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Classification
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Family Moerisiidae
Hydroid with moniliform or modified moniliform tentacles, scattered or
in one whorl around middle part of the hydranth body; medusa buds
between or just under the tentacles; polyp buds produced from lower
part of hydranth; hydrocaulus short, ending in pedal disc forming podocysts, or with short
stolon-like tubes ending in podocysts or
hydranths.
Medusa with prismatic manubrium, manubrium with basal radial lobes
giving into radial canals; no gastric peduncle; with or without
centripetal canals; mouth simple, cruciform; gonads on manubrium and
surrounding manubrial lobes or only on manubrial lobes overlying the
radial canals; with either four to several hundreds moniliform or
modified moniliform hollow marginal tentacles with adnate bulbs;
abaxial ocelli present.
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Family Sphaerocorynidae
Hydroid colony stolonal or erect, creeping stolons;
hydrocaulus long, unbranched or slightly branched; hydranth vasiform with bulbous base and long,
conical hypostome; no oral tentacles but numerous solid, single or
trifid capitate tentacles in 3-5 closely set whorls around broadest
part of hydranth body; gonophores borne singly or on short, branching
blastostyles between or below tentacles, free medusae or eumedusoids.
Medusa umbrella bell-shaped, apical mesoglea thick, apical projection
conical or dome-shaped, with broad apical chamber; manubrium either
flask-shaped or cruciform, base quadrate; mouth round or cruciform; in
non mature specimens gonads interradial, may be divided in adradial
masses by longitudinal median grooves, gonads adnate along perradii in
adult specimens; four hollow, marginal tentacles with either adaxial or
spirally-arranged nematocyst clusters and terminating in an ellipsoid
nematocyst knob; marginal bulbs clasping exumbrella, with an adaxial
expansion; one abaxial ocellus per tentacle.
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Family Corynidae
Hydroid colonial, monomorphic. With one or several whorls of capitate
tentacles, with or without one whorl of fine filiform tentacles.
Gonophores on hydranths, either liberated as free medusae or retained
as medusoids or sessile sporosacs. Exceptionally gonophores completely
reduced and gametes stored in hydranth wall. Free medusa with
four radial canals, four tentacle bulbs, and four tentacles. Each bulb
normally with one abaxial ocellus only. Manubrium cylindrical, without
lips, gonads encircle manubrium without perradial interruptions.
Exumbrella without nematocyst pouches. Nematocysts as in polyp stage,
but additionally with desmonemes,macrobasic euryteles absent. Newly
released immature medusae with manubrium always shorter than bell
cavity.
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Family Hydrocorynidae
Hydroid colony forming a chitinized hydrorhizal stolonal plate;
hydranths columnar, with thickened, ridged mesogloeal lamella;
tentacles hollow, capitate, in one or five to six close-set whorls
around a conical hypostome; gonophores in clusters on proximal part of
hydranth body or developing from from hydrorhiza. Gonophores free
medusae or sessile sporosacs.
Medusa umbrella evenly rounded; with or without gastric peduncle; four
marginal tentacles with scattered nematocyst knobs and a small
capitation; tentacular bulbs clasping, with ocelli; manubrium broadly
flask-shaped or tubular, quadrate or cruciform; oral part of manubrium
prismatic, ending in a cruciform mouth with or without nematocyst
clusters; gonads interradial, without longitudinal groove, surrounding
nearly the whole manubrium.
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Family Polyorchidae
Hydroid: unknown.
Medusa rather large, gastric peduncle usually well developed; manubrium
prismatic, with perradial pouches, 4 oral lips studded with
nematocysts; 4 radial canals with or without blind side branches;
gonads either spiral or sausage-shaped on perradial manubrial pouches
only; tentacles numerous (24-260) with stout elongate bulbs; abaxial ocelli.
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Family Halimedusidae
Hydroid: polyps small (<0.2 mm), solitary; 3-8, generally 4 distal
whorls of capitate tentacles, with a few scattered nematocysts along
their length; medusa buds single, just below tentacles.
Medusa usually with a low gastric peduncle and with distinct
interradial peaks in jelly above manubrium base; manubrium cruciform,
with basal perradial lobes; mouth quadratic to cruciform, with lips
lined by nematocysts; four radial canals; either with four perradial
marginal tentacles or with four perradial marginal tentacles and four
interradial groups of tentacles, all hollow at base; gonads either on
manubrium or on manubrium and perradial lobes; no mesenteries; marginal
bulbs cylindrical, with abaxial ocelli.
