Placozoa A phylum based on Trichoplax adhaerens, a small, multicellular organism first discovered in 1883 in a marine aquarium in Naples; since then two more species have been discovered. T. adhaerens is flat, amoeboid, 2–3 mm in diameter, and has a dorsal and ventral layer of ciliated (cilium) cells with a layer of loose, contractile cells between. It produces eggs and also reproduces by fission and budding. The DNA content is less than that in any other animal.

Placozoa A phylum of simple aquatic animals containing just a single known species, Trichoplax adhaerens. This has a transparent round flattened body, between 0.2 and 3 mm in diameter, without head, tail, or appendages. It is covered in cilia, which it uses to crawl over surfaces, and it feeds by secreting enzymes from its ventral surface, part of which it may invaginate to form a temporary stomach. An adult comprises a few thousand cells of just four types, whose DNA content is the smallest of any animal. Placozoans can reproduce asexually by binary fission or budding, and sexual reproduction has been observed in laboratory cultures, although full details of the life cycle under natural conditions are unknown. The evolutionary relationships of placozoans with other animals, especially the sponges (Porifera), remain speculative. Molecular studies of ribosomal RNA sequences have indicated that the Placozoa may be secondarily simplified descendants of more complex ancestors and are not closely related to the sponges.