Trichoplax adhaerens is a simple balloon-like marine animal with a body cavity filled with pressurized fluid. It is given its own phylum, called Placozoa; the only other species, Treptoplax reptans, was described in 1896 and has not been seen since, leading to doubts about its existence.
Trichoplax lacks organs and most tissues, including the nervous system, although evidence suggests that they evolved from species with nerves. It is comprised of a few thousand cells in three distinct layers, the outermost of which each have a single cilium, which allow the adult to move. They feed by absorption, and have been observed to form temporary bulges to trap food. Eggs and sperm are produced but there are no sexual organs. Asexual reproduction also occurs. It has the smallest amount of DNA yet measured for any animal. Their native habitat is unknown: they were discovered on the walls of a marine aquarium in the 1880s, and have not been seen outside the laboratory environment.
Indiviual Trichoplax are soft-bodied, about 0.5 mm across, and somewhat resemble a large amoeba. The name T. adhaerens was given because it tends to stick to its substrate, including glass pipettes and microscope slides. Its evolutionary relationships are still being investigated, but it may be allied with the cnidarians and ctenophores.
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