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. It appears that Trichoplax feeds by phagocytosis of organic detritus. (Brusca and Brusca 2003) Although the discovery and description of Trichoplax adhaerens in 1883 (in saltwater tanks in Austria) sparked controversy among zoologists about what its significance might be with respect to inferring the characteristics of early metazoans, interest faded away as the (incorrect) idea took hold that these enigmatic organisms were simply abnormal larvae of hydrozoan cnidarians. Although strong evidence against this view was published in 1912 and 1914, there was no more discussion of Trichoplax in the zoological literature--nor citation of the key critiques of 1912 and 1914-- for more than half a century (although the misconception that it was a cnidarian was repeated in textbooks). Beginning in the 1960s, several researchers re-focused attention on Trichoplax, demonstrating that it is an adult form of a new phylum dubbed, in 1971, Placozoa. It became clear that placozoans could be found worldwide in the shallow waters of subtropical and tropical regions (Pearse 1989; Eitel and Schierwater 2010). (Syed and Schierwater 2002) Recent data all indicate that the Placozoa represent one of the earliest branching lineages in the metazoan tree, but the exact placement of this branch remains uncertain (Schierwater et al. 2009; Ball and Miller 2010 and references therein).
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