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Eschenholtz, 1829

Siphonophora are very polymorph members of the cnidarian Superclass Hydrozoa. The fully developed siphonophore is organised as a colony, made up of highly modified medusae and polyps. Polymorphism is in this class far much further carried out than in other hydrozoan classes, the colony is a real superorganism. Furthermore, in contrast to most other hydrozoans, Siphonophora have no alternation of generations with a free living medusa and an attached polypoid stage, but both developmental stages are attached to the stem of the free floating colony. About 150 siphonophore species are currently known; they are strictly marine and rather stenohaline and appear only rarely in inshore waters or seas where salinities are below 33-44 ä. Siphonophora are holoplanktonic, except for the Families Physaliidae (pleuston) and Rhodaliidae (benthos). They occur in all worlds’ oceans from the surface to the deep-sea and are most common in warmer waters. A few species tend to be more neritic than oceanic, or regularly occur in coastal waters, entered with ocean currents or strong winds.
Siphonophora, as coelenterates, consist of two cell layers: the outer ectoderm and the inner endoderm. Both layers are separated by the mesogloea, a more or less thickened amorphous layer. To keep afloat and help to maintain vertical position, siphonophores have a gas-filled float (pneumatophore) at the very top, or swimming bells (nectophore) in the upper part of the colony, or both. The pneumatophore is thought to be developed directly from the larval stage and probably is a highly modified polyp. The gas content in floats can be regulated in order to keep the colony at particular depth. The nectophore is an asexual medusoid individual, with many of the structures common to free-swimming medusae, although mouth, tentacles and sense organs are lost. The swimming bell, with its musculated wall, functions for the jet propulsion, and also give buoyancy to the colony due to a large amount of mesogloea in the bells.
There are three kinds of polypoid zooids: (1) the digestive zooids, or gastrozooid, located on the siphosome, with a mouth and usually with a single branching tentacle, (2) the protective and prey-catching zooids, or dactylozooid, they are slender and tentacle-like, without a mouth and full with nematocysts, and finally (3) the reproductive zooids, or gonozooid, producing sexual medusoids: gonophores that are retained to the colony and form either eggs or sperm (they resemble gastrozooids, but have no mouth).

See also page Siphonophorae where the chapter starts.

Identification
The keys to families of Siphonophorae starts at Page 7: Siphonophorae.

Classification
The classification adopted here is basically that used by Totton (1965). That taxonomy is well established, although a few more recent authors (e.g. Alvariño, 1981) still use a few outmoded names. Other useful references include Bigelow (1911), Kirkpatrick and Pugh (1984) and Totton (1954). The only change that has been adopted here is the re-establishment of the Siphonophorae as a subclass of the class Hydrozoa (see Bouillon et al., 1992), and the consequent raising of the 3 old suborders to order status. Totton (1965) recognised approximately 130 species and since then about 70 other species and subspecies either have been described or resurrected. However, it is certain that several of Totton's "doubtful" species, and many of the subsequent new ones, are not valid. This is because many aberrant forms have been described as new species, or the reasons given for distinguishing the new material from extant species are insufficient. Nonetheless, it is clear from submersible collections that there are still many species yet to be described.

Phylum Cnidaria Verril, 1865
Class Hydrozoa Owen, 1843
Subclass Siphonophorae Eschscholtz, 1829
Order Cystonectae Haeckel, 1887
Family Physaliidae Brandt, 1835
Family Rhizophysidae Brandt, 1825
Order Physonectae Haeckel 1888
Family Apolemidae Huxley, 1859
Family Agalmatidae Brandt, 1835
Family Pyrostephidae Moser, 1925
Family Physophoridae Eschscholtz, 1829
Family Athorybiidae Huxley, 1859
Family Rhodaliidae Haeckel, 1888
Family Forskaliidae Haeckel, 1888
Order Calycophorae Leuckart, 1854
Family Prayidae Kölliker, 1853
Subfamily Amphicaryoninae Chun, 1888
Subfamily Prayinae Haeckel, 1888
Subfamily Nectopyramidinae Bigelow, 1911
Family Hippopodiidae Kölliker, 1853
Family Diphyidae Quoy and Gaimard, 1827
Subfamily Sulculeolariinae Totton, 1954
Subfamily Diphyinae Moser, 1925
Subfamily Giliinae Pugh and Pagès, 1995
Family Clausophyidae Totton, 1954
Family Sphaeronectidae Huxley, 1859
Family Abylidae L. Agassiz, 1862
Subfamily Abylinae L. Agassiz, 1862
Subfamily Abylopsinae Totton, 1954.

The only family not dealt with herein is the Rhodaliidae whose species are benthic.

Subclass Siphonophorae