Hydroidolina
Creators: name: Daly, Marymegan
name: Brugler, Mercer R.
name: Cartwright, Paulyn
name: Collins, Allen G.
name: Dawson, Michael N.
name: Fautin, Daphne G.
name: France, Scott C.
name: Mcfadden, Catherine S.
name: Opresko, Dennis M.
name: Rodriguez, Estefania
name: Romano, Sandra L.
name: Stake, Joel L.
Publisher: name: Zenodo
Year: 2007
Subjects: Biodiversity
Taxonomy
Animalia
Cnidaria
Hydrozoa

description: SUBCLASS HYDROIDOLINA   Hydroidolina Collins, 2000 includes Anthoathecata, Leptothecata, and Siphonophorae (see Marques & Collins 2004; Collins et al. 2006a). The monophyly of Hydroidolina is well supported by phylogenetic analyses of molecular (Bridge et al. 1995; Collins 2002; Collins et al. 2006a) and morphological (Bouillon & Boero 2000a; Marques & Collins 2004) data. The statocysts of Hydroidolina, when present, are ectodermal in origin. Hydroidolina polyps may be solitary or colonial, and the colonies may be polymorphic, a state that is only present in one genus (Monobrachium) of Trachylina. The relationships between major groups of Hydroidolina (Leptothecata, Anthoathecata, Siphonophorae) are uncertain (Collins 2002; Collins et al. 2006a).   
Order Anthoathecata  
 Anthoathecata Cornelius, 1992 comprises two suborders, Filifera and Capitata, and approximately 1,140 valid species (Bouillon et al. 2006). The polyps do not have a skeletal covering and can be solitary or colonial. The medusae do not have statocysts and the gametogenic tissue is confined to the manubrium. Molecular phylogenetic studies do not support monophyly of Anthoathecata, suggesting instead that Anthoathecata is a paraphyletic assemblage that gave rise to one or more of the other suborders of Hydroidolina (Collins et al. 2006a).   
Suborder Filifera  
 Filifera comprises 22 families (Schuchert 2007a) and approximately 765 valid species (Bouillon et al. 2006). Although the suborder has the putative morphological synapomorphies of filiferan tentacles on the feeding polyps and desmoneme and eurytele nematocysts, molecular phylogenetic analyses do not support its monophyly (Collins 2002; Collins et al. 2005, 2006a).   
 Included families   
 Australomedusidae Russell 1971 comprises three genera (Schuchert 2007a) and approximately five valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and its members have never been the subject of a phylogenetic analysis. This family is distinguished by polyps with large extensible hypostomes and medusae with (usually) four radial canals and four clusters of tentacles at the perradii of the bell margin.   
 Balellidae Stechow 1922 is a monospecific family (Bouillon et al. 2006) whose sole species has not been included in molecular phylogenetic analyses. This family is distinguished by polyps with two widely separated whorls of filiform tentacles.   
 Bougainvilliidae Lütken, 1850 comprises 13 genera (Schuchert 2007a) and approximately 100 valid species (Bouillon et al. 2006). Bougainvilliidae includes Rhizorhagium, which is not found within Schuchert's (2007b) classification (2007a). Molecular phylogenetic analyses that include two species from two genera were ambiguous with regard to monophyly (Collins et al. 2006a) and the group awaits a thorough phylogenetic investigation. Bougainvilliidae lacks morphological synapomorphies and shares many features with other families (Calder 1988; Schuchert 2007c). Some genera classified elsewhere by Schuchert (2007a), e.g., Lizzia in Rathkeidae, have been considered part of Bougainvilliidae (Bouillon et al. 2006).   
 Bythotiaridae Maas, 1905 comprises nine genera (Schuchert 2007a) and approximately 25 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of a phylogenetic analysis. Hydroids from this family can be distinguished by their habit of living inside the prebranchial cavity of ascidians. Medusae are recognized by marginal tentacles that have tiny or absent basal bulbs and terminate in a cluster of cnidae, but these features are present in Eucodoniidae.   
 Clathrozoellidae Peña Cantero, Vervoort & Watson, 2003 is a monogeneric family (Schuchert 2007a) with four valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. This family is distinguished by a pseudohydrotheca covering the polyps.   
 Cytaeididae Agassiz, 1862 comprises three genera (Schuchert 2007a) and approximately 20 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. There are no known morphological synapomorphies for this group and the validity of one of the genera (Perarella) is questionable (Bouillon et al. 2006; Schuchert 2007c).   
 Eucodoniidae Schuchert, 1996 is a monospecific family (Schuchert 1996) that has not been included in molecular phylogenetic analyses. This family is distinguished by four clusters of embedded nematocysts around the mouth margin of medusae (Schuchert 1996).   
 Eudendriidae Agassiz, 1862 comprises two genera (Schuchert 2007a) and approximately 85 valid species (Marques 1996). Only a single representative has been included in molecular phylogenetic analyses (Collins et al. 2006a). Possible synapomorphies for this group include the absence of desmoneme nematocysts, a styloid gonophore, and trumpet-shaped hypostome (Marques 1996).   
 Hydractiniidae Agassiz, 1862 comprises seven genera (Schuchert 2007a) and approximately 100 valid species (Bouillon et al. 2006). Eleven species representing two genera have been included in molecular phylogenetic analyses, which supported monophyly of the group (Cunningham & Buss 1993). However, taxon sampling in this analysis was not broad. The hydractiniid genus Clava is sometimes classified together with genera of Oceanidae in the family Clavidae (see Bouillon et al. 2006), suggesting that its phylogenetic status is uncertain. Hydroids of the family are distinguished by stolonal, polymorphic colonies that may bear spines.   
 Laingiidae Bouillon, 1978 comprises three genera and four valid species (Bouillon 1978; Bouillon et al. 2006). One species has been included in molecular phylogenetic analyses (Collins et al. 2006a). This group was originally classified as its own subclass by Bouillon (1978) but molecular phylogenetic analyses have shown that at least one member of the group, Fabienna sphaerica, is nested within Hydroidolina and closely related to Proboscidactylidae (see Collins et al. 2006a). This hypothesis is supported by morphological data, including a solid radial canal and macrobasic euryteles (Schuchert 1996).   
 Niobiidae Peterson, 1979 is a monospecific family (Bouillon et al. 2006) whose sole species has not been included in molecular phylogenetic analyses. The hydroid stage is unknown and the medusae are distinguished by marginal tentacle bulbs that develop into medusae buds (Petersen 1979).   
 Oceanidae Eschscholtz, 1829 comprises eight genera (Schuchert 2004) and approximately 25 valid species (Bouillon et al. 2006). Representatives of the family have not been included in any phylogenetic analyses. The family is distinguished by scattered filiform tentacles on the polyps, but this character is not specific to this group (Calder 1988; Schuchert 2004). Several genera of this family are sometimes classified with Clava in the family Clavidae (see Bouillon et al. 2006), suggesting that its phylogenetic status is uncertain.   
