Trichoplax adhaerens is one of the four named species in the phylum Placozoa. The others are Hoilungia hongkongensis, Polyplacotoma mediterranea and Cladtertia collaboinventa. Placozoa is a basal group of multicellular animals, possible relatives of Cnidaria. Trichoplax are very flat organisms commonly less than 4 mm in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated "cover cells" flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these layers is the fibre syncytium, a liquid-filled cavity strutted open by star-like fibres.
Trichoplax
Ultrastructure of Trichoplax adhaerens. The upper epithelium (blue bar) with monociliated cells (light blue). The intermediate layer (green bar) consists of nonciliated fiber cells (labeled “fc” in light green). The lower epithelium (orange bar) is mostly made up of monociliated cylinder cells (light red). Scale bar is 2 μm.
Extracorporeal food uptake by Trichoplax adhaerens
Placozoa is a phylum of marine and free-living (non-parasitic) animals. They are simple blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among the most basal animals, thus, constituting a primitive metazoan phylum.
Crawling motility and food uptake by Trichoplax adhaerens