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 highly specialised individual animals called zooids. Most colonies are long, thin, transparent pelagic floaters. Some siphonophores superficially resemble jellyfish, one example being the venomous Portuguese man o' war (Physalia physalis).