 Pandeidae Haeckel, 1879 comprises 23 genera (Schuchert 2007a) and approximately 75 valid species (Bouillon et al. 2006). One species has been included in molecular phylogenetic analyses (Collins et al. 2006a), but no explicit analyses of its phylogeny have been attempted. There are no known synapomorphies for this group and it is probably not monophyletic, as it encompasses a diverse assemblage of genera (Calder 1988).   
 Proboscidactylidae Hand & Hendrickson, 1950 is a monogeneric family (Schuchert 2007a) with six valid species (Bouillon et al. 2006). One species has been included in molecular phylogenetic analyses (Collins et al. 2006a). Hydroids of this family are distinguished by polymorphic stolonal colonies bearing gastrozooid polyps with two filiform tentacles.   
 Protiaridae Haeckel, 1879 comprises five genera (Schuchert 2007a) and approximately eight valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. Medusae of this family are distinguished by large, hollow tentacular bulbs and four well-developed tentacles that are often interspersed with short, solid tentacles.   
 Ptilocodiidae Coward, 1909 comprises six genera (Schuchert 2007a) and eight valid species (Bouillon et al. 2006). The group has not been the subject of a phylogenetic analysis. Hydroids of this family are distinguished by the absence of tentacles on the feeding polyps.   
 Rathkeidae Russell, 1954 comprises six genera (Schuchert 2007a) and approximately 20 valid species (Bouillon et al. 2006). Three species representing three genera have been included in molecular phylogenetic analyses and in phylogenetic analyses, and these support familial monophyly (Schuchert 2007a). Species of this family are distinguished by primary medusae buds arising from stolons and secondary medusae buds arising interradially from the medusa manubrium.   
 Rhysiidae Brinckmann, 1965 is a monogeneric family (Schuchert 2007a) with three valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. This family is distinguished by dactylozooids covered with perisarc to the capitate apical tip and female gonozooids that transform into a sporosac-like structure.   
 Russelliidae Kramp, 1957 is a monospecific family (Bouillon et al. 2006) whose sole species has not been included in molecular phylogenetic analyses. The hydroid stage is unknown and the medusa possesses marginal tentacles in groups of three: one large and two small hollow tentacles. The large tentacles are sunk into the umbrella margin, forming a furrow.   
 Stylasteridae Gray, 1847 comprises 26 genera (Schuchert 2007a) and approximately 260 valid species (Bouillon et al. 2006). Cairns (1984b) published a cladistic analysis of the genera of this diverse group, but its monophyly was not tested because only one outgroup was considered. Nevertheless, species of the group are readily distinguished by a massive calcareous exoskeleton, often brightly pigmented, and they very likely form a clade.   
 Trichydridae Hincks, 1868 is a monospecific family (Bouillon et al. 2006) whose sole species has not been included in molecular phylogenetic analyses. No putative synapomorphies are known in the hydroid stage, but medusae are distinguished by the possession of many tiny, anastomosing centripetal canals.   
 Tubiclavoididae Moura, Cunha & Schuchert, 2007 is a monospecific family (Moura et al. 2007) whose sole species has been included in molecular phylogenetic analyses (Moura et al. 2007), the results of which have not been published. The species is characterized by elongate polyps with scattered filiform tentacles and hydrocauli covered with striated perisarc.   
Suborder Capitata  
 Capitata comprises 26 families (Schuchert 2007a) and approximately 375 valid species (Bouillon et al. 2006). The putative synapormorphies are stenotele nematocysts and capitate tentacles on the polyps or filiform tentacles in separated whorls (Bouillon & Boero 2000b). Molecular phylogenetic analyses suggest that Capitata is a paraphyletic assemblage containing two clades, Aplanulata and the other capitates (Collins 2002; Collins et al. 2005, 2006a). The synapomorphy for Aplanulata is the absence of a ciliated planula larva (Petersen 1990). Four families, Tubulariidae, Corymorphidae, Candelabridae, and Hydridae have been sampled in molecular phylogenetic analyses that support monophyly of Aplanulata (Collins et al. 2005; Collins et al. 2006a), but it is likely that other families are part of this group (Petersen 1990; Collins et al. 2006a). Mosaics of features, very few of which appear to be unique to any particular family, distinguish the medusa stages of capitate families.   
 Included families   
 Acaulidae Fraser, 1924 comprises three genera and five valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. Members of this family are distinguished by scattered capitate tentacles on the distal portions of their solitary polyps.   
 Boeromedusidae Bouillon, 1985 is a monospecific family (Bouillon et al. 2006) whose sole species has not been included in molecular phylogenetic analyses. Hydroids are unknown; medusae have an apical projection, four tentacles terminating in nematocyst clusters, and four perradial pouches bearing gametes hanging from the manubrium.   
 Boreohydridae Wesblad, 1947 comprises two genera (Bouillon 1985) and two valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. This family is distinguished by small solitary polyps that possess a whorl of three to five diminutive tentacles (Schuchert 2006).   
 Candelabridae Stechow, 1921 comprises three genera (Schuchert 2006) and approximately 20 valid species (Bouillon et al. 2006). Just a single representative has been included in molecular phylogenetic analyses (Collins et al. 2005) and no explicit tests of the group’s monophyly have been conducted. The family is distinguished by its solitary or pseudo-colonial polyps that are relatively large and bear numerous scattered capitate tentacles (Schuchert 2006).   
 Cladocorynidae Allman, 1872 comprises two genera (Schuchert 2006) and seven valid species (Bouillon et al. 2006). Only a single representative has been included in molecular phylogenetic analyses (Collins et al. 2005) and no explicit tests of the group’s monophyly have been conducted. The putative synapomorphy for the family is patches of macrobasic euryteles on the body wall of the polyp (Petersen 1990).   
 Cladonematidae Gegenbaur, 1856 comprises four genera (Schuchert 2006) and approximately 20 valid species (Bouillon et al. 2006). Phylogenetic analyses including three species representing two genera support monophyly of the group (Collins et al. 2005). The family is distinguished by benthic medusae with branched tentacles and adhesive structures at the tips of the medusae tentacles (Petersen 1990).   
 Corymorphidae Allman, 1872 comprises 10 genera (Schuchert 2007a) and approximately 45 valid species (Bouillon et al. 2006). Two species from one genus were included in molecular phylogenetic analyses, the results of which contradicted monophyly (Collins et al. 2005). There are no known synapomorphies for this group (Petersen 1990).   
