Acaulis primarius Stimpson, 1853 (redescription)
Bougainvillia muscoides (Sars, 1846) (redescription)
Bougainvillia muscus (Allman, 1863) (redescription)
Bougainvillia principis (Steenstrup, 1850) (redescription)
Bougainvillia superciliaris (L. Agassiz, 1849) (redescription)
Candelabrum phrygium (Fabricius, 1780) (additional source)
Catablema vesicarium (A. Agassiz, 1862) (redescription)
Clava multicornis (Forsskål, 1775) (redescription)
Cordylophora caspia (Pallas, 1771) (redescription)
Coryne pusilla Gaertner, 1774 (redescription)
Dicoryne conferta (Alder, 1856) (redescription)
Ectopleura L. Agassiz, 1862 (redescription)
Ectopleura crocea (Agassiz, 1862) (redescription)
Ectopleura dumortierii (Van Beneden, 1844) (redescription)
Ectopleura larynx (Ellis & Solander, 1786) (redescription)
Eudendrium album Nutting, 1898 (redescription)
Eudendrium annulatum Norman, 1864 (redescription)
Eudendrium arbuscula Wright, 1859 (redescription)
Eudendrium capillare Alder, 1856 (redescription)
Eudendrium rameum (Pallas, 1766) (redescription)
Eudendrium ramosum (Linnaeus, 1758) (additional source)
Eudendrium ramosum (Linnaeus, 1758) (redescription)
Eudendrium vaginatum Allman, 1863 (redescription)
Euphysa aurata Forbes, 1848 (redescription)
Euphysa tentaculata Linko, 1905 (redescription)
Garveia franciscana (Torrey, 1902) accepted as Calyptospadix cerulea Clarke, 1882 (redescription)
Halitholus cirratus Hartlaub, 1914 (redescription)
Hybocodon prolifer Agassiz, 1860 (redescription)
Hydractinia echinata (Fleming, 1828) (redescription)
Leuckartiara nobilis Hartlaub, 1914 (redescription)
Leuckartiara octona (Fleming, 1823) (redescription)
Nemopsis bachei L. Agassiz, 1849 (redescription)
Neoturris breviconis (Murbach & Shearer, 1902) (redescription)
Neoturris pileata (Forsskål, 1775) (redescription)
Pachycordyle michaeli (Berrill, 1948) (additional source)
Plotocnide borealis Wagner, 1885 (redescription)
Podocoryna borealis (Mayer, 1900) (redescription)
Podocoryna carnea M. Sars, 1846 (redescription)
Proboscidactyla stellata (Forbes, 1846) (redescription)
Protohydra leuckarti Greeff, 1870 (redescription)
Rathkea octopunctata (M. Sars, 1835) (redescription)
Rhizorhagium roseum M. Sars, 1874 (redescription)
Sarsia lovenii (M. Sars, 1846) (redescription)
Sarsia tubulosa (M. Sars, 1835) (redescription)
Slabberia halterata Forbes, 1846 (redescription)
Stauridiosarsia producta (Wright, 1858) (redescription)
Trichydra pudica Wright, 1857 (redescription)
Tubularia indivisa Linnaeus, 1758 (redescription)
Velella velella (Linnaeus, 1758) (redescription)

 Diagnosis

 for Codonium Haeckel, 1879

Comprises Codonium proliferum (Forbes, 1884) and similar Corynidae with medusa that bud secondary medusae from ... [details]

 Diagnosis

 for Rhizogeton Agassiz, 1862

Stolonal hydroid colonies, polyps monomorphic, hydranths either sessile or with a perisarc-covered pedicel. ... [details]

 Distribution

 for Stylaster gemmascens (Esper, 1790)

North Atlantic from Denmark Strait, east of Greenland, northwest of Iceland through Faroes-Hebrides area to Norway ... [details]

 Taxonomy

 for Protohydridae Allman, 1888

The taxonomic position is unclear due to the very simple morphology. [details]