Taken primarily from Kozloff, 1987, 1996 p. 44 (Copyright 1987, 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
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| Lower taxonomic level | Main Page | Alphabetic Index | Systematic Index | Glossary |
| 1a | Benthic (solitary or colonial); polyps transparent or variously pigmented; with fixed gonophores or releasing free-swimming medusae | 2 |
| 1b | Pelagic; with an oval float and upright triangular sail; polyps deep blue or purple; releases medusae containing zooxanthellae | Velella velella |
| 2a | Hydranth not enclosed by a hydrotheca or a pseudohydrotheca | 3 |
| 2b | Hydranth at least partly enclosed by a hydrotheca or a pseudohydrotheca | 39 |
| 3a | Hydranth with tentacles; polyp solitary or colonial | 4 |
| 3b | Hydranth without tentacles, polyp solitary (in sediment or in mats of blue-green algae) | Protohydra ?leuckarti |
| 4a | Hydranth with 1 tentacle only (colony always found subtidally on bivalve shells; with knobbed "defensive" zooids in addition to feeding zooids; releases medusae) | Monobrachium parasitum |
| 4b | Hydranth with more than 1 tentacle | 5 |
| 5a | Hydranths usually with 2 or 3 tentacles when mature | 6 |
| 5b | Hydranths usually with more than 3 tentacles when mature | 7 |
| 6a | ||
| 6b | ||
| 7a | Tentacles on the hydranths arranged in 1 or 2 whorls | 8 |
| 7b | Tentacles on the hydranths scattered over the surface, not in distinct whorls | 18 |
| 8a | Tentacles of hydranths limited to 1 whorl | 26 |
| 8b | Tentacles of hydranths arranged in 2 whorls (1 whorl oral, the other aboral) | 9 |
| 9a | ||
| 9b | ||
| 10a | ||
| 10b | ||
| 11a | ||
| 11b | ||
| 12a | ||
| 12b | ||
| 13a | ||
| 13b | ||
| 14a | ||
| 14b | ||
| 15a | ||
| 15b | ||
| 16a | ||
| 16b | ||
| 17a | ||
| 17b | ||
| 18a | ||
| 18b | ||
| 19a | ||
| 19b | ||
| 20a | ||
| 20b | ||
| 21a | ||
| 21b | ||
| 22a | ||
| 22b | ||
| 23a | ||
| 23b | ||
| 24a | ||
| 24b | ||
| 25a | ||
| 25b | ||
| 26a | Polyps solitary, less than 2 mm tall | 27 |
| 26b | Polyps colonial, usually more than 2 mm tall | 29 |
| 27a | ||
| 27b | ||
| 28a | ||
| 28b | ||
| 29a | Stalks of hydranths enclosed by a perisarc for most of their length; colonies branched or unbranched above the stolons | 37 |
| 29b | Stalks of hydranths not enclosed by perisarc for most of their length; colonies not branched above the stolons | 30 |
| 30a | Stolons anastomosing to form a basal perisarcal crust from which the polyps emerge, the crust usually with spines | 31 |
| 30b | Stolons not forming a basal perisarcal crust and without spines | 34 |
| 31a | Gastrozooids usually with 8 tentacles | 33 |
| 31b | Gastrozooids usually with 12-24 tentacles | 32 |
| 32a | Spines smooth and long; adult gastrozooids usually with 12-20 tentacles; female gonophores with 1 egg | Hydractinia milleri |
| 32b | Spines jagged and sometimes coalescing to form ridges; adult gastrozooids usually with 20-224 tentacles; female gonophores with several to many eggs (usually more than 6) | Hydractinia aggregata |
| 33a | Spines numerous, smooth, short and slightly curved; gastrozooids uniformly pink in color (female gonophores with 1 egg) | Hydractinia laevispina |
| 33b | Spines not numerous, usually smooth and long; gastrozooids with a white hypostome (female gonophores with 1 egg) | Hydractinia sp. |
| 34a | ||
| 34b | ||
| 35a | ||
| 35b | ||
| 36a | ||
| 36b | ||
| 37a | ||
| 37b | ||
| 38a | ||
| 38b | ||
| 39a | Perisarc enclosing or partly enclosing the hydranth constituting a true hydrotheca (of consistent form, and often with toothed margins, devices for closing the aperture, and other elaborations) | 43 |
| 39b | Perisarc enclosing or partly enclosing the hydranth constituting a pseudohydrotheca rather than a true hydrotheca (pseudohydrothecae are generally of inconsistent and somewhat irregular form, and have transverse wrinkles) | 40 |
| 40a | ||
| 40b | ||
| 41a | ||
| 41b | ||
| 42a | ||
| 42b | ||
| 43a | Hydrotheca wider than deep, flaring, with the margin entire, not large enough to accomodate the hydranth when it contracts | Halecium spp or
Hydrodendron spp. |
