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Classification
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Family Aequoreidae
Hydroid colony stolonal or sparingly branched; hydrotheca delicate, tubular, elongated, radially symmetrical, operculum a continuation of hydrothecal wall, composed of several long, triangular flaps, not delimited by crease-line; in older colonies operculum generally lost and hydrotheca reduced to a perisarcal collar; hydranth with intertentacular web, tentacles amphicoronate, moniliform-like when completely extended; gonotheca large, pedicellate, cylindrical, giving rise to one or two medusae.
Medusa with wide, circular manubrium; usually no gastric peduncle; numerous simple or branched radial canals; gonads on radial canals, separated from manubrium; marginal tentacles hollow; usually with excretory pores or papillae on bulbs; no marginal or lateral cirri; statocysts closed; no ocelli.
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Family Barcinidae
Hydroid phase unknown.
Medusa with narrow manubrium, no peduncle; closed marginal vesicles; adaxial ocelli; four simple radial canals.
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Family Blackfordiidae
Hydroid colony stolonal, rarely slightly ramified; hydrotheca with diaphragm; operculum composed of numerous, converging, triangular flaps, showing no clear demarcation from hydrothecal margin; hydranth with one whorl of 12-16 filiform tentacles, with membranous intertentacular web; gonothecae developing on stem or on hydranth stalk, one medusa in each gonotheca.
Medusa with small and short manubrium; mouth with four long, drawn-out lips; numerous hollow tentacles; tentacle gastrodermis extending inwards from bell margin into bell mesogloea; four radial canals; gonads cover radial canals completely; no permanent rudimentary tentacles; numerous closed statocysts.
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Family Campanulinidae
Hydroid colony stolonal or branched; hydrotheca usually campanulate or cylindrical, adnate or upright, with or without pedicel, always covered by operculum of several triangular flaps, distinctly demarcated from hydrotheca or not; with or without diaphragm; with or without nematophores; gonophores as fixed sporosacs or as free medusae.
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Family Cirrholoveniidae
Hydroid colony stolonal, hydrotheca sessile, tubular turning gradually into a pyramidal operculum formed by numerous converging flaps; no intertentacular web; gonotheca unknown.
Medusa with small manubrium; four simple radial canals; gonads on radial canals, separated from manubrium; marginal tentacles hollow; with marginal cirri; four or more closed statocysts.
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Family Clathrozoidae
Hydroid colony arborescent through skeleton of complexly fused chitinous stolons; hydrotheca tubular, entierly or nearly entierly embedded in skeleton; hydranth cylindrical, deeply retractile into hydrotheca; nematotheca tubular, scattered on surface of skeleton; gonophore as fixed sporosacs or free eumedusoids, gonotheca developed within skeleton.
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Family Dipleurosomatidae
Hydroid phase only insufficiently known, colony stolonal, hydrotheca sessile, tubular turning gradually into a pyramidal operculum formed by numerous converging flaps.
Medusa manubrium with narrow base; three, four or more radial canals, radial canals either branched or if not so then irregularly arranged; gonads on radial canals separated from manubrium; ocelli may be present.
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Family Eirenidae
Hydroid colony of benthic species stolonal or ramified erect; bivalve-inhabiting species without perisarc, with pedal disc, usually solitary; planktonic species solitary, polyp budding totally into a single medusa; hydrotheca cylindrical in young colonies of erect forms, with diaphragm and folded pleated operculum formed by convergent flaps not demarcated from the hydrothecal rim; in older colonies of this type, operculum generally lost and hydrotheca reduced to perisarcal collar, of haleciid type; hydrotheca usually reduced or absent in stolonal colonies, hydranth naked, borne directly on hydrorhiza or on short pedicels; hydranth of commensal species elongated, extensile, with filiform tentacles in a single amphicoronate whorl; intertentacular web present; gonophores on hydranths, hydrocaulus, or hydrorhiza, naked or more usually at least initially in a gonotheca, in form of medusae or medusoids with gonads on radial canals.
Medusa manubrium small, on rather well differentiated gastric peduncle; four to six simple radial canals running from subumbrella along peduncle to manubrium; with or without excretory papillae or pores on tentacle bulbs; tentacles hollow; with or without cirri or marginal warts; gonads on radial canals, separated from manubrium, eight and more statocysts; without ocelli.
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Family Haleciidae
Hydroid colony usually erect and branching, rarely stolonal, arising from a creeping hydrorhiza; hydrotheca sessile or pedicellate (on socalled hydrophore), shallow; hydranth much larger than hydrotheca, often robust, with or without intertentacular web, hydrothecal rim usually even, hydrotheca lacking operculum; renovation common; bottom of hydrotheca formed by concentric diaphragm, above diaphragm a ring of small knobs; endoderm of hydranths differentiated into proximal digestive part and distal non digestive pert, nematophores, nematothecae and nematodactyls present or absent; gonophores fixed or rarely free sporosacs; gonothecae solitary or grouped into a glomulus, infrequently with naked gonophores.
