SymbranchiiSym*bran"chi*i\, n. pl. [NL., fr. Gr. sy`n with + ? a gill.] (Zo["o]l.) An order of slender eel-like fishes having the gill openings confluent beneath the neck. The pectoral arch is generally attached to the skull, and the entire margin of the upper jaw is formed by the premaxillary. Called also Symbranchia.