An order of teleost fishes comprising eight families, 88 genera, and about 810 species that inhabit fresh and brackish waters of tropical and temperate North and Middle America, Eurasia, and Indo-Malaysia. Several features unify the order: a symmetrical caudal skeleton of one epural supporting an unlobed (round) caudal fin; low-set pectoral fins with a large, scalelike postcleithrum; sensory pores chiefly on the head; and the upper jaw bordered by protractile premaxillae. Sexual dimorphism is common, with the males brightly colored, and, in the livebearers, an anal fin that is modified as a gonopodium (intromittent organ) for internal fertilization. The top of the head is usually flat and the mouth is usually terminal to superior, features that adapt the fishes to feed on or near the surface of the water.