 Corynidae Johnston, 1836 comprises seven genera and approximately 90 valid species (Schuchert 2001). A molecular phylogenetic analysis sampling 13 species from four genera strongly contradicted monophyly of the group, with some species being closely allied to members of Polyorchidae (Collins et al. 2005). Not surprisingly, no morphological synapomorphies have been identified for Corynidae (Schuchert 2001).   
 Halimedusidae Arai & Brinckmann-Voss, 1980 comprises three genera (Mills 2000; Schuchert 2007a), each with a single valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. Species of this family have small solitary polyps that give rise to medusae with distinct interradial peaks in jelly above the manubrium base, a feature also present in medusae of Boeromedusidae (Mills 2000).   
 Hydridae Linnaeus, 1758 is a monogeneric family (Schuchert 2007a) with approximately 30 valid species (Bouillon et al. 2006). A molecular phylogenetic analysis sampling three species representing both the green and brown hydra groups supported monophyly of the family (Collins et al. 2006a). Hemmrich et al. (2007) sampled additional taxa (mainly focused on laboratory strains) and also found the group to be monophyletic, although this study aimed at elucidating relationships within the group. Hydridae is distinguished by the absence of medusae, its freshwater habitat, and lateral budding of polyps.   
 Hydrocorynidae Rees, 1957 comprises two genera (Schuchert 2007a) and three valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. Hydroids of the group are colonial, with hydranths arising from a chitinized hydrorhizal plate. No putative synapomorphies have been identified for the medusa stage.   
 Margelopsidae Uchida, 1927 comprises three genera (Schuchert 2006) and six valid species. No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. The family is distinguished by its small, pelagic, solitary polyps.   
 Milleporidae Fleming, 1828 is a monogeneric family with approximately seven valid species (Razak & Hoeksema 2003). One species has been sampled for molecular analysis (Collins et al. 2006a). This family is distinguished by colonies that build massive calcareous skeletons, polyps with capitate tentacles, and dimorphism with gastrozooids and dactylozooids.   
 Moerisiidae Poche, 1914 comprises three genera (Schuchert 2007a) and fewer than 10 valid species (Bouillon et al. 2006). One representative has been included in molecular phylogenetic analyses (Collins et al. 2005), but the family has never been the subject of an explicit phylogenetic analysis. Polyp stages of this family, where known, are solitary with scattered filiform tentacles. Medusae are recognized by a manubrium with radial lobes that extend toward and connect with the radial canals.   
 Paracorynidae Picard, 1957 is a monospecific family (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. This family is distinguished by a flat, highly organized colony, with polymorphic zooids. Bouillon (1974) suggested that Paracoryne could be interpreted as an individual flattened polyp, rather than as a colony.   
 Pennariidae McCrady, 1859 is a monogeneric family with two valid species (Schuchert 2006). A single, widespread representative has been included in molecular phylogenetic analyses and no explicit tests of the group’s monophyly have been conducted. The family is distinguished by a pinnate hydroid colony with polyps that contain an aboral whorl of filiform tentacles and capitate tentacles scattered towards the oral end.   
 Polyorchidae Agassiz, 1862 comprises three genera (Schuchert 2007a) and five valid species (Bouillon et al. 2006). Three species from two genera have been included in molecular phylogenetic analyses, which strongly supported monophyly of the group (Collins et al. 2005). The hydroid is unknown. Its large medusae are distinguished by a conspicuous gastric peduncle, a manubrium with four oral lips studded with nematocysts, numerous tentacles, and abaxial ocelli.   
 Porpitidae Goldfuss, 1818 comprises two genera and three valid species (Kirkpatrick & Pugh 1984). One species from each genus was sampled in a molecular phylogenetic analysis, and the family was found to be monophyletic (Collins et al. 2005). This family is distinguished by a highly polymorphic, pelagic colonial stage, although it has been argued that this is not a colony but a modified individual polyp (Petersen 1990).   
 Protohydridae Allman, 1888 is a monogeneric family with two valid species (Schuchert 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. The family is distinguished by small polyps that lack tentacles and gonophores and a pedal disk that is epidermal in origin (Petersen 1990).   
 Solanderiidae Marshall, 1873 is a monogeneric family (Schuchert 1996) with approximately seven valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. Species of the family are easily recognized by their large, branching colonies with chitinous, anastomosing internal skeletons.   
 Sphaerocorynidae Prévot, 1959 comprises two genera (Petersen 1990) and approximately five valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. Hydroids of the family are long-stemmed and scattered with capitate tentacles at the broadest part of the hydranth. Medusae possess apical projections with an apical chamber and a single abaxial ocellus on each of four tentacles; each tentacle terminates in an elliptical nematocyst pad.   
 Teissieridae Bouillon, 1974 comprises three genera (Petersen 1990) and approximately 10 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. Two genera are sometimes classified in a separate family (Bouillon et al. 2006). This family is distinguished by a colony with a unique basal plate of periderm that forms spines.   
 Tricyclusidae Kramp, 1949 is a monospecific family (Schuchert 2006) whose sole member has not been included in molecular phylogenetic analyses. This family is distinguished by solitary polyps that have a gelatinous perisarc and three whorls of capitate tentacles.   
 Tubulariidae Fleming, 1828 comprises six genera (Schuchert 2007a) and approximately 60 valid species (Bouillon et al. 2006). Four species from three different genera have been sampled in a molecular phylogenetic analysis and found to be monophyletic (Collins et al. 2005). Although no explicit test of the group’s monophyly was conducted, a larger study subjected 32 species to morphological phylogenetic analyses (Marques & Migotto 2001). This study did support reciprocal monophyly of two subfamilies, Ectopleurinae and Tubulariinae. The family is distinguished by polyps possessing two sets of tentacles, an aboral and oral whorl with gonophores developing between the sets of tentacles. The medusae often have a manubrium surrounded by gametogenic tissue. Based on these features and the molecular data, monophyly of the family is likely.   
 Zancleidae Russell, 1953 comprises four genera (Schuchert 2007a) and approximately 25 valid species (Bouillon et al. 2006). Two species from one genus have been included in molecular phylogenetic analyses (Collins et al. 2005), but no detailed analyses of the groups' phylogenetic status have been conducted. Phylogenetic analyses contradict monophyly, although the node separating these species was poorly supported (Collins et al. 2005). Hydroids of the family possess no obvious putative synapomorphies. Zancleid medusae have four perradial exumbrellar nematocyst pads.   
 Zancleopsidae Bouillon, 1978 comprises two genera (Petersen 1990) and six valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of an explicit phylogenetic analysis. The hydroid phase is unknown. Medusae have a conical or dome-shaped umbrella without an apical chamber, and usually have two opposed capitate tentacles that typically have capitate side branches.   
