Phylum Coelenterata Leuckart, 1847 see Infrakingdom Coelenterata Leuckart, 1847
   Class Hydrozoa Owen, 1843
     Order Actinulida Swedmark & Teissier, 1959 - actinulidans see Suborder Actinulida Swedmark & Teissier, 1959
     Order Hydroida Johnston, 1836 - hydroids see Order Anthoathecatae
     Order Milleporina - millepore hydrocorals see Suborder Capitata Kühn, 1913 () sensu stricto
     Order Pteromedusae see Family Tetraplatidae
     Order Siphonophorida Eschscholtz, 1829 see Order Siphonophora Eschscholtz, 1829
     Order †Spongiomorphida Alloiteau, 1952
     Order Stylasterina - stylasterine hydrocorals see Suborder Filifera Kühn, 1913
     Order Trachylinida see Order Trachylina Haeckel, 1879