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Boeck, 1866

Description
Somewhat delicate. Umbrella usually flatter than a hemisphere, with thin jelly [D.typicum-medusa ]. Velum fairly broad, width about 1/8 bell radius.
Stomach short, varied in shape, usually lobed; mouth with four simple lips. Radial canals straight, narrow, varying in number: 5-18+ [D.typicum-variation ]; irregularly spaced around bell, entering lobes of stomach; often giving appearance of branching basally; in some to many specimens incomplete centripetal canals growing inwards from circular canal towards stomach, and/or outwards from stomach centrifugally.
Gonads 1-12, usually 5, on inner half of most but not all radial canals; in form of sac in which planulae develop.
Marginal tentacles up to 100, each with bulbous base and adaxial ocellus; spirally coiled when contracted, extending up to 70-100 mm [D.typicum-margin ]. No marginal cirri or other sense organs.
Note — the radial canal system is most unusual, hardly any two specimens being alike. Its variation and development were studied in detail by E.T. Browne (summary in Russell, 1953a). Edwards, 1973d showed the medusa to propagate in the plankton by repeated transverse fission which, he considered, might account for both the radial asymmetry and the occurrence of dense swarms.

Size
Diameter recorded as up to 8 mm and as up to 12 mm.

Colour
In British Isles, colour of stomach and marginal tentacle bases recorded as yellowish-brown, brown through reddish brown; ocelli black; base of stomach ‘having appearance of ground glass’ (Russell, 1953a). British Columbian medusae recorded as colourless excepting black mouth-lips and base of stomach (Arai and Brinckmann-Voss, 1980).

Ecology
Reproductive season: Medusae recorded at Valencia (SW Ireland) April-October, but most commonly June-September (summary in Russell, 1953a). In Washington State (NW U.S.A.) a fully grown medusa was collected in March (Arai and Brinckmann-Voss, 1980).

Depth range
Medusa apparently coastal; no other information.

Distribution in the North Sea
Probably not in the North Sea, but recorded off Shetland as most adjacent to the area.

World distribution
Recorded in the Atlantic from the British Isles, the Faeroes-Shetland Channel (Fraser, 1972), north to mid-Norway and Newfoundland. In the Pacific, from SW Canada and NW United States (Arai and Brinckmann-Voss, 1980) and from Japanese waters (Yamada and Hirano, 1983) suggesting a boreal-circumpolar distribution.

Remark
Hydroid known only from The Clyde, Scotland (Kramp, 1961, Russell, 1953a; Russell, 1970a). Polyp known as Cuspidella sp.

[Description after Cornelius, 1995a]

Dipleurosoma typicum