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The Placozoa are simple balloon-like marine animals with a body cavity filled with pressurized fluid. They have no organs, no mouth, and no digestive tract, and are characterized by hair-like cilia on both the upper and lower surfaces. They are one of very few groups of animals to lack a nervous system, although evidence suggests that they evolved from species with nerves. Eggs and sperm are produced but there are no sexual organs. Asexual reproduction also occurs. The only confirmed species is Trichoplax adhaerens[?], with no fossil record. Another species, Treptoplax reptans, was described in 1896 and has not been seen since, leading to doubts about its existence.

Placozoans have the smallest amount of DNA yet measured for any animal. Their native habitat is unknown: they were discovered on the walls of a lab aquarium in the 1880s, and have not been seen outside the laboratory environment.

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