Key:  Hydroid Polyps

Phylum Cnidaria
 Class Hydrozoa

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



Taxonomic Levels Represented in This Key:

  Aglaophenia sp.
  Hydractinia laevispina
  Hydractinia milleri
  Obelia dichotoma
  Orthopyxis everta
  Velella velella



Page created by Dave Cowles, 2006
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