(A) Manubrium: It is a handle-shaped process, hangs down from the centre of sub-umbreller surface of the medusa. It bears four-sided mouth, surrounded by four oral lobes, at its free distal end. (B) Velum: Velum is a very narrow rudimentary fold that makes circular margin of the medusa.
In obelia it is not prominent but well developed in other Hydrozoans. (C) Tentacles: The circular margin of medusa bears, highly contractile tentacles. Their swollen bases are called tentacular bulbs, that may lodge the sense organs statocysts and serve as the sites for nematocyst-formation. A young medusa bears 16 tentacles. (D) Gastrovascular cavity: In medusa gastrovascullar cavity starts from mouth and leads into gullet that runs along manubrium. Gullet followed by a dilated stomach lying at the base of manubrium and central part of umbrella.
Along with circular margin (velum) a circular canal or ring canal runs and is linked with stomach through 4 radial canals. Radial canals run equidistant and at right angles to each other. (E) Gonads: Medusa is dioecious, i.e., testes and ovaries are borne by separate individuals. There are 4 gonads, formed as ventral diverticula of each radial canal in the middle. Each gonad is a knob-like body, has same epidermis and gastrodermis continuous with that of radial canal.
The space between these two layers is filled with a mass of sex cells, which become differentiated into sperms or ova, as the case may be.