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This is the smallest of all animal databases yet; it contains a single nominal species, Trichoplax adhaerens. A second species that is sometimes mentioned, Treptoplax reptans, has never been found again after its first and only report and is most likely a fragmented placozoan. With only one species, the phylum knows only three taxonomic ranks: Phylum (Placozoa), genus (Trichoplax), and species (T. adhaerens). This situation is soon to change radically. Genetic analyses have suggested that Placozoa harbor at least seven major species clades, which may represent different orders, each with multiple families. The placozoan species database will be updated immediately when a relevant manuscript becomes accepted. Please send information to Bernd Schierwater.
Placozoa occur in the littoral of all warm oceans and are distributed globally in tropical and sub-tropical waters. They reproduce by (i) binary (sometimes trinary) fission, (ii) budding off small swarmers (iii) and sexual reproduction.
The placozoan Trichoplax adhaerens is more simply organized than any other living metazoan. This tiny marine animal, with a size of up to 2mm, looks like an irregular "hairy plate" ("tricho plax") whose unique bauplan is based on a simple, irregular sandwich organization. An upper and a lower epithelium surround a loose network (not an epithelium) of so-called fiber cells. Traditionally only four cell types have been described in Trichoplax, upper and lower epithelial cells, gland cells within the lower, feeding epithelium, and fiber cells sandwiched between the epithelia. No organs or specialized nerve or muscular cells are present. A basal lamina and extracellular matrix are likewise lacking. All these simple bauplan characteristics make placozoans more similar to protozoans than to any other existing metazoans. Body shape is irregular and changes constantly. No symmetry of any kind is seen, and nothing like an oral-aboral or even a dorso-ventral polarity exists. All of the above justified the construction of an own phylum, Placozoa.
After its original description by F.E. Schulze 1883, Trichoplax attracted particular attention as a potential candidate representing the basic and ancestral state of metazoan organization. The simplest of all metazoan morphologies suggested a basal position for T. adhaerens. Presently the phylogenetic position of Placozoa is subject of hottest debates.