The Placozoaires are Métazoaire S (animals) of very simple organization.

The group of the Placozoaires contains only one known species, Trichoplax adhaerens , discovered in 1883 in sea water an aquarium of the Université of Graz, in Austria. Another species, Treptoplax reptans , would have been described in 1896, but found forever since, which makes it possible to doubt its existence. However recent studies of genomes (published in 2004) seem to indicate the existence of several species in spite of a similar morphology.

The phylogenetic position of Trichoplax adhaerens is dubious: certain primitive characters (absence of permanent digestive cavity, two layers of cells, etc) seem to bring this species closer to the sponges, but the analyzes of the ARNr 18S would rather place it among the Eumétazoaire S.

Description of Trichoplax adhaerens

The organization of very small size, consists of a few organized thousands of cell S following two layers. The dorsal layer presents ciliées cells flattened the made-to-order of a epithelium suitable for paving. The ventral layer consists of higher cells of two principal types: cells ciliées as on the dorsal face, but presenting in addition to the Microvillosité S, and of the glandular cells. These peripheral cells delimit a space containing of the mésenchymateuses cells drowned in a extracellular Matrice.

Refringent bodies are observed between the cells of the dorsal epithelium: of lipidic origin, they come from the degeneration of a cell épithélio-dorsal. An intercalated layer of thread-like cells contains symbiotic bacteria.

The épithélio-ventral cells are phagocytic, after an external prédigestion of food by the enzymes which these cells secrete.

Ethology

The locomotion is carried out thanks to the lashes and with the deformation of the body.

The Digestion is external (exodigestion). Trichoplax adhaerens includes prey (organic remains, unicellular algae, etc) in a digestive cavity whose walls are consisted of the cells of the sub-base. The digestive Enzyme S are poured in this cavity and the food thus digested. The products of digestion are then absorbed.

The reproduction can be asexual (by simple fission) or sexuée.

With reading

phylogenetic Classification of alive the by Guillaume Lecointre and Herve Guyader with the editions Belin

See too

External bonds

External bonds

• Drawing of Trichoplax adhaerens (overall picture): * Drawing of cut of Trichoplax adhaerens (left supérieur= dorsal face; part inférieure= ventral face): *

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