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Family Cladonematidae
Hydroid colony stolonal; hydranth with one whorl of oral capitate
tentacles, with or without aboral whorl of filiform tentacles; mouth
giving into a preoral chamber formed by epidermal gland cells;
medusa buds at base of hydranth or above filiform tentacles if
such
are present.
Medusa adapted to benthic mode of life and able to walk, some species
still able to swim freely; with or without a thickened continuous or
broken ring of nematocysts around umbrellar margin, with or without
brood pouch above manubrium; manubrium cylindrical, with or without
perradial pouches; mouth either with short lips, or with oral
nematocyst clusters, or with ramified oral tentacles; gonads either
completely surrounding manubrium, or on manubrial extrusions, or in
special brood pouches; number of radial canals variable, some may be
branched;
marginal branched, with upper branches bearing
nematocyst clusters and lower branches having adhesive organs; with
abaxial ocelli on tentacle base.
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Family Boeromedusidae
Medusa with apical projection, manubrium cylindrical, mouth simple,
tubular; four radial canals; four conical marginal bulbs; four simple,
hollow tentacles with many nematocyst clusters including a terminal
ovoid cluster; gonads on manubrium as four large perradial pouches
hanging freely in subumbrellar cavity; no ocelli. Hydroid:
unknown.
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Family Hydridae
Hydroid solitary, with hollow filiform tentacles, but often moniliform
distally, in one whorl under hypostome; lower part of hydranth with
simple pedal disc, no perisarc except on encysted embryos.. Without
medusa phase or gonophore development, eggs and sperm developed
directly in epidermis of polyps in wart-like protuberances; asexual
reproduction by lateral buds, leading only to temporary colonies
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Family Candelabridae
Hydroid solitary or forming pseudo-colonies. Hydrocaulus short, with
tubular or root-like adhesive processes, with or without perisarc.
Hydranth large and elongated, covered in distal half by numerous
scattered, hollow capitate tentacles, tentacles solitary or grouped adnate
basally;
gonophores fixed, developing on aboral part of hydranth below tentacle
covered region, either directly on hydranth or on spindle-shaped blastostyles. Gonophores fixed
sporosacs.
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Family Acaulidae
Hydroid solitary, body divided into basal and distal part. Basal part conical to cylindrical, covered or not by gelatinous perisarc which may form anchoring filaments. Distal part with tentacles, tentacles capitate or indistinctly capitate. Upper part of hydranth with one or two oral whorls of capitate tentacles and scattered capitate tentacles below; below capitate tentacles with or without one whorl of thick filiform tentacles Gonophores fixed sporosacs in the lower or middle part of the hydranth, sometimes with asexual reproduction by transverse fission.
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Family Boreohydridae
Hydroids solitary, small, with one whorl of reduced tentacles, capitate or not, located in the oral or median part of column; perisarc covering of base filmy or absent; gametes in body wall.
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Family Tubulariidae
Hydroid solitary or colonial; hydrocaulus divided into distal neck
region covered by thin perisarc, and proximal stem which may be either
short and thick with tuber-like aboral processes, or long and
cylindrical, or cone-shaped with basal disc, or with stolons covered by
thicker perisarc; neck perisarc secreted from a groove on the hydranth
proper; hydranth broadly vasiform, tentacles in two sets, oral ones
filiform or pseudofiliform in one to several close-set whorls,
sometimes capitate or moniliform (especially transiently in juveniles);
aboral tentacles long, in one whorl, pseudofiliform or filiform,
sitting on a more or less developed parenchymatic cushion of
gastrodermal tissue; gonophores develop above aboral tentacles, free
medusae or fixed sporosacs; embryonic development without planula but
an actinula larvae.
Medusae with or without exumbrellar nematocyst tracks, bell margin
oblique or straight, radial radial canals; mouth usually circular;
gonads covering manubrium entirely; one to four marginal tentacles;
with or without asexual medusae budding from marginal bulbs; ocelli
absent.
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Family Corymorphidae
(sensu Petersen 1990)
Hydroids solitary, hydrocaulus long, hollow or more or less filled with
parenchymatic gastrodermis, lower part of caulus with short papillae
or/and longer anchoring filaments; tentacles arranged into two separate
sets; oral tentacles composed either of one whorl of moniliform, one
whorl of capitate tentacles, or several whorls of filiform tentacles;
aboral tentacles arranged in one to three whorls of moniliform or
filiform tentacles; gonophores borne above aboral tentacles, either
directly issuing from hydranth wall or on blastostyles, gonophores
develop into free medusae or fixed sporosacs.