Order Leptothecata  
 Molecular and morphological phylogenetic analyses support monophyly of Leptothecata Cornelius, 1982 (e.g., Collins et al. 2006a; Leclère et al. 2007). The synapomorphy for Leptothecata is the theca, a skeletal covering on the polyps (hydrotheca) and gonophores (gonotheca). However, some phylogenetic analyses have identified a theca-less taxon as the earliest diverging lineage of Leptothecata, suggesting that the theca may have evolved after the origin of Leptothecata. Leptothecate polyps are always colonial and the tentacles of the polyps are arranged in a single whorl. The medusae usually have shallow bells with gametogenic tissue restricted to the radial canals. The approximately 2,000 valid species are classified into two suborders, Conica and Proboscidoidea (Bouillon et al. 2006). When families of Leptothecata are distinguished based on features of the medusa stage, suites of characters are typically used although very few individual characters are unique to any particular family.   
Suborder Conica  
 Conica is the more diverse of the two orders of leptothecates, comprising approximately 1,770 valid species (Bouillon et al. 2006) in 29-31 families (Bouillon et al. 2006; Schuchert 2007a). The presumed synapomorphy of the suborder is a conical hypostome on the polyps. Leclère et al. (2007) did not recover a monophyletic Conica; however this work addressed a different question and support in the relevant part of the topologies was weak.   
 Included families   
 Aequoreidae Eschscholtz, 1829 comprises four genera (Schuchert 2007a) with approximately 25 valid species (Bouillon et al. 2006). One species has been included in a molecular phylogenetic analysis (Collins et al. 2006a). No putative synapomorphies are known in the hydroid stage. The family is distinguished by relatively large medusae that possess a large number of (greater than 16) radial canals (Cornelius 1992).   
 Aglaopheniidae Agassiz, 1862 comprises eight genera (Schuchert 2007a) and approximately 250 valid species (Bouillon et al. 2006). Eleven species, including four genera, have been included in molecular phylogenetic analyses; their monophyly was strongly supported (Leclère et al. 2007). The family is distinguished by one median nematotheca below each hydrotheca and a pair of lateral nematotheca fused with the hydrotheca. Fused lateral nematotheca are thought to be a pleisiomorphy for the family (Leclère et al. 2007).   
 Barcinidae Bouillon, Gili, Pagès, Palanques & Puig, 1999 is a monospecific family (Gili et al. 1999) whose sole member has not been included in molecular phylogenetic analyses. Hydroids of the family are unknown; medusae are distinguished by the presence of closed statocysts and adaxial ocelli (Gili et al. 1999).   
 Blackfordiidae Bouillon, 1984 is a monogeneric family (Schuchert 2007a) with three valid species (Bouillon et al. 2006). A single representative has been included in molecular phylogenetic analyses. No putative synapomorphies are known in the hydroid stage; the medusae have four long fluted lips and numerous marginal tentacles with endodermal cores that extend into the mesoglea of the bell.   
 Campanulinidae Johnston, 1836 comprises 13 genera (Schuchert 2007a) and approximately 40 valid species (Bouillon et al. 2006). None of its species have been included in molecular phylogenetic analyses. No putative synapomorphies are known in the hydroid stage and the group is almost certainly a polyphyletic assemblage of genera that have a tubular hydrotheca with a pointed, segmental or pleated operculum, and that lack a medusa stage (Cornelius 1992; Bouillon et al. 2006).   
 Cirrholoveniidae Bouillon, 1984 is a monogeneric family (Schuchert 2007a) with two valid species (Bouillon et al. 2006). Neither of its species have been included in molecular phylogenetic analyses, and no putative synapomorphies have been proposed for either the hydroid or medusa stage.   
 Clathrozoidae Hirohito, 1967 comprises two monospecific genera (Bouillon et al. 2006), neither of which have been included in molecular phylogenetic analyses. Members of this family are distinguished by a hydroid skeleton consisting of anastomosed chitinous stolons.   
 Dipleurosomatidae Boeck, 1866 comprises four genera and approximately eight valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and no putative synapomorphies are known in the hydroid stage. Medusae of this family are distinguished by branched or irregularly arranged radial canals and a manubrium with a narrow base.   
 Eirenidae Haeckel, 1879 comprises nine (Schuchert 2007a) or 10 genera (Bouillon et al. 2006) and approximately 65 valid species (Bouillon et al. 2006). None of its species have been included in molecular phylogenetic analyses. No putative synapomorphies have been proposed for either the medusa or hydroid stages, the latter of which is recognized by elongate polyps and diminutive or absent hydrothecae.   
 Haleciidae Hincks, 1868 comprises four genera (Schuchert 2007a) and approximately 120 valid species (Bouillon et al. 2006). Two species representing three genera have been included in molecular phylogenetic analyses, which contradict monophyly of this group, albeit with low support for the nodes separating its representatives (Leclère et al. 2007). This family is distinguished by large polyps that are unable to retract into the hydrotheca. The hydrotheca often appears as a collar at the base of the polyp.   
 Halopterididae Millard, 1962 comprises 12 genera and approximately 85 valid species (Schuchert 1997; Bouillon et al. 2006). Eleven species representing three genera have been included in molecular phylogenetic analyses, which strongly support monophyly of the group (Leclère et al. 2007). This family is distinguished by one median nematotheca and a pair of lateral nematotheca associated with each hydrotheca. The hydrothecae are found on stems and terminal branches. These morphological characters are interpreted as pleisiomorphies for the family (Leclère et al. 2007).   
 Hebellidae Fraser, 1912 comprises six genera (Schuchert 2007a) and approximately 40 valid species (Bouillon et al. 2006). Two species representing two genera have been included in molecular phylogenetic analyses, which provide low support for monophyly (Leclère et al. 2007). Monophyly of Hebellidae (without the genus Staurodiscus, which is sometimes considered to be a part of Laodiceidae) was also supported in a phylogenetic analysis based on morphology (Marques et al. 2006). None of its diagnostic characters are unique to the group, many of them being present in members of Lafoeidae or putative outgroups (Marques et al. 2006).   
 Kirchenpaueriidae Stechow, 1921 comprises five genera (Schuchert 2007a) and 40 valid species (Bouillon et al. 2006). Four species from three genera have been sampled for molecular analysis, which strongly support monophyly of the group (Leclère et al. 2007). This family is distinguished by having one median nematotheca below each hydrotheca.   
 Lafoeidae Agassiz, 1865 comprises nine genera (Marques et al. 2006) and approximately 100 valid species (Bouillon et al. 2006). A tenth genus, Billardia, is sometimes classified in the family (Bouillon et al.   
 2006), but its phylogenetic affinity is unclear. Lafoeidae (excluding Billardia) has been shown likely to be monophyletic based on morphology, though putative synapomorphies were dependent on outgroup choice (Marques et al. 2006). Most species in the family have gonothecae closely packed in a coppinia.   
