Hydromedusae with considerable variation in form, with umbrella usually deep bell-shaped; with gonads almost invariably situated on stomach, very rarely extending perradially on subumbrella; with or without ocelli; without statocysts. Hydroids always athecate.

Hydromedusae with umbrella usually hemispherical or flattened; with gonads on radial canals; marginal sense organs, when present, in form of cordyli or marginal vesicles of ectodermal origin; occasionally with ocelli.

Hydromedusae with umbrella margin entire and not divided into lobes; with thickened marginal nematocyst ring; with radial canals; with gonads usually confined to radial canals; with solid marginal tentacles, or with both solid and hollow tentacles, situated on the margin of the umbrella; with sensory clubs with endodermal axis which may be free or enclosed.

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.

Scyphomedusae with the umbrella margin cleft into lappets; with a single mouth opening provided with simple lips; with sense organs (rhopalia) and solid marginal tentacles arising from clefts between the lappets; with a circular, coronal furrow in the exumbrella, and peripheral to this a zone of gelatinous thickenings (pedalia) divided by radiating clefts alternating in position with the tentacles and rhopalia.

Scyphomedusae with umbrella margin cleft into lappets; without a coronal furrow and without pedalia; with (or without) hollow marginal tentacles and with marginal rhopalia; with a single central mouth opening provided with four large curtain-like or gelatinous lips; with gonads in sac-like folds of the endodermal wall of the subumbrella.