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Family Hebellidae
Hydroid colony stolonal or erect branched, hydrotheca campanulate with smooth margin, pedicel present; hydrothecal base with annular perisarcal thickening and membranous diaphragm or thick diaphragm and no annular thickening; gonotheca solitary, with or without operculum, arising from hydrorhiza; exceptionally with nematothecae; gonophores as fixed sporosacs, liberated meduoids, eumedusoids, or free medusae.
Medusa with flat umbrella; manubrium short and flat, mouth with irregular lips; with four or more branched radial canals; marginal tentacles hollow; gonads linear to sinuous on radial canals; with marginal cordyli; with or without marginal cirri; with or without adaxial ocelli; without statocysts.
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Family Laodiceidae
Hydroid colony stolonal, hydrotheca sessile, tubular, turning gradually into a pyramidal operculum formed by numerous converging flaps, with visible crease-line basally; hydrotheca often with transversal growth-rings, sometimes with basal constriction at origin, or exceptionally a poorly delimited pedicel; hydranth without intertentacular web, tentacles amphicoronate; gonotheca like hydrotheca but larger.
Medusa with marginal cordyli, cordyli with or without nematocysts; with or without marginal cirri; with four or eight simple radial canals; marginal tentacles hollow; gonads on radial canals, on radial canals and lobes of manubrium, or on manubrial pouches; with or without adaxial ocelli; without statocysts.
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Family Lovenellidae
Hydroid colony stolonal or erect sympodial; hydrotheca pedicellate, elongate, everted-conical to bell-shaped, with diaphragm; operculum conical, formed either by many triangular plates in embayments of hydrothecal margin and demarcated from hydrothecal wall by crease line, or pleated continuation of the hydrothecal wall; hydrothecae in old specimens often reduced to a collar-shaped sheath around hydranth base; with or without intertentacular web; gonophores free medusae, gonothecae peduncled. Medusa manubrium short; no gastric peduncle; no excretory pores; four simple radial canals; marginal tentacles hollow, with lateral cirri; no marginal cirri; gonads on radial canals, not reaching manubrium; 16 or more statocysts when adult; no ocelli.
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Family Malagazziidae
Hydroid colony stolonal; hydrotheca pedicellate, with a conical operculum formed by numerous convergent segments that are not clearly demarcated from hydrothecal wall; hydranth with intertentacular web; gonothecae claviform, arising from stolons.
Medusa with small manubrium; no gastric peduncle; four to eight, sometimes up to 12 radial canals; gonads completely surrounding radial canals, separated from manubrium; tentacle bulbs with adaxial excretory papillae; no permanent rudimentary marginal, only transient ones; with closed statocysts; no ocelli and cirri.
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Family Melicertidae
Hydroid stolonal or sparingly branched, arising from creeping hydrorhiza stolonal; perisarc thinning away completely below hydranth base, no hydrotheca; hydranth naked, large, spindle-shaped, amphicoronate tentacles; no intertentacular web; gonophores borne on hydranth column, no gonothecae.
Medusa with broad manubrium base; eight simple or bifurcated radial canals; marginal tentacles hollow; no cirri, statocysts or cordyli; with or without ocelli.
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Family Mitrocomidae
Hydroid stolonal, hydrotheca tubular, sessile, with pyramidal operculum made either of several triangular flaps, or of pleats in the continuation of hydrothecal tube, all not well demarcated from hydrothecal wall, lacking a crease-line at base of flaps or pleats; hydranth extensile, with a usually amphicoronate tentacle whorl; no intertentacular web; no nematophores; gonophores as free medusae, gonotheca pedicellate, arising from stolons.
Medusae with open marginal vesicles; with four, eight or more simple radial canals; with gonads on radial canals separated from stomach; with hollow marginal tentacles; with or without marginal cirri; with or without ocelli.
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Family Octocannoidae
Hydroid phase unknown.
Medusa with eight simple radial canals; eight normal marginal tentacles and16-32 short, club-shaped tentaculae, all marginal structures with black pigmented spots; manubrium short; mouth with eight simple lips; no gastric peduncle and excretory papillae; gonads along radial canals, split into two lateral halves; with numerous statocysts, no ocelli.
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Family Orchistomatidae
Hydroid phase unknown.
Medusa with very short manubrium on large gastric peduncle; mouth with 8-30 sinuous or crenulated folds; eight or more radial canals, simple, ramified, or in clusters of four; up to 64 marginal tentacles, laterally compressed; no marginal cirri, but in intertentacular space filiform, tentaculiform structures devoid of marginal bulbs, not in contact with circular canal; gonads usually on proximal parts of radial canals; numerous (up to 800) adaxial ocelli; without statocysts, without cordyli, without excretory pores or papillae.
