(Forbes and Goodsir, 1851)
Description
Umbrella high when young [E.gracilis-young ], expanding lateral to become just flatter than hemispherical when fully grown [E.gracilis-habitus]; velum 1/4 - 1/3 of bell radius; stomach short, cross-shaped in section, with long narrow gastric peduncle on small conical base, peduncle extending far beyond umbrella margin in fully grown specimens; mouth with four simple lips [E.gracilis-insignis ].
Radial canals four, extending along gastric peduncle; these and ring canal narrow.
Gonads restricted to parts of radial canals along gastric peduncle, extending almost right along peduncle and in female making it cross-shaped in section.
Marginal tentacles usually only two [E.gracilis-medusa ] or four [E.gracilis-insignis ], several more in occasional specimens; without distinct swellings at base; 50-60 marginal swellings. Lateral cirri spiral, one on each side of marginal tentacle bases and usually one on each side of most marginal swellings. Eight closed marginal vesicles each with 1-6, usually 3 concretions.
Young specimens E.gracilis-juvenile; E.gracilis-young.
Size
Umbrella diameter up to ca 30 mm (Russell, 1963d) but perhaps more usually up to ca 13 mm (Russell, 1953a; Kramp, 1959); apparently no more detailed information, but Russell (1953a) discussed development in relation to diameter.
Colour
Stomach and gonads pale pink or greenish, marginal swellings with fine black pigment granules, but colouration possibly to due local diet; remainder of medusa extremely transparent, glass-like.
Ecology
[After Russell, 1953a] Medusa recorded off Plymouth every month of year but most commonly May-July with a lesser peak August-September; elsewhere in British Isles reported at different places from April to October, in latter month for example by Hamond (1963) off Norfolk.
Depth range
Medusa in coastal plankton.
Distribution in the North Sea
Northern North Sea, Skagerrak, Kattegat, probably absent from the SW North Sea.
World distribution
Medusa recorded from European coasts from Gulf of Trieste north to North Sea; common in W English Channel. In all Scottish coastal waters but not abundant (Fraser, 1972); recorded twice from China.
Remark
Hydroid only known from culture.
[After Cornelius, 1995a]