An order of jellyfish of the class Hydrozoa of the phylum Cnidaria. These jellyfish are of moderate size. They differ from other hydrozoan jellyfish in having balancing organs which develop partly from the digestive epithelium and in having only a small polyp stage or none at all. Many authorities now recognize three distinct orders of trachylines, Limnomedusae, Trachymedusae, and Narcomedusae, and in this case the older term Trachylina is abandoned.