 Laodiceidae Agassiz, 1862 comprises six genera (Schuchert 2007a) and approximately 25 valid species (Bouillon et al. 2006). The family has not been the subject of phylogenetic analysis, and no putative synapomorphies are known in the hydroid stage. Medusae of Laodiceidae have marginal cordyli and lack statocysts, but cordyli and cordyli-like structures are known in other families of Leptothecata.   
 Lineolariidae Allman, 1864 comprises three genera and four valid species (Bouillon et al. 2006). The group has never been the subject of a phylogenetic analysis. Species are recognized by having hydrothecae and gonothecae that are adherent to the substrate for most of their lengths.   
 Lovenellidae Russell, 1953 comprises five genera (Schuchert 2007a) and approximately 30 valid species (Bouillon et al. 2006). Two representatives from two genera, Lovenella and Eucheilota (sometimes considered to be in a separate family, Eucheilotidae), have been sampled for molecular data and found to form a strongly supported monophyletic group (Govindarajan et al. 2006). Further sampling is needed for a more explicit test of monophyly. The family is distinguished only by medusae characters, including lateral cirri, numerous statocysts, and a short manubrium.   
 Malagazziidae Bouillon, 1984 comprises three genera (Schuchert 2007a) and approximately 20 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of any phylogenetic analyses. No putative synapomorphies are known in the hydroid stage. Medusae of this family have gametogenic tissue completely surrounding the radial canals and tentacle bulbs with adaxial excretory papillae, features that are also exhibited by other leptothecate groups.   
 Melicertidae Agassiz, 1862 comprises four genera (Schuchert 2007a) and as many as six valid species (Bouillon et al. 2006). One species from the family has been included in molecular phylogenetic analyses, and so monophyly of the group remains untested. Phylogenetic analyses suggest that this family may be the sister taxon to the rest of Leptothecata (Collins et al. 2006a). This family is distinguished by the absence of a hydrotheca (though the hydroid is known only for one of the genera), suggesting that the theca may have evolved after the divergence of Melicertidae from the rest of the leptothecates. Medusae are recognized by eight simple or bifurcating radial canals, a manubrium with a broad base, and the absence of cirri, statocysts and cordyli.   
 Mitrocomidae Haeckel, 1879 comprises 10 genera (Schuchert 2007a) and approximately 30 valid species (Bouillon et al. 2006). The group has never been part of any analysis explicitly assessing its monophyly. Members of the family are distinguished by tubular, sessile hydrothecae in the hydroid stage and by a manubrium attached to the subumbrella along a continuum of the radial canals and open statocysts in the medusa stage. Genera with ocelli associated with open statocysts (Octogonade, Tiaropsidium, and Tiaropsis) are sometimes classified separately in family Triaropsidae (Bouillon et al. 2006).   
 Octocannoididae Bouillon, Boero & Seghers, 1991 is a monospecific family (Bouillon et al. 2006). This family is only known from the medusa stage and is distinguished by multiple club shaped tentaculae at the bell margin in addition to eight normal tentacles; all have dark pigment spots. The gametogenic tissue is divided into lateral halves along the radial canals.   
 Orchistomatidae Bouillon, 1984 is a monogeneric family (Schuchert 2007a) with five valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the family’s monophyly has not been tested. Hydroid stages are unknown; medusae are distinguished by a short manubrium, a large gastric peduncle, a mouth with many crenulated lips, adaxial ocelli, and laterally compressed tentacles.   
 Phialellidae Russell, 1953 comprises two genera (Schuchert 2007a) and approximately 10 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. This family is distinguished by the absence of marginal cirri from the medusa and a crease line at the base of the hydrothecal operculum. It is questionable whether these characters are synapomorphies for the group (Cornelius 1982).   
 Plumulariidae McCrady, 1859 comprises six genera (Schuchert 2007a) with approximately170 valid species (Bouillon et al. 2006). Fourteen species representing five genera have been sampled for molecular phylogenetic analysis, which strongly supports monophyly of the group (Leclère et al. 2007). The family is distinguished by attributes of the paired nematothecae, which are either absent or never fused with hydrothecae.   
 Sertulariidae Lamouroux, 1812 comprises 26 genera (Schuchert 2007a) and approximately 600 valid species (Bouillon et al. 2006). Five species representing four genera have been included in molecular phylogenetic analyses, which contradicted monophyly of this group, albeit with low support for the nodes separating the different lineages (Leclère et al. 2007). This family is characterized by erect colonies and sessile hydrotheca with a hinged operculum.   
 Sugiuridae Bouillon, 1984 is a monospecific family (Bouillon et al. 2006) whose sole member has never been included in molecular phylogenetic analyses. No putative synapomorphies are known in the hydroid stage. This family is characterized by medusae with multiple manubria.   
 Syntheciidae Marktanner-Turneretscher, 1890 comprises three genera (Schuchert 2007a) and approximately 35 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group’s monophyly has not been tested. Species of this family usually have erect colonies that are unbranched or with pinnate stems, with hydrotheca on both sides of the stem.   
 Teclaiidae Bouillon, Gili, Pagès, Palanques & Puig, 1999 comprises two genera (Schuchert 2007a), each with a single valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. The hydroid stage is unknown and the medusae possess marginal tentacles separated by cordiliform structures.   
 Thyroscyphidae Stechow, 1920 comprises five genera (Schuchert 2007a) and approximately 20 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses and the group has not been the subject of a phylogenetic analysis. Members of this family possess an ectodermal annular fold on the polyp, but this feature is also present in some species of Aglaopheniidae and Sertulariidae.   
 Tiarannidae Russell, 1940 comprises six genera (Schuchert 2007a) and approximately 15 valid species (Bouillon et al. 2006). No representatives have been included in molecular phylogenetic analyses. Medusae of this family are distinguished by having the gametogenic tissue folded on the adradial walls of the manubrium and/or on the perradial manubrial pouches.   
Suborder Proboscidoidea  
 Proboscidoidea comprises three families (Schuchert 2007a) and approximately 150 valid species (Bouillon et al. 2006). The putative synapomorphy for this suborder is a flared hypostome forming a pregastric cavity. The group was not revealed as monophyletic in the analyses of Govindarajan et al. (2006), but this study was not focused on this question and leptothecate taxon sampling was relatively narrow.   
 Included families   
 Bonneviellidae Broch, 1909 is a monogeneric family (Schuchert 2007a) with approximately 10 valid species (Bouillon et al. 2006). Four representatives, only one of which was identified to species, have been included in molecular phylogenetic analyses. These analyses supported monophyly of the family (Govindarajan et al. 2006), but more explicit tests await further sampling. This family has a unique hypostome morphology, with the pregastric cavity separated by the base of the tentacles, projecting into the gastric cavity and fusing to form a ring.   
