The sur-snake was a poisonous aquatic creature native to the planet Argo. The source of the sur-snake's poison was from a venom shot from its mouth. It was noted for having dual respiratory systems, both lungs and gills, and a similar skeletal structure to that of the Denebian whale.
Despite its poisonous nature, the sur-snakes' venom was used in 2269 to reverse the surgo-op, a mutation Spock and James T. Kirk underwent that transformed them into water-breathers. (TAS: "The Ambergris Element")