Placozoa

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 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".

Biology

Trichoplax is a small, flattened, animal around 1 mm (0.039 in) 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 flagella, which the animal uses to help it creep along the seafloor.

Trichoplax

Trichoplax adhaerens is the only extant representative of phylum Placozoa, which is a basal group of multicellular animals (metazoa). Trichoplax are very flat organisms around a millimetre in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated "cover cells" flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these layers is the fibre syncytium, a liquid-filled cavity strutted open by star-like fibres.

Trichoplax feed by absorbing food particles—mainly microbes—with their underside. They generally reproduce asexually, by dividing or budding, but can also reproduce sexually. Though Trichoplax has a small genome in comparison to other animals, nearly 87% of its 11,514 predicted protein-coding genes are identifiably similar to known genes in other animals.

Discovery

Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze, in a seawater aquarium at the Zoological Institute in Graz, Austria. The generic name is derived from the classical Greek θρίξ (thrix), "hair", and πλάξ (plax), "plate". The specific epithet adhaerens comes from Latin "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.