 Campanulariidae Johnston, 1836 comprises 11 genera (Schuchert 2007a) with approximately 140 valid species (Bouillon et al. 2006). Forty six species representing eight genera have been sampled for molecular phylogenetic analysis. The results fail to support monophyly of the group: members of Bonneviellidae nest inside Campanulariidae (Govindarajan et al. 2006). Members of Campanulariidae are distinguished by the campanulate-shaped skeleton (theca) surrounding the polyp and a peduncled hypostome. Bonneviellidae does not have this feature.   
 Phialuciidae Bouillon, 1984 is a monospecific family (Bouillon et al. 2006) whose sole species has not been included in molecular phylogenetic analyses. Hydroids are similar to those of campanulariids, but with a rounded hypostome. Medusae possess rudimentary bulbs between hollow marginal tentacles.   
Order Siphonophorae  
 Siphonophora Eschscholtz, 1829 comprises 16 families (Schuchert 2007a) and approximately 160 valid species (Dunn et al. 2005) divided into three suborders: Calycophorae, Cystonectae, and Physonectae. Molecular phylogenetic analysis strongly support siphonophore monophyly (Collins 2002; Dunn et al. 2005). Siphonophores are characterized by their holopelagic, highly polymorphic colonial organization.   
Suborder Calycophorae  
 Calycophorae comprises six families and approximately 100 valid species (Pugh 1999). Molecular phylogenetic analyses support monophyly of Calycophorae (Dunn et al. 2005). The putative synapomorphies for the group are the absences of an apical pneumatophore and palpons (Dunn et al. 2005).   
 Included families   
 Abylidae Agassiz, 1862 comprises five genera and approximately 10 valid species (Pugh 1999). Only a single representative has been included in molecular phylogenetic analyses (Dunn et al. 2005), and no explicit tests of the group’s monophyly have been conducted. This family is distinguished by having two nectophores, with the posterior one lacking a somatocyst.   
 Clausophyidae Totton, 1954 comprises four genera and approximately 10 valid species (Pugh 1999). Three species representing two genera have been sampled for molecular analysis, the results of which support monophyly of the group (Dunn et al. 2005). This family is distinguished by the possession of two nectophores, each of which has a somatocyst.   
 Diphyidae Quoy & Gaimard, 1827 comprise eight genera and approximately 50 valid species (Pugh 1999). Seven representatives, representing six genera, have been included in molecular phylogenetic analyses, the results of which strongly support monophyly of the group (Dunn et al. 2005). The possession of two dissimilar nectophores appears to be a synapomorphy for the group (Dunn et al. 2005).   
 Hippopodiidae Kölliker, 1853 comprises two genera and five valid species (Pugh 1999). Four representatives including species from both genera have been included in molecular phylogenetic analyses, the results of which strongly support monophyly of the group (Dunn et al. 2005). This family is distinguished by multiple nectophores of one type and the absence of bracts; both of these features appear to be synapomorphies for this group (Dunn et al. 2005).   
 Prayidae Kölliker, 1853 comprises seven genera and 12 valid species (Pugh 1999). Six species from five genera have been included in molecular phylogenetic analyses. The results of these phylogenetic analyses contradict monophyly of the group; Hippopodiidae nests within Prayidae (Dunn et al. 2005). The family is distinguished by two relatively large nectophores.   
 Sphaeronectidae Huxley, 1859 is a monogeneric family with four valid species (Pugh 1999). A single representative has been included in molecular phylogenetic analyses and no explicit tests of the group’s monophyly have been conducted (Dunn et al. 2005). This family is distinguished by a single spherical nectophore.   
Suborder Cystonectae  
 Cystonectae comprises two families and five recognized species (Pugh 1999). Molecular phylogenetic analyses demonstrate that Cystonectae is monophyletic and the sister group to all other siphonophores (Dunn et al. 2005). The putative synapomorphy of Cystonectae is the absence of a nectosome or bracts and a relatively large pneumatophore.   
 Included families   
 Physaliidae Brandt, 1835 is a monospecific family (Pugh 1999). Although its sole species has been included in molecular phylogenetic analyses (Dunn et al. 2005; Collins et al. 2006a), there have been no explicit investigations of cryptic diversity within the group. This family is distinguished by a large, horizontal pneumatophore.   
 Rhizophysidae Brandt, 1825 comprises two genera and four recognized species (Pugh 1999). Two species have been sampled for molecular analysis, the results of which do not support monophyly: Physaliidae nests within this group (Dunn et al. 2005). Further sampling is needed to test this result. This family is distinguished by an apical and vertical pneumatophore.   
Suborder Physonectae  
 Physonectae comprises seven families and approximately 50 valid species (Pugh 1999). The putative synapomorphies are the relatively small apical pneumatophore and a series of identical nectophores located beneath the pneumatophore. Molecular phylogenetic analyses suggest that the Physonectae is paraphyletic with respect to Calycophorae (Collins 2002; Dunn et al. 2005).   
 Included families   
 Agalmatidae Brandt, 1835 comprises nine genera and approximately 25 valid species (Pugh 1999). Nine species representing three genera have been included in molecular phylogenetic analyses, the results of which contradict monophyly and indicate that Agalmatidae is a polyphyletic assemblage (Dunn et al. 2005). There are no unique morphological features for this family.   
 Apolemidae Huxley, 1859 comprises three monospecific genera (Pugh 1999). Four representatives from one genus (Apolema) have been included in molecular phylogenetic analyses, although none were identified to species. The results of these phylogenetic analyses strongly support monophyly of the group (Dunn et al. 2005), although further sampling is needed to explicitly test this hypothesis. This family is distinguished by a hollowed nectophore that forms a pair of large axial wings.   
 Athorybiidae Huxley, 1859 comprises two genera and three known species (Pugh 1999). Two representatives have been included in molecular phylogenetic analyses, but these represent Atlantic and Pacific populations of the same species. Phylogenetic analyses support monophyly (Dunn et al. 2005), but further taxon sampling is needed to test this hypothesis. The family is distinguished by the absence of a nectophore, a feature which appears to be a synapomorphy for this group (Dunn et al. 2005).   
 Forskaliidae Haeckel, 1888 is a monogeneric family with six valid species (Pugh 2003). Six representatives of four species have been included in molecular phylogenetic analyses; one of the species (Forskalia edwardsi) was sampled from two locations in the Pacific and one from the Atlantic. Phylogenetic analyses support monophyly (Dunn et al. 2005). The putative synapomorphy for the group is the possession of four types of bracts (Dunn et al. 2005).   
