells bear cilia, 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. Sexual reproduction has been reported to occur in one clade of the placozoa. Intergenic recombination was observed as well as other hallmarks of sexual reproduction. Some Trichoplax species contain Rickettsiales endosymbionts. One of the at least 20 described species turned out to have two bacterial endosymbionts; Grellia which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the first described Margulisbacteria, that lives inside cells used for algal digestion. It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return. There is no convincing fossil record of the placozoa, although the Ediacaran biota (Precambrian, ) organism "Dickinsonia" may be allied with this phylum.