Pelvic fins and skeleton absent; pectoral fins and girdle absent in some; pectoral fins, when present, at least midlateral in position or higher and skeleton lacking bony connection to skull (posttemporal absent); dorsal and anal fins confluent with caudal fin (caudal fin rayless or lost in some); scales usually absent or, if present, cycloid and embedded; body very elongate (eel-like); gill openings usually narrow; gill region elongate and gills displaced posteriorly; gill rakers absent; pyloric caeca absent; maxilla toothed, bordering mouth; the two pre-maxillae (rarely absent), the vomer (usually), and the ethmoid united into a single bone; branchiostegal rays 6-49; swim bladder present, duct usually present; oviducts absent; opisthotic, orbitosphenoid, mesocoracoid, gular plate, posttemporal, postcleithra, supramaxilla, and extrascapular bones absent; ossified symplectic absent (cartilaginous one present in Synaphobranchidae); hyomandibular united with quadrate; ribs present or absent.