Swamp eels are almost entirely finless; the pectoral and pelvic fins are absent, the dorsal and anal fins are vestigial, reduced to rayless ridges, and the caudal fin ranges from small to absent, depending on species. Almost all of the species lack scales. The eyes are small, and in some cave-dwelling species, they are beneath the skin, so the fish is blind. The gill membranes are fused, and the gill opening is either a slit or pore underneath the throat. The swim bladder and ribs are also absent. These are all believed to be adaptations for burrowing into soft mud during periods of drought, and swamp eels are often found in the mud underneath a dried-up pond.