Acaulis primarius Stimpson, 1853 (redescription)
Acauloides ammisatum Bouillon, 1965 (redescription)
Boreohydra simplex Westblad, 1937 accepted as Plotocnide borealis Wagner, 1885 (redescription)
Candelabrum cocksii (Cocks, 1854) (redescription)
Corymorpha nana Alder, 1857 (redescription)
Corymorpha nutans M. Sars, 1835 (redescription)
Coryne eximia Allman, 1859 (additional source)
Coryne muscoides (Linnaeus, 1761) (additional source)
Coryne pusilla Gaertner, 1774 (additional source)
Ectopleura L. Agassiz, 1862 (redescription)
Ectopleura crocea (Agassiz, 1862) (redescription)
Ectopleura dumortierii (Van Beneden, 1844) (redescription)
Ectopleura larynx (Ellis & Solander, 1786) (additional source)
Ectopleura wrighti Petersen, 1979 (redescription)
Eucodonium brownei Hartlaub, 1907 (redescription)
Eudendrium album Nutting, 1898 (redescription)
Eudendrium annulatum Norman, 1864 (redescription)
Eudendrium arbuscula Wright, 1859 (redescription)
Eudendrium capillare Alder, 1856 (redescription)
Eudendrium capillaroides Schuchert, 2008 (redescription)
Eudendrium glomeratum Picard, 1952 (redescription)
Eudendrium racemosum (Cavolini, 1785) (redescription)
Eudendrium rameum (Pallas, 1766) (additional source)
Eudendrium ramosum (Linnaeus, 1758) (additional source)
Eudendrium unispirum Schuchert, 2008 (redescription)
Eudendrium vaginatum Allman, 1863 (redescription)
Eudendrium vervoorti Marques & Migotto, 1998 (additional source)
Euphysa aurata Forbes, 1848 (redescription)
Euphysa tentaculata Linko, 1905 (redescription)
Hybocodon prolifer Agassiz, 1860 (redescription)
Hydra circumcincta Schulze, 1914 (redescription)
Hydra oligactis Pallas, 1766 (redescription)
Hydra oxycnida Schulze, 1914 (redescription)
Hydra viridissima Pallas, 1766 (redescription)
Hydra vulgaris Pallas, 1766 (redescription)
Hydractinia proboscidea (Hincks, 1868) (redescription)
Lizzia blondina Forbes, 1848 (redescription)
Margelopsis haeckelii Hartlaub, 1897 (redescription)
Paratiara digitalis Kramp & Damas, 1925 (redescription)
Pliobothrus symmetricus Pourtalès, 1868 (redescription)
Plotocnide borealis Wagner, 1885 (redescription)
Protohydra leuckarti Greeff, 1870 (redescription)
Psammohydra nanna Schulz, 1950 (redescription)
Stylaster gemmascens (Esper, 1790) (redescription)
Stylaster norvegicus (Gunnerus, 1768) (redescription)
Trichydra pudica Wright, 1857 (redescription)
Tricyclusa singularis (Schulze, 1876) (redescription)
Tubularia indivisa Linnaeus, 1758 (redescription)

East Greenland Shelf for Stylaster gemmascens (Esper, 1790) 
Faroe Plateau for Stylaster gemmascens (Esper, 1790) 
North and East Iceland for Stylaster gemmascens (Esper, 1790) 
Southern Norway for Stylaster gemmascens (Esper, 1790) 

 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]