The "football jellyfish". Bell hemispherical, diameter typically to ca. 40 cm, exceptionally to ca. 90 cm. Exumbrella surface finely granular, jelly thick, stiff over central 2/3, thinner and flexible in outer 1/3; translucent. Marginal lappets 8-12 per octant. Rhopalia 8; rhopalial lappets smaller than inter-rhopalar, pointed. Subumbrellar musculature in eight distinct peripheral muscle fields. Manubrium short, massive, translucent; concealed by 16 scapulets upon it. Each scapulet inverted Y-shaped in section, bearing numerous mouthlets. Mouth-arms inverted Y-shaped in section, supporting two long, massive, outwardly-directed blades also bearing numerous mouthlets. Terminally on each is a large, translucent terminal club. Stomach occupying central 1/3 of bell, roughly square with concave sides; from it 16 substantial radial canals connect to bell edge; younger specimens have narrow ring canal which follows closely outline of each marginal lappet; in many older specimens ring canal apparently absent in places and perhaps in some is completely lacking; an intermediate ring-canal about 1/3 of radius in from margin, broad; centripetal to this a coarse, irregular anastomosing network of canals, connecting only with intermediate ring-canal and not with radial canals. Peripheral to intermediate ring-canal a similar but finer meshwork, its branchings becoming increasingly more fine towards perimeter. Gonads 4, each a much convoluted lobe fundamentally forming most of a circle but this not obvious due to its convolutions. In older animals surface of gonad bearing grooves extending to its edge.
Coloration: bell translucent, surface recalling frosted glass; typically pale-grey to colorless but some specimens or populations also variously described as washed with pale milk-white, opalescent, and rose-red. In near-mature and older specimens the largely white gonads show diffusely through as four large internal patches. Marginal lappets neatly and narrowly edged with ultramarine blue, violet, or brown line of dense color, constant in width and following margin; rhopalar lappets less strongly colored, or colorless. "Frills" of scapulets and mouth-arms pink off west Scotland, also bluish, violet, yellowish and reddish recorded; and in terminal clubs of mouth arms canals may be bright blue, yellowish-brown, pink, deep purple-brown, or colorless. Maturing gonads tinged brown to reddish-brown or blue. All other structures translucent, colorless to pale greyish-white.
Remark: the validity of Rhizostoma luteum (Quoy and Gaimard, 1827) is unclear.