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Haeckel, 1880

Description
Umbrella flattened and disc-like [A.wyvillei-habitus ] ; exumbrella with a deep circular, coronal furrow situated nearer umbrella margin than apex; rim of central lens-shaped disc overhanging coronal furrow, rim occasionally with up to 21 notches; jelly thick. [A.wyvillei-umbrella ].
Usually 44 pedalial thickenings arranged in an innermost series of 22 solid tentacles alternating with an equal numbered outermost series of rhopalia. Tentacular pedalia may have longitudinal furrows; rhopalial pedalia together with hood and statocyst forming a sense organ.
Base of stomach in the shape of a four-leaved clover, stomach wall attached interradially to umbrella disc over four flattened triangular gastric septa; four perradial ostia from stomach leading into annular gastrovascular sinus. Gastric septa with small cavities and canals. Gastrovascular sinus typically with 22 pairs of closely adjacent radial septa, between which narrow channels run each to base of one rhopalium; septa diverging at inner ends and extending beyond inner margin of coronal muscle.
Manubrium wide, projecting below disc, with four leaf-shaped, somewhat pointed, perradial lips each thickened by bulging, longitudinal, mesogloeal ridge.
The eight gonads first develop as crescent-shaped endodermal thickenings on the subumbrellar wall of the gastro- vascular sinus opposite the ends of the gastric septa at about the level of the coronal furrow. In mature females each gonad is distinct, oval or bean-shaped; in mature males the gonads are irregularly folded and lengthened appearing as a more or less continuous circle of gonad tissue.

Size
Umbrella up to 150 mm in diameter, sexually mature specimens all over 40 mm in diameter.

Colour
In nature completely covered with a thin, very delicate, dark reddish brown epithelial layer; stomach walls always intensely dark.

Ecology
May reach sexual maturity at any time of year.

Depth range
In open waters in daytime at a depth of 500-1,500 m, closer to the surface at night and in upwelling areas.

Distribution in the North Sea
This deep-water species is likely to be absent from the northern North Sea, but is present in the Faeroe-Shetland and Faeroe-Iceland areas, off Greenland, Norwegian Sea, and S and W off Ireland.

World distribution
A deep-water cosmopolitan.

Remark
Polypoid stage absent.

[After Russell, 1970a; Russell, 1976]

Atolla wyvillei