Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida". Their closest living relatives, according to the modern view, are the Anthomedusae which are similar enough to have always been considered closely related, and the very apomorphic Siphonophora which were placed outside the "Hydroida". Given that there are no firm rules for synonymy for high-ranked taxa, alternative names like Leptothecata, Thecaphora or Thecata, with or without the ending emended to "-ae", are also often used for the Leptomedusae. World Hydrozoan Database currently uses the name Leptothecata.