A zooid or zoöid is a single animal that is part of a colonial animal. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooids can either be directly connected by tissue (e.g. corals, Catenulida, Siphonophorae, Pyrosome or Ectoprocta) or share a common exoskeleton (e.g. Bryozoa or Pterobranchia). The colonial organism as a whole is called a zoon , plural zoa (from Greek zòon ζώον meaning animal; plural zòa, ζώα).