Kingdom Animalia Linnaeus, 1758 - animals
   Phylum Cnidaria Hatschek, 1888 - cnidarians
     Class Hydrozoa Owen, 1843
       1 Order Hydroida Johnston, 1836 - hydroids see Order Anthoathecatae
       2 Order Milleporina - millepore hydrocorals see Suborder Capitata Kühn, 1913 () sensu stricto
       3 Order Stylasterina - stylasterine hydrocorals see Suborder Filifera Kühn, 1913
       4 Order Trachylina Haeckel, 1879 - trachyline medusae
       5 Order Siphonophora Eschscholtz, 1829 - siphonophorans
       6 ?Order Chondrophora Gray, 1849 - chondrophorans see Suborder Capitata Kühn, 1913 () sensu stricto
       7 Order Actinulida Swedmark & Teissier, 1959 - actinulidans see Suborder Actinulida Swedmark & Teissier, 1959