 Physophoridae Eschscholtz, 1829 is a monospecific family (Pugh 1999) whose sole species has been included in molecular phylogenetic analyses (Dunn et al. 2005). One of the distinguishing features of the species, short stemmed physonects, is a pleisiomorphy (Dunn et al. 2005).   
 Pyrostephidae Moser, 1925 comprises two genera and four valid species (Kirkpatrick & Pugh 1984). Two species of Bargmannia have been included in molecular phylogenetic analyses; the results of these analyses support monophyly (Dunn et al. 2005). This family is distinguished by long stems, dioecy, and the absence of palpons. The absence of palpons is likely a synapomorphy that arose convergently in Calycophorae (Dunn et al. 2005); the other traits are pleisiomorphies. Although it has not been tested explicitly, the molecular evidence and the absence of palpons suggest that monophyly of this group is likely.   
 Rhodaliidae Haeckel, 1888 comprises seven genera and 10 valid species (Pugh 1983). Only a single representative has been included in molecular phylogenetic analyses (Dunn et al. 2005) and no explicit tests of the group’s monophyly have been conducted. The distinguishing features of the family are short stemmed physonects and a benthic life style.   
 SUBCLASS TRACHYLINA   
 The hydrozoan subclass Trachylina comprises all species classified in Actinulida, Limnomedusae, Narcomedusae, and Trachymedusae (Schuchert 2007). These four orders presently contain about 150 valid extant species (Bouillon et al. 2006).   
 The phylogenetic hypotheses of Bouillon and Boero (2000) and Marques and Collins (2004) recognize a close relationship between Actinulida, Narcomedusae, and Trachymedusae, all of which are direct developing and have ecto-endodermal statocysts. However, the position of Limnomedusae has been somewhat controversial. Bouillon and Boero (2000) maintain that the presence of a polyp stage in Limnomedusae indicates that it shares a common ancestry with Anthoathecata, Leptothecata, and Siphonophora, but its position was equivocal in cladistic analyses of morphological and life history characteristics (Marques & Collins 2004). Molecular phylogenetic analyses including samples from Limnomedusae, Narcomedusae, and Trachymedusae have provided strong evidence for their close relationship (Collins 2002; Collins et al. 2006a). Actinulida has yet to be sampled for molecular data. The clearest diagnostic apomorphy for Trachylina is statocysts of ecto-endodermal origin (Haeckel 1879).   
Order Actinulida  
Actinulida Swedmark & Teissier, 1959 was created for Halammohydridae and Otohydridae, two groups of interstitial species. Integral to their decision was the documentation that species of both groups have direct development via an actinula-like stage. Swedmark and Teissier (1966) regarded this life cycle as ancestral for Hydrozoa, and therefore rejected earlier ideas that Halammohydridae was a derived group of Narcomedusae (Remane 1927). However, subsequent authors have suggested that these animals are most likely descendants of trachyline species with free-swimming medusa stages (e.g., Werner 1965; Salvini-Plawen 1987; Bouillon & Boero 2000; Marques & Collins, 2004). Trachymedusae and Narcomedusae are also direct developers with ontogenetically similar (ecto-endodermal) statocysts. However, characters affiliating the two actinulid families to any particular group within Trachylina, or even to each other, are lacking.  
 Included families   
 Halammohydridae Remane, 1927 is a monogeneric family with ten valid species (Bouillon et al., 2006), all of which live interstitially. No species of this family have ever been included in a phylogenetic analysis. Members of Halammohydridae possess a distinctive aboral adhesive organ, a nerve ring, and two amphicoronate whorls of aboral tentacles; these unique features suggest that the group is monophyletic.   
 Otohydridae Swedmark & Teissier, 1958 is a monogeneric family with two valid species (Bouillon et al. 2006). Its members have not been included in any phylogenetic analyses. The species of Otohydridae are readily distinguished from those of Halammohydridae because they lack a nerve ring and have one whorl of dimorphic oral tentacles.   
Order Limnomedusae  
Limnomedusae Kramp, 1938 has a complicated taxonomic history. Kramp (1938; Browne & Kramp 1939) erected the taxon for the hydrozoan families Moerisiidae, Olindiasidae (= Olindiidae), and Proboscidactylidae to accommodate species with a biphasic life-cycle that did not readily fit in the Anthoathecata (=Anthomedusae or Athecata) because their medusae had either ecto-endodermal statocysts or gametogenic tissue along their radial canals, and also failed to fall in Leptothecata (=Leptomedusae or Thecata) because their polyps were not covered by a theca. Naumov (1960) added Monobrachiidae, whose species meet the criteria above. Two other families, Armorhydridae and Microhydrulidae, have also been classified within Limnomedusae, for lack of better alternatives (Bouillon 1985).  
 Broader considerations of more characteristics, including the cnidom, have indicated that Moerisiidae has a closer relationship to members of the anthoathecate group Capitata (Rees 1958; Petersen 1990). Likewise, the absence of statocysts and the presence of desmonemes strongly suggest that Proboscidactylidae shares a recent common ancestor with anthoathecate species classified in Filifera (Edwards 1973; Schuchert 1996). Molecular data have confirmed that Moerisiidae and Proboscidactylidae are more closely related to anthoathecate species than they are to those of Limnomedusae (Collins 2002; Collins et al. 2006a). Thus, Limnomedusae is presently limited to Armorhydridae, Microhydrulidae, Monobrachiidae, and Olindiasidae.   
 The molecular phylogenetic analyses of Collins et al. (2006a) included representatives of Monobrachiidae and Olindiasidae. These analyses did not support monophyly of the order, instead favoring (but with modest support values) the hypothesis that the group is a paraphyletic grade at the base of Trachylina. Additionally, these data provided relatively strong support for the hypothesis that the trachymedusan family Geryoniidae is derived within Limnomedusae. It is unclear precisely why Armorhydridae is included in Limnomedusae, as it appears to not share any putative synapomorphies with other limnomedusans. Their medusae, which inhabit the interstices of coarse sediments, differ from those of other limnomedusans by the presence of hollow tentacles and the absence of radial canals (gametes borne on the manubrium), statocysts, or other sense organs. The position of Microhydrulidae in Limnomedusae is also somewhat tentative. Adult stages of species of Microhydrulidae are unknown. Their polyps are solitary, minute, without tentacles, and armed with just a general nematocyst type, microbasic euryteles.   
 Included families   
 Armorhydridae Swedmark & Teissier, 1958 is a monospecific family whose single valid species, Armorhydra janowiczi, lives interstitially in coarse sediments. This species has never been included in a population-level analysis, which could presumably reveal cryptic diversity.   
