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(Forbes, 1848)

Description
Umbrella usually somewhat flatter than hemisphere [Cosmetira pilosella ], jelly fairly thick in upper region thinning towards margin. Velum broad, ca. 1/5 bell radius.
Stomach short, quadrate, with broad base attached to subumbrella along arms of perradial cross. Mouth with four lips, margins crenulate. Radial canals four, narrow.
Gonads on radial canals, linear, 1/2 - 3/4 length of radial canal, slightly nearer ring canal than stomach, with median division in both sexes.
Marginal tentacles short, ca 65-100, with large round basal bulbs [C.pilosella-margin ]; without ocelli; 6-10 marginal cirri between adjacent tentacles, older cirri extending up onto exumbrellar surface. Eight open marginal vesicles with 12+ concretions sometimes arranged in two rows.
Young stages: C.pilosella-young-habitus; C.pilosella-young2-habitus.

Size
Diameter when fertile 20-48 mm; length of marginal cirri when contracted ca 0.3-2.0 mm, longest typically being oldest.

Colour
Stomach and gonads reddish violet, marginal tentacle bases deep purple with reddish centre.

Ecology
Medusa recorded March-October, commonest April-August (Russell, 1953a). Penetration of medusa into Skagerrak and Kattegat might indicate some tolerance of reduced salinity

Depth range
Planktonic, coastal to offshore; the species has been regarded as typically occurring on and over the Continental Shelf (Edwards, 1968), although the medusa has recently been found in mid-Atlantic Ocean (see next section).

Distribution in the North Sea
Recorded from Dover to Bergen, though scarce south of 56-59°N; also in Skagerrak and Kattegat.

World distribution
Medusa only recorded from the NE Atlantic Ocean. (Largely after Edwards, 1968; Fraser, 1972): Abundant but somewhat patchy off NW Scotland and out to the Porcupine Bank, becoming scarcer southwards; occasional Roscoff, found NW Spain (Ramil, 1988), reportedly uncommon Portugal. In the North Sea, recorded from Dover to Bergen though scarce south of a line from Stavanger to the Scottish border (Fraser); also occurs Skagerrak and Kattegat. Recorded in mid-Atlantic Ocean west of the Azores at 50-100 m (38°N, 35°W; Winkler, 1982).
Hydroid seldomly confidently identified (summary in Russell, 1953a). Probably from Faeroes to about Portugal, the relatively long-lived medusa stage being carried far from its point of release (Edwards, 1968).

Remark
Hydroid reared in aquarium but seldom confidently identified from nature (summary in Russell, 1953a).

[Description after Cornelius, 1995a]

Cosmetira pilosella