Hydrozoa

Class Hydrozoa

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diagnosis	Cnidaria with mostly epidermal gonads. Original life cycle includes a polyp and a medusa stage. The medusa stage develops the gonads. Polyps always without internal septae and pharynx, mostly radially symmetric. Medusae produced by budding from polyp stage, if such a stage is present. Medusa nearly always with a velum, without rhopalia. The original life cycle has been modified extensively: the medusa stage is often reduced to a mere appendage of the polyp stage, rarely completely reduced. In some orders the polyp stage is strongly or completely reduced. Polyp stages form often colonies of macroscopic size. Integrated colonies including polypoid and medusoid structures can also form complex colonies, giving them it the appearance of an individual organism.
other characteristic features	size of individual polyps usually in the range of one mm polyps often form colonies, these with stolons attaching them to the substrate, sizes 2- 1000 mm size of the individual medusa usually in the range of mm, rarely more than a few cm
higher classification	Metazoa, Cnidaria, Medusozoa
Remarks	Scyphozoa differ by: medusa production through strobilation, the presence of rhopalia, gastric septae the polyp stage. Their gonads are gastrodermal. Cubozoa lack medusae budding and transform their polyp entierly or nearly so into a medusa. Cubomedusae lack a true velum and possess rhopalia. Their gonads are gastrodermal. Although there are no very obvious synapomorphies, the Hydrozoa are a monophyletic group. The class falls into two clear subclasses, the Trachylinae and the Hydroidolina (see below).
References	Bouillon, J. 1985a. Essai de classification des hydropolypes-hydroméduses (Hydrozoa-Cnidaria). Indo Malayan Zoology 2: 29-243. Bouillon, J. 1994. Les Hydrozoaires. In Traité de Zoologie, vol. 3(2) (ed. P. Grassé & J. Doumeng), pp. 29-416. Paris: Masson. Bouillon, J., & Boero, F. 2000b. Synopsis of the families and genera of the Hydromedusae of the world, with a list of the worldwide species. Thalassia Salentina 24: 47-296. Cornelius, P.F.S. 1995a North-west European thecate hydroids and their medusae. Part 1. Introduction, Laodiceidae to Haleciidae. Synopses of the British Fauna New Series 50: 1-347. Collins, A. G. 2002. Phylogeny of Medusozoa and the evolution of cnidarian life cycles. Journal of Evolutionary Biology 15: 418-432. Cornelius, P. F. S. 1995b. North-west European thecate hydroids and their medusae. Part 2. Sertulariidae to Campanulariidae. Synopses of the British Fauna New Series 50: 1-386. Hyman, L. H. 1940. The Invertebrates: Protozoa through Ctenophora. New York and London: McGraw-Hill. Marques, A. C. 2001b. Simplifying hydrozoan classification: inapproriateness of the group Hydroidomedusae in a phylogenetic context. Contributions to Zoology 70: 175-179. Millard, N. A. H. 1975. Monograph on the Hydroida of southern Africa. Annals of the South African Museum 68: 1-513. Schuchert, P. 1993. Phylogenetic analysis of the Cnidaria. Zeitschrift für Zoologische Systematik und Evolutionsforschung 31: 161-173. Werner, B. 1984. 4. Stamm Cnidaria, Nesseltiere. In Wirbellose Tiere (ed. H.-E. Gruner), pp. 10-305. Stuttgart: Gustav Fischer.

Classification
Subclass Trachylinae
Order Trachymedusae Medusa bell hemispherical to higher than broad, margin not lobed, with ring of thickened tissue studded with nematocysts, mostly with 8 radial canals, 4, 6 or more than 8 also possible, gonads on radial canals or at junction of radial canals and manubrium, statocysts of endo-ectodermal origin. Polyp stage absent, development direct. Cnidome can include stenoteles. Predominantly forms of the open sea and deeper waters.
Order Limnomedusae Hydrozoa without polyp stage or only very reduced and modified polyp stage. Meduse with bell composed of a lens-shaped mass of jelly and much thinner sides, tentacles inserting above bell margin, no tentacular bulbs, bell margin lobed, stomach big, not forming a distinct manubrium, without radial canals; Statocysts almost always external, of endo-ectodermal origin. No stenoteles. Predominantly forms of the open sea and deeper waters.
Order Narcomedusae Hydrozoa without polyp stage or only very reduced and modified polyp stage. Meduse with bell composed of a lens-shaped mass of jelly and much thinner sides, tentacles inserting above bell margin, no tentacular bulbs, bell margin lobed, stomach big, not forming a distinct manubrium, without radial canals; Statocysts almost always external, of endo-ectodermal origin. Some abberrant forms without bell, wormlike, with four swimming lappets. No stenoteles. Predominantly forms of the open sea and deeper waters.
Order Actinulida Very small, medusoid Hydrozoa without polyp phase living in the sand interstitial, solitary, bell entirely or very much reduced, epidermis ciliated throughout, 1-2 whorls of tentacles, statocysts present or not, club shaped, of endo-ectodermal origin; cnidome may include stenoteles.
Subclass Hydroidolina
Order Leptothecata Polyps always colonial, hydranth protected by hydrotheca, the latter can be so short that hydranth is unable to retract into it, hydrotheca can be shed in few species. Tentacles always in one whorl. Gonophores on much reduced hydranths and nearly always in periderm capsule, the gonotheca; exceptionally medusae buds on fully formed hydranths without gonotheca. Gonophores develop either into medusae or sessile sporosacs. Medusae with shallow bell, gonads on radial canals, usually more than 4 tentacles, often with statocysts. Statocysts formed only from epidermal tissue. Cnidome never includes stenoteles.
Order Anthoathecata Polyps mostly colonial, more rarely solitary. Hydranths in colonial forms connected by periderm covered stolons or plate like tissue growths, the latter rarely calcified. Hydranth not in firm hydrotheca, only occasionally covered by filmy, soft periderm. Tentacles in one or more whorls or scattered. Gonophores develop on hydranth body, pedicels or stolons. Gonophores develop either into medusae or sessile sporosacs. Medusae always without statocysts, gonads on manubrium, umbrella spherical or bell-shaped.
Order Siphonophorae Colonial, nearly always pelagial, freely movable Hydrozoa, composed of medusoid and polypoid zooids that are morphologically and functionally specialized. The integration of the zooids is so strong that the colony attains the character of an individual. Medusae are only occasionally released as medusoids, mostly they remain part of the colony; individual medusoids are not radially symmetric, often bilaterally symmetric or irregular, radial canals curved. Skeleton absent. Sizes 2 mm to 30000 mm, usually 5-20 mm. Cnidome can include desmonemes and stenoteles.