Tiny and degenerate, having several vestigial features, and ephemeral; lacking mouth, manubrium and stomach [Orthopyxis integra ]. Umbrella bell-shaped, higher than wide; jelly fairly thick; velum broad, more than 1/2 bell radius. Four narrow radial canals, either meeting at apex or each closing before they meet; each with one irregularly-lobed sac-like outgrowth midway along each side; ring canal narrow.
Gonads on lobes of radial canal outgrowths [O.integra-habitus ]. No marginal tentacles or cirri; eight adradial marginal vesicles each with one concretion.
Sexually mature on release; swimming actively but probably living for less than a week; assumed not to feed. Limited evidence, from the two colonies studied, suggested that medusae of either sex are sometimes retained on the colony and not released, reportedly under environmental influence (Stefani, 1956; Stefani, 1959).
Umbrella ca 1 mm high.
Radial canal outgrowths and marginal bulbs dark brown; with brown pigments in radial and ring canals.
(After Cornelius, 1982a). Free medusae seen May-November in UK waters, June-September in NW France, December-February and July at Naples. Medusa release may occur more towards end of reproductive season than beginning. Reproductive structures on hydroid stage May-October in NW France. Hydroid on a wide variety of algae, hydroids, other animals and inorganic substrates; not reported from brackish waters.
Hydroid lowermost intertidally, rarely in large cool mid-shore rock pools, to a little below Continental Shelf depths.
Distribution around the British Isles
Throughout, but patchy. Infrequently reported around the coasts of Ireland, W Scotland, and Irish Sea, but more commonly from the coastal waters of England and S Wales.
Distribution in the North Sea
Nearly cosmopolitan, occurring in all oceans, from the tropics to 76°40'N (Greenland) and the whole northern coastline of Eurasia. Throughout Europe except Black Sea and lesser saline waters.
[After Cornelius, 1995b]