Trichoplax adhaerens, the only described species in the phylum Placozoa, is the structurally simplest metazoan (multicellular animal). It is quite small (just 2 to 3 mm in diameter) and consists of several thousand cells arranged as a double-layered plate. It lacks anterior-posterior polarity and symmetry. However, the cells of the upper and lower layers differ in shape and there is a consistent dorsal-ventral orientation of the body relative to the substrate. Trichoplax moves by ciliary gliding, changing its shape along the edges as it moves, like an amoeba. Very small (presumably young) individuals can swim, but larger individuals crawl.
(Brusca and Brusca 2003)
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