 Microhydrulidae Bouillon & Deroux, 1967 comprises two genera and three described species, all considered valid (Bouillon et al. 2006). Its members have not been included in any phylogenetic analyses. The species are only known from the minute polyp stage (<500 μm), which are distinguished from those of other Limnomedusae by their lack of tentacles and mouths. There is no evidence of monophyly.   
 Monobrachiidae Naumov, 1960 is a monogeneric family containing three valid species (Bouillon et al. 2006). Only a single species has been sampled for molecular data, and the species have never been the subject of a phylogenetic analysis. The family can be differentiated from other limnomedusans by its polymorphic hydroid colonies, which live on bivalve molluscs. Monophyly of the family is likely.   
 Olindiasidae Haeckel, 1879 comprises 16 genera (Schuchert 2007) and approximately 40 valid species (Bouillon et al., 2006). Six species and genera were included in the analyses of Collins et al. (2006a); results of these analyses indicate that the family is paraphyletic, having given rise to the trachymedusan family Geryoniidae. Olindiasidae differs from Geryoniidae only in the presence of a polyp stage, so non-monophyly of the group is not surprising.   
Order Narcomedusae  
 Narcomedusae Haeckel, 1879 currently contains four families (Schuchert 2007): Aeginidae, Cuninidae, Solmarisidae, and Tetraplatiidae. Investigations of narcomedusan relationships are still in their infancy. The most detailed published analysis is that of Collins et al. (2006b), which included representatives of Aeginidae, Cuninidae, and Tetraplatiidae and corroborated the narcomedusan affinities suggested by Hand (1955) for the worm-shaped Tetraplatia. Molecular data consistently support the monophyly of Narcomedusae (Collins 2002; Collins et al. 2006a, b). Even as taxon sampling increases in such studies, monophyly of Narcomedusae is likely to be upheld because its members share a number of distinctive features that are likely to be synapomorphies. For instance, the oral-aboral axes in adult medusae are derived from the transverse axes of their respective planulae (Bouillon 1987) and the tentacles arise from the exumbrella rather than at the margin.   
 Included families   
 Aeginidae Gegenbaur, 1857 comprises six genera (Schuchert 2007) with fewer than ten accepted species (Bouillon et al. 2006). Only two species have been sampled for molecular data, and these do not form a clade (Collins et al. 2006b) because Tetraplatia is derived within them. The paraphyly of Aeginidae is not surprising, as it is differentiated from Cuninidae and Solmarisidae by the presence of interradial manubrial pouches that are very similar to pouches that run up into the interradially located flying buttresses of Tetraplatia. This character may be a synapomorphy of Aeginidae plus Tetraplatiidae.   
 Cuninidae Bigelow, 1913 comprises four genera (Schuchert 2007) and approximately 20 valid species (Bouillon et al. 2006). A single species has been sampled for molecular data (Collins et al. 2006b); no test of the monophyly of the group has been conducted. Cuninidae is distinguished from other narcomedusans by manubrial pouches located in the perradii.   
 Solmarisidae Haeckel, 1879 comprises two genera (Schuchert 2007) and approximately 10 valid species (Bouillon et al., 2006). No representatives have been sampled for molecular data and monophyly of the group has not been assessed. Solmarisidae is separated from other narcomedusans because its species lack manubrial pouches.   
 Tetraplatiidae Schuchert, 2007 is a monogeneric family containing two valid species (Rees & White 1957), one of which has been sampled for molecular data (Collins et al. 2006b). While monophyly of the group has not been tested, the distinctive morphology of its members (Hand 1955; Rees & White 1957) would suggest that the species of Tetraplatia have a single evolutionary origin.   
Order Trachymedusae  
 As presently constituted, Trachymedusae Haeckel, 1866 contains five families (Schuchert 2007): Geryoniidae, Halicreatidae, Petasidae, Ptychogastriidea, and Rhopalonematidae. No detailed analyses of their relationships have been conducted. Only four species, representing Geryoniidae, Halicreatidae, and Rhopalonematidae, have been sampled for molecular data (Collins et al. 2006a). These data indicate that the group may be diphyletic; a representative of Geryoniidae was found to have a close relationship to some members of Limnomedusae. As a practical matter, the two groups are difficult to distinguish. Members of Geryoniidae, like other trachymedusans and unlike limnomedusans, lack polyp stages. However, geryonids share several characters (e.g., centripetal canals, hollow marginal tentacles, and four or six radial canals) with members of Limnomedusae. Disentangling the relationships among limnomedusans and trachymedusans is clearly a priority for future studies in trachyline systematics.   
 Included families   
 Geryoniidae Péron & Lesueur, 1810 comprises two monospecific genera (Bouillon et al. 2006). Each of the geryonid species has a cosmopolitan distribution and neither has been the subject of population-level genetic studies, which could detect cryptic diversity. The two genera are distinctive, differing mainly in symmetry: one is four-parted and the other six-parted. Monophyly is likely.   
 Halicreatidae Fewkes, 1886 comprises five genera (Schuchert 2007) and approximately 10 valid species (Bouillon et al. 2006). A single representative has been sampled for molecular data (Collins et al. 2006a). Halicreatidae is distinguished from other trachymedusans by a wide circular manubrium that lacks lips, and tentacles that are stiff distally.   
 Petasidae Haeckel, 1879 comprises two monospecific genera (Bouillon et al. 2006; Schuchert 2007). No representatives have been sampled for molecular data and monophyly of the group has not been assessed. Petasidae is distinguished from other trachymedusans in having tentacles that terminate in a clubshaped knob of cnidae and four radial canals (shared with Liriope of Geryoniidae and Varitentaculata of Halicreatidae).   
 Ptychogastriidae Mayer, 1910 comprises two genera (Schuchert 2007) and three valid species (Bouillon et al. 2006). No representatives have been sampled for molecular data and monophyly of the group has not been assessed. Members of Ptychogastriidae are benthopelagic and distinguished from other trachymedusan families by having adhesive discs on some tentacles.   
 Rhopalonematidae Russell, 1953 comprises 16 genera (Schuchert 2007) and approximately 35 valid species (Bouillon et al. 2006). Two species have been sampled for molecular data (Collins et al. 2006a), and although they did not form a clade, no convincing test of the group's monophyly has been conducted. The family is distinguished from other trachymedusans by the possession of eight (usually) narrow radial canals and a narrow manubrium that terminates in a mouth with distinct lips.
description: Published as part of Daly, Marymegan, Brugler, Mercer R., Cartwright, Paulyn, Collins, Allen G., Dawson, Michael N., Fautin, Daphne G., France, Scott C., Mcfadden, Catherine S., Opresko, Dennis M., Rodriguez, Estefania, Romano, Sandra L. & Stake, Joel L., 2007, The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus *, pp. 127-182 in Zootaxa 1668 on pages 153-168, DOI: 10.5281/zenodo.180149