Lesueur, 1813
Body flat, ribbon-shaped, strongly compressed in the tentacular axis, with the sides strongly extended in the stomodeal axis. Subtentacular comb rows reduced, represented by 4 short tracts close to sides of sense organ. At a short distance, 4 long stomodeal comb rows commence, extending along sides of arched aboral edge of animal. Tentacles with tentilla, closely set in a double row, extending along sides of oral edge of animal. Subtentacular mesodermal canals arise under the subtentacular comb rows.
Color: young animals transparent, older ones often becoming violet with greenish-blue or ultra-marine fluorescence. Length along the stomodeal axis: up to 2 m.