The following description applies to adult or near-adult individuals. Disc flat, notably circular; diameter up to ca. 150 mm; mesoglea of central region lens-shaped, its edge with 21 (not 22) notches; coronal groove slightly closer to periphery than to centre; jelly thick, unbending; tentacles short. Marginal lappets typically 44, shortly elongate, rounded. Rhopalia 22, alternating with 22 marginal tentacles which are short, stout and sharply tapered; one "hypertrophied" tentacle (noticeably larger than the rest), not in exactly same radius as septum between lobes of stomach. Each tentacle inserted onto prominent basal pedalium. Other numbers of marginal tentacles occur, but recorded somewhat rarely. Coronal musculature robust, an annulus typically 5-10 mm wide, whitish, visible when pigmented epidermis sloughed off; outer half of annulus thicker than inner. Manubrium wide, projecting below disc, lips somewhat pointed, each thickened by bulging, longitudinal, mesogleal ridge. Stomach outline of 4 rounded lobes; 4 vertical gastric septa, triangular in horizontal section; 4 perradial ostia from stomach leading into annular gastrovascular sinus. Gastrovascular sinus typically divided by 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. Gonads 8, elongate-oval with rounded ends, forming broken annulus just internal to tentacle bases.
Coloration: reddish-brown; stomach deeply pigmented, epidermis over central mesogleal "lens" pale to dark; coronal muscle when exposed white to greenish-yellow; tentacles uncolored. Thin, colored epidermis often lost, revealing transparent mesoglea.
See Russell (1970) for discrimination of the four species. Atolla gigantea Maas, 1897, has been held distinct by some recent authors (e.g. Larson, 1986).
(Mainly after Russell, 1970).