Trachyline medusae whose body is divisible into aboral and oral halves, separated by a line of tissue lying in the bottom of a constriction, this tissue representing the margin of the umbrella which is divided into four flaps acting as swimming lappets; the subumbrellar part of the body is highly reduced and consists of only the manubrium and the tissues on the adoral surfaces of the lappets. Without tentacles, but with statocysts on the oral sides of the lappets; statoliths endodermal. Gonads of ectodermal origin; development direct.