Medusa with or without apical projection; manubrium usually not
extending beyond umbrella margin, cylindrical or exceptionally with
sac-like processes; mouth simple, circular; gonads undivided
surrounding manubrium completely over its entire length, exceptionally
also on sac-like processes of manubrium; one to four hollow, usually
filiform tentacles, tentacles all uniform or differentiated into two
types, differing in size and structure, tentacles exceptionally
branched or rudimentary.
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Family Tricyclusidae
Hydroid solitary, attached by small pedal disk; hydrocaulus thin, as
long as hydranth, perisarc covering hydrocaulus inflated, gelatinous;
hydranth pear-shaped; with one oral whorl of up to six capitate
tentacles and two widely spaced whorls each of 8-14 stout, solid aboral, imperfectly moniliform tentacles; hydroid buds produced from
lower part of hydranth; actinuloid larvae arising from under aboral
tentacles; gonophores as fixed sporosacs, only male observed.
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Family Pennariidae
Hydroid colony pinnate, occasionally bushy, stem monosiphonic, giving rise alternately from opposite sides to two series of hydrocladia; hydrocaulus and hydrocladia with terminal hydranths (monopodial); hydranths on short pedicels on upper side of the hydrocladia; hydranths pear-shaped; tentacles of two types: in distal half of hydranth more or less capitate tentacles in one oral whorl and more in indistinct whorls below, on lower par of hydranth one aboral whorl of semifiliform to slightly capitate aboral tentacles; gonophores developing above aboral tentacles, eumedusoids, liberated or not.
Medusa a simple eumedusoid; manubrium not extending beyond umbrella margin; gonads completely surrounding manubrium; four radial canals; four permanently rudimentary tentacles, usually reduced to mere bulbs, without ocelli.
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Family Margelopsidae
Capitata with solitary, pelagic hydroids; hydrocaulus absent or reduced to a small process; hydranth vasiform, tentacles filiform to moniliform, arranged into two separate sets, oral tentacles in one or several whorls; aboral tentacles either in two to three alternating whorls or numerous and scattered over most of body; medusa buds among the tentacles or over the aboral tentacles.
Medusa manubrium with simple mouth; gonads surrounding manubrium entirely; four radial canals; tentacles solid, generally moniliform, two or more tentacles per marginal bulb, in some genera tentacles also issuing at different levels on exumbrella; without ocelli.
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Family Paracorynidae
Hydroid with polymorphic hydranths, differentiated into gastrozooids, gonozooids, and
dactylozooids; colony flat, basal plate divided into
upper layer of broad gastrodermal cavities and basal layer of large,
parenchymatic gastrodermal cells continuous with those in dactylozooids, transversed by mesogloeal lamellae, all enveloped by a
layer of epidermis, perisarc absent; gastrozooid short, stout, with one
to four whorls of solid capitate tentacles; gonozooids short, lacking
tentacles and mouth, bearing cryptomedusoid gonophores; eggs develop
into actinulae within gonophore or into encysted resting stage;
dactylozooids around edge of colony, long, finger-shaped, filled with
parenchymatic gastrodermis.
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Family Cladocorynidae
Hydroid colonial, stolonal simple or sparingly branched, hydranth club-shaped, one whorl of moniliform or
capitate oral tentacles, aboral tentacles moniliform or branched capitate, scattered or in several whorls; with nematocysts on body wall
arranged in conspicuous pads or scattered around the base of oral and
aboral tentacles; gonophores carried singly or on short, branched
pedicels on lower or middle part of hydranth; gonophores developing
into free medusae or fixed sporosacs.
Medusa with two opposite perradial tentacles possessing stalked
cnidophores and two non-tentacular bulbs; above each atentaculate
marginal bulb an exumbrellar pads containing macrobasic euryteles;
tentaculate marginal bulbs large, without nematocyst pads; gonads
interradial on manubrium.
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Family Zancleidae
Hydroid colonial, stolonal, pedicels present or very short; stolons creeping; polyps monomorphic or
polymorphic; gastrozooids cylindrical
or spindle-shaped, either with one whorl of oral capitate tentacles and
many scattered capitate or moniliform tentacles, or with reduced
capitate tentacles, or without tentacles; gonozooids and dactylozooids
- when present - with capitate tentacles or these reduced to nematocyst
patches.
