Siphonophorae is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 species described thus far.
Siphonophorae
Portuguese man o' war (Physalia physalis)
Plate 7
Plate 37
Hydrozoa is a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in freshwater habitats. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.
Hydrozoa
The hydroid Tubularia indivisa, fertile, Gulen Dive resort, Norway
The highly apomorphic Siphonophorae—like this Portuguese man o' war (Physalia physalis)—have long misled hydrozoan researchers.
Limnomedusae like the flower hat jelly (Olindias formosa) were long allied with Anthomedusae and Leptomedusae in the "Hydroida".