Placozoa is a phylum of multicellular animals that are only a few mm in diameter and are continuously changing shape by forming folds at the dorsal surface. Its natural habitat is seawater.

Keywords: placozoa; Trichoplax adherens; Treptoplax reptans; evolution; metazoa

Figure 1.

Drawing of T. adherens (adapted from Wim van Egmond).

Figure 2.

Alternative trees for the evolution of metazoa. (a) Tree supported by parsimony and minimum evolution of 18S ribosomal ribonucleic acid (rRNA). Modified from Collins (1998). (b) Competing phylogeny based on morphological characters and simplicity arguments. Modified from Grell and Ruthmann (1991).


Further Reading

Cavalier‐Smith T (1998) A revised six‐kingdom system of life. Biological Reviews 73: 203–266.

Christen R, Ratto A, Baroin A et al., (1991) An analysis of the origin of metazoans, using comparisons of partial sequences of the 28S RNA, reveals an early emergence of triploblasts. EMBO Journal 10: 499–503.

Collins AG (1998) Evaluating multiple alternative hypotheses for the origin of Bilateria: an analysis of 18S rRNA molecular evidence. Proceedings of the National Academy of Sciences of the USA 95: 15458–15463.

Grell KG (1971) Trichoplax adherens F.E. Schulze und die Entstehung der Metazoen. Naturwiss. Rundschau 24: 160–161.

Grell KG and Benwitz G (1981) Erganzende Untersuchungen zur Ultrastruktur von Trichoplax adherens F.E. Schulze (Placozoa). Zoomorphology 98: 47–67.

Grell KG and Ruthmann A (1991) Placozoa. In: Frederick WH (ed.) Microscopic Anatomy of Invertebrates, pp. 13–27. New York: Wiley‐Liss.

Kobayashi M, Takahashi M, Wada H and Satoh N (1993) Molecular phylogeny inferred from sequences of small subunit ribosomal DNA, supports the monophyly of the metazoa. Zoological Sciences 10: 827–833.

Ruthmann A, Behrendt G and Wahl R (1986) The ventral epithelium of Trichoplax adherens (Placozoa): cytoskeletal structures, cell contacts and endocytosis. Zoomorphology 106: 115–122.

Thiemann M and Ruthmann A (1991) Alternative modes of asexual reproduction in Trichoplax adherens (Placozoa). Zoomorphology 110: 165–174.

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How to Cite close
Thiemann, Otavio H(Jun 2001) Placozoa. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001581]