Placozoa
| Placozoa | |
|---|---|
 |
|
| Trichoplax adhaerens | |
| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Parazoa |
| Phylum: |
Placozoa Grell, 1971 |
The Placozoa are a basal form of invertebrate. They are the simplest in structure of all non-parasitic multicellular animals (Animalia). They are generally classified as a single species, Trichoplax adhaerens, although there is enough genetic diversity that it is likely that there are multiple, morphologically similar species. Although they were first discovered in 1883 by the German zoologist, Franz Eilhard Schulze (1840-1921) and since the 1970s more systematically analyzed by the German protozoologist, Karl Gottlieb Grell (1912-1994), a common name does not yet exist for the taxon; the scientific name literally means "flat animals".
Trichoplax is a small, flattened, animal around 1 mm (0.039 in) across. Like an Amoeba, it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple epithelium enclosing a loose sheet of stellate cells resembling the mesenchyme of some more complex animals. The epithelial cells bear flagella, which the animal uses to help it creep along the seafloor.
The lower surface engulfs small particles of organic detritus, on which the animal feeds. It reproduces asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme.
Some sponges
Other sponges
Homoscleromorphs
Placozoa
Eumetazoa
There is no convincing fossil record of the placozoa, although the Ediacaran biota (Precambrian, 550 million years ago) organism Dickinsonia may be allied with this phylum.
Traditionally, classification was based on their level of organization: i.e. they possess no tissues or organs. However this may be as a result of secondary loss, so is inadequate to demark a clade. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the sponges and the eumetazoa. In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.
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