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Bryozoans, or Ectoprocta, are a phylum of small aquatic animals living in colonies. The colonies usually have a skeleton of calcium carbonate. Bryozoans have a long fossil history, starting in the Ordovician. In their life-style they resemble the polyps which form coral. Bryozoa are known informally as moss animals or sea mats. They generally like warm, tropical waters but live all over the world. There are about 5,000 living species, and 15,000 fossil species are known.

The colonies are formed by tiny (~0.2mm) members called zooids. They secrete tubes, usually of lime (CaCO₃), sometimes of chitin, an organic compound. The zooids in a colony are all clones, produced by asexual reproduction. Despite this, most species produce different morphs: zooids with different functions.