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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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