The Hydrozoa, on the basis of embryological, developmental and morphological features, are considered as a superclass of the phylum Cnidaria comprising three classes: the Automedusa (with the subclasses: Actinulidae, Narcomedusae and Trachymedusae), characterised by direct development of the planula into a medusa; the Hydroidomedusa (with the subclasses: Anthomedusae, Laingiomedusae, Leptomedusae, Limnomedusae, and Siphonophorae), characterised by a polyp stage budding medusae through a medusary nodule; and the Polypodiozoa, with complex endocellular parasitic life cycles
