The Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine animals belonging to the phylum Cnidaria. Although a siphonophore appears to be a single organism, each specimen is actually a colony composed of many individual animals called zooids, all of which have a specific role for survival. Most colonies are long, thin, transparent pelagic floaters. Some siphonophores superficially resemble jellyfish, one example being the venomous Portuguese man o' war (Physalia physalis).