Forbes and Goodsir, 1851
Description
Umbrella usually just flatter than a hemisphere with fairly thin jelly [L.undulata-habitus ]; stomach quadrate, attached to subumbrella along arms of perradial cross. Velum broad, about 1/3 of bell radius. Mouth with four lips with folded margins. The four radial canals and ring canal narrow [L.undulata-subumbrellar ].
Gonads on upper parts of sides of four pouches formed by radial canals and lateral extensions of stomach, extending nearly to umbrella margin, with 5-8 pairs of lateral folds, in lateral view giving impression that radial canal is deep in upper part.
Marginal tentacles '200-300+' (Russell, 1953a) or up to '400-600' (Kramp, 1961), with very slight basal swellings with abaxial spur-like gastrodermal processes on underside of exumbrella seen in lateral view [L.undulata-margin ]. Small single ocelli on adaxial sides of some marginal tentacle bases. One to two spiral cirri between adjacent marginal tentacles, sometimes missing [L.undulata-margin detail ]. Usually one cordylus without nematocysts between adjacent marginal tentacles.
Young specimens: L.undulata-newly-habitus; L.undulata-young stage; L.undulata-young-habitus.
Size
Diameter recorded up to 37 mm (Kramp, 1961) but in British waters usually slightly less. Edwards (1968) stated that in the Firth of Clyde and (citing Kramp) off the Belgian coast the medusa 'is dwarfed, not attaining the large size it reaches in other places'.
Colour
Colour of stomach, gonads and tentacle bases usually slight, varying between yellow, brown and pink.
Ecology
At Plymouth, medusa recorded February-November, though considered a 'spring and summer form, reaching its maximum abundance in late summer'.
Depth range
Coastal to offshore, medusae occurring right across the West European Continental Shelf and in deeper water beyond. Depth range of hydroid not known.
Distribution in the North Sea
Northern and southern North Sea though not common (medusa); southern Bight of the North Sea, and in deeper, offshore waters (hydroid).
World distribution
Recorded from most of the Atlantic Ocean between ca. 70°N and 55-30°S, including Adriatic and Black Seas; China Sea; perhaps likely to occur nearly worldwide in coastal to shelf waters.
[Description after Cornelius, 1995a]