| 43b | Hydrotheca deeper than wide, large enough to accomodate the hydranth when it contracts | 44 |
| 44a | Hydranths generally on distinct, slender stalks (except in certain species of Lofoea) | 54 |
| 44b | Hydranths not on distinct, slender stalks, but attached directly to the main stem or its branches | 45 |
| 45a | Hydrotheca without an operculum; with nematophores (specialized small polyps in which nematocysts are concentrated; there is often a nearly constant number of these in relation to each hydranth) | 52 |
| 45b | Hydrotheca with an operculum consisting of 1 to 4 flaps; without nematophores | 46 |
| 46a | ||
| 46b | ||
| 47a | ||
| 47b | ||
| 48a | ||
| 48b | ||
| 49a | ||
| 49b | ||
| 50a | ||
| 50b | ||
| 51a | ||
| 51b | ||
| 52a | Thecae of nematophores articulating with the branch from which they arise and thus movable to some extent | Plumularia spp. |
| 52b | Thecae of nematophores not articulating with the branch from which they arise, and thus not movable | 53 |
| 53a | Gonophores situated within corbulae, which consist of several pairs of leaflike expansions located on some of the branches | Aglaophenia spp. |
| 53b | Gonophores not situated within corbulae, but associated with slender branches (phylactogonia) that lack hydranths, but that have nematophores | Cladocarpus spp. |
| 54a | Hydrotheca with an operculum of 4 or more converging flaps | 55 |
| 54b | Hydrotheca without an operculum | 59 |
| 55a | ||
| 55b | ||
| 56a | ||
| 56b | ||
| 57a | ||
| 57b | ||
| 58a | ||
| 58b | ||
| 59a | Hydrotheca rimmed to strongly toothed, usually bell-shaped or goblet-shaped, a diaphragm always present in the lower part of the hydrotheca (except in the genus Rhizocaulus) | 60 |
| 59b | Hydrotheca with a rim, generally tubular (but sometimes bell-shaped or goblet-shaped); diaphragm usually absent (but sometimes present) | 69 |
| 60a | Colony consisting largely of free stems (these may be fascicled) that bear many hydranths | 61 |
| 60b | Colony consisting largely of stolons applied to the substratum, the free branches rarely bearing more than 3 or4 hydranths | 67 |
| 61a | Colony branching sympodially, the hydranths being produced alternately on one side of a branch and then on the other; stems usually not fascicled | 62 |
| 61b | Colony branching irregularly, the hydranths more or less forming whorls, stems fascicled (lacking a true hydrothecal diaphragm) | Rhizocaulus verticillatus |
| 62a | Gonangia releasing free medusae | 63 |
| 62b | Gonangia with fixed gonophores, not releasing free medusae | 66 |
| 63a | Released medusae disk-shaped and with 16 or more tentacles that are more or less of fixed length (not highly extensile) and have conspicuous rings of nematocysts | 64 |
| 63b | Released medusae spherical or bell-shaped and with 4 or more tentacles that are highly extensile and have scattered nematocysts | Clytia spp. |
| 64a | Rim of hydrotheca always even, not sinuous or toothed; colony usually unbranched; usually on brown algae, rarely on animals or inert substrata | Obelia geniculata |
| 64b | Rim of hydrotheca even, sinuous, or toothed; colony usually branched; usually on animals or inert substrata, less often on algae | 65 |
| 65a | Rim of hydrotheca with prominent teeth, perisarc not darkening as the colony ages | Obelia bidentata |
| 65b | Rim of hydrotheca usually even, but may be sinuous or slightly toothed; perisarc of older colonies becoming dark brown or black | Obelia dichotoma |
| 66a | ||
| 66b | ||
| 67a | Colony consisting of creeping stolons with 1 hydranth per upright stem; each gonangium producing a single thimble-shaped medusa that may either be retained or swim free; the medusa is less than 2 mm high and has mature gonads, but no stomach | Orthopyxis spp. |
| 67b | Colony slightly branched with a few hydranths on each upright stem; gonangia and medusae not as described in choice 67a | 68 |
| 68a | ||
| 68b | ||
| 69a | ||
| 69b | ||
| 70a | ||
| 70b | ||
| 71a | ||
| 71b | ||
| 72a | ||
| 72b | ||
| 73a | ||
| 73b | ||
| 74a | ||
| 74b |
Aglaophenia sp.
Hydractinia laevispina
Hydractinia milleri
Obelia dichotoma
Orthopyxis everta
Velella
velella