Placozoa

Placozoans Temporal range: Middle Triassic–Recent PreꞒ Ꞓ O S D C P T J K Pg N [1]
Trichoplax adhaerens
Scientific classification
Domain:	Eukaryota
(unranked):	Filozoa
Kingdom:	Animalia
Phylum:	Placozoa Grell, 1971
Type species
Trichoplax adhaerens
Classes[2]
Class Polyplacotomia Order Polyplacotomea Family Polyplacotomidae Class Uniplacotomia Order Trichoplacea Family Trichoplacidae Order Cladhexea Undescribed species Order Hoilungea Family Cladtertiidae Family Hoilungidae

Placozoa (/plækəˈzoʊə/ PLAK-ə-ZOH-ə, "flat animals")^[3] is a phylum of marine and free-living (non-parasitic) animals.^[4][5] They are simple blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth."^[6] Structural and molecular analyses have supported them as among the most basal animals,^[7][8] thus, constituting a primitive metazoan phylum.^[9]

The first known placozoan, Trichoplax adhaerens, was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921).^[10][11] Describing the uniqueness, another German, Karl Gottlieb Grell (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century,^[12][13] new species began to be added since 2018. So far, three other species have been described, in two distinct classes: Uniplacotomia (Hoilungia hongkongensis in 2018 and Cladtertia collaboinventa in 2022^[14]) and Polyplacotomia (Polyplacotoma mediterranea, the most basal, in 2019^[15]).