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Family Phialellidae
Hydroid colony stolonal or erect sympodial, arising from a creeping hydrorhiza; hydrotheca pedicellate, tubular to deeply campanulate, persistent, with a cone-shaped operculum formed by separate triangular flaps demarcated or not from hydrothecal margin by a basal crease line; diaphragm present; gonophores as free medusae, gonotheca usually on stolons, sometimes on erect shoot.
Medusa with small small manubrium; no gastric peduncle; four radial canals; gonads on radial canals, separated from manubrium and divided into two moieties by a median groove; tentacles hollow, smooth or moniliform; without excretory pores, without cirri; eight closed statocysts, usually each on a bulbous-like swellings; without ocelli.
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Family Lafoeidae
Hydroid colony stolonal or erect, arising from creeping hydrorhiza; hydrotheca tubular to campanulate, radially or bilaterally symmetrical, often adnate for some distance basally, seldom pedicellate, margin entire, operculum usually absent, with or without diaphragm, without annular perisarcal thickening; hydranth with or without abcauline diverticulum; nematophores present or absent; cnidome: microbasic mastigophores; gonophores fixed sporosacs; gonothecae aggregated into coppinia or scapus, exceptionally solitary.
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Family Syntheciidae
Hydroid colony erect, arising from a creeping hydrorhiza, shoots unbranched or with stem and pinnately arranged hydrocladia, hydrothecae sessile, bilaterally symmetrical, in two or more longitudinal rows on hydrocaulus and hydrocladia, alternate or opposite, partly adnate, no real diaphragm but with perforated basal floor, hydrothecal rim even, operculum and nematophores absent; gonophore as fixed sporosacs, gonothecae arising from within hydrothecal cavity or from fenestrae below hydrothecae or from hydrorhiza.
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Family Sugiuridae
Hydroid colony stolonal; hydrotheca cylindrical, tapering distally, on a short pedicel; operculum pyramidal, formed by numerous convergent flaps that are not demarcated by a basal crease line; hydranth with intertentacular web; gonothecae unknown.
Medusa umbrella more or less elliptical, with up to six manubria (exceptionally 9), generally with four radial canals per well developed manubrium, in mature animals usually all joining circular canal but often incomplete canals; no centripetal canals; two to six ovoid gonads on some radial canals; marginal tentacles numerous; numerous statocyst; asexual reproduction by fission.
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Family Teclaiidae
Hydroid phase is not known.
Medusa with four simple radial canals, tentacles hollow, manubrium with four simple lips; elongated, sac-like gonads on radial canals, separated from manubrium; one to three cordyliform structures between successive tentacles; with or without open statocysts.
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Family Tiarannidae
Hydroid colony stolonal or erect; hydrotheca pedicellate or sessile, deep, asymmetric-tubular; operculum formed by two pleated membranes which meet one another like a gabled roof; gonothecae usually resembling hydrothecae, but larger; gonophores as free medusae or fixed sporosacs.
Medusa without apical projection, no gastric peduncle; with large, cruciform stomach with perradial basal lobes; mouth with four simple or crenulated lips; four simple radial canals; gonads folded on adradial walls of manubrium and or on perradial manubrial pouches; marginal tentacles numerous, hollow; hollow cordyli-like structures bearing nematocysts; no ocelli.
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Superfamily Sertularioidea
Hydroid colony usually erect, rarely stolonal, branched or not, hydrothecae on stems and branches, in two or more longitudinal rows, sometimes apparently uniseriate through secondary modification; hydrotheca stalked or sessile, if sessile then adnate to a varying degree, bilaterally symmetrical, usually with a toothed margin and a hinged operculum of one and more valves, may be lost in older hydrotehcae, hydrotheca with a diaphragm in stalked species and a definite floor perforated by an asymmetrical pore in sessile species; internal wall of hydrotheca often covered by a layer of epidermal epithelium that may contain nematocysts; hydranths with a single whorl of filiform tentacles, gastrodermis differentiated into an aboral and oral region, the former sometimes forming an abcauline sac on contraction; nematothecae absent. Gonothecae solitary, gonophore as fixed sporosacs, exceptionally as medusoids.
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Superfamily Plumularioidea
Hydroid colonies erect, branched or not, stems mono- or polysiphonic; hydrothecae always uniseriate, sessile, and partly adnate, rim cusped or even; no true diaphragm, but a well-defined floor with usually excentric hydropore; hydranths with conical hypostome and single whorl of filiform tentacles; with gastrodermis differentiated into digestive and non-digestive portions. Nematophores always present, usually with protective nematotheca; gonotheca borne singly or grouped, unprotected or protected by modified hydrocladia or appendages; gonophores usually fixed sporosacs, rarely much reduced medusoid.
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