The siphonophores are an order of the hydrozoans, a class of marine animals belonging to the phylum Cnidaria. According to the World Register of Marine Species, the order contains 188 species.  Although a siphonophore may appear to be a single organism, each specimen is, in fact, a colonial organism composed of small individual animals called zooids that have their own special function for survival. Most colonies are long, thin, transparent pelagic floaters. Some siphonophores, such as the venomous Portuguese man o' war and the Indo-Pacific man o' war, superficially resemble jellyfish.