Medusa umbrella evenly rounded; four perradial exumbrellar nematocyst
pads, pads either oval, clavate, elongate or linear, usually containing
stenoteles; manubrium cylindric, mouth simple, circular; gonads usually
interradial, rarely in a single mass around manubrium; four radial
canals; marginal tentacles 0, 2 or 4, hollow, each with numerous
abaxial stalked cnidophores containing macrobasic euryteles; without ocelli.
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Family Zancleopsidae
Hydroid phase so far unknown.
Medusa umbrella conical or dome-shaped, without apical chamber;
manubrium broadly flask-shaped, base quadrate or cruciform, mouth
square or circular, with or without faint lips; gonads either
surrounding manubrium or in four interradial pads, gonads have deep
interradial furrows which may divide them into eight adradial patches;
two to four capitate marginal tentacles, with or without lateral
capitate branches; marginal bulbs clasping umbrella margin and with
adaxial hemispherical projection armed with nematocysts; marginal bulbs
with or without ocelli either on marginal bulbs, or on proximal part of
tentacles.
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Family Teissieridae
Colony forming a basal plate or upright, branching hydrorhiza consisting
of basal or central periderm provided with spines that penetrate
overlying coenosarc, with or without peridermal ridges, overlaid by
gastrodermal tubes enclosed in common epidermis and covered externally
by thin, filmy periderm; hydranths polymorphic; broad, sausage-shaped,
with numerous scattered capitate tentacles, with or without a clear
whorl of oral tentacles; with or without simple dactylozooids with
scattered capitate tentacles; medusae bud carried singly among
tentacles on proximal part of gastrozooids; cnidome with macrobasic heteronemes.
Medusa with or without apical projection; four radial canals; four
perradial exumbrellar nematocyst patches on base of radial canals,
containing stenoteles, each pouch with an apical ocellus; two opposite
perradial tentacles with large bulbs without ocelli, abaxial
cnidophores on tentacles, non tentaculate perradial bulbs small or
absent;gonads on interradial of simple manubrium; one ocellus in the
most apical part of the exumbrellar pouches.
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Family Porpitidae
Hydroid colonial, mobile, when mature floating on water surface, with a
chitinous internal skeleton, forming a floating chamber, covered by
mantle, hydranths polymorphic on underside of float, one central,
large, atentaculate gastrozooid; surrounded by gastro-gonozooids and
dactylozooids; tentacles capitate or reduced to nematocyst patches.
Medusa with evenly-rounded bell; four or eight radial canals and an
equal number of exumbrellar nematocystcyst tracks containing stenoteles,
nematocyst tracks originating from marginal bulbs; circular canal
present; manubrium short, conical; mouth circular; gonads perradial or irregularly arranged perradially
and interradially; two opposite thin marginal tentacles terminating in
large spherical nematocyst knob, with or without an additional
adaxially oriented short tentacle developed from tentaculiferous bulbs;
two or six small marginal bulbs without tentacles; ocelli lacking;
cndome comprising spherical stenoteles, macrobasic euryteles and
atrichous haplonemes; zooxanthellae generally present.
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Family Solanderidae
Hydroid forming large, branching colony, with chitinous internal
anastomosing skeleton formed by coalescence and modification of
adjacent hydrocauline tubes; coenosarc covering entire colony and
penetrating skeletal interstices; hydranths over whole colony surface,
uniform in structure, cylindrical, with a single whorl of capitate
tentacles around mouth and numerous similar tentacles scattered over
body; gonophores, where known, cryptomedusoid or eumedusoid, arising
directly from coenosarc.
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Family Milleporidae
Hydroid colony forming massive, calcareous exoskeletons (= coenosteum)
of varied shape; coenosteum with an internal complex network of
coenosarcal tubes and covered externally by a thin ectodermal layer,
coenosteal surface perforated by pores; margins of pores not elevated;
large gastropores surrounded by smaller dactylopores, forming
indistinct cyclosystems; polyps polymorphic; gastrozooids relatively
short and stout, with an oral whorl of four to seven short capitate
tentacles, arising from gastropores; dactylozooids long, slender, mouthless, with scattered capitate tentacles, arising from
dactylopores; cnidome with stenoteles and macrobasic mastigophores;
gonophores arising from coenosarc within chambers embedded entirely in
the coenosteum.
Medusa free swimming eumedusoids with exumbrellar nematocyst patches,
radial and circular canals present, without tentacles and sense organs;
gonads occupying the place of an indistinct manubrium.
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Family Protohydridae
Solitary, small, elongated hydroids usually living in brackish-waters; without tentacles and gonophores, pedal disc formed by epidermal tissue only; gametes bulging into gastric cavity, nematocysts: stenoteles and isorhizas.
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