page_title: Marine life
section_title: Placozoa
hierarchical_section_title: Marine life / Basal invertebrate animals / Placozoa
context_page_description: Marine life, or sea life or ocean life, is the plants, animals and other organisms that live in the salt water of the sea or ocean, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. Marine organisms produce oxygen and sequester carbon. Shorelines are in part shaped and protected by marine life, and some marine organisms even help create new land. The term marine comes from the Latin mare, meaning sea or ocean.
  Most life forms evolved initially in marine habitats. By volume, oceans provide about 90 percent of the living space on the planet. The earliest vertebrates appeared in the form of fish, which live exclusively in water. Some of these evolved into amphibians which spend portions of their lives in water and portions on land. Other fish evolved into land mammals and subsequently returned to the ocean as seals, dolphins or whales. Plant forms such as kelp and algae grow in the water and are the basis for some underwater ecosystems. Plankton forms the general foundation of the ocean food chain, particularly the phytoplankton which are key primary producers.
context_section_description: Placozoa (from Greek for flat animals)  have the simplest structure of all animals. They are a basal form of free-living (non-parasitic) multicellular organism that do not yet have a common name. They form a phylum containing sofar only three described species, of which the first, the classical Trichoplax adhaerens, was discovered in 1883.  Two more species have been discovered since 2017, and genetic methods indicate this phylum has a further 100 to 200 undescribed species.
  Trichoplax is a small, flattened, animal about one mm across and usually about 25 µm thick. Like the amoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement. Trichoplax lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers. The outer layer of simple epithelial cells bear cilia which the animal uses to help it creep along the seafloor. Trichoplax feed by engulfing and absorbing food particles – mainly microbes and organic detritus – with their underside.
caption: Placozoa have the simplest structure of all animals
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