A cannula for insertion through an anatomical cavity wall to establish communication with the anatomical cavity includes an elongate tubular body having a distal end adapted to be disposed within the anatomical cavity and a proximal end adapted to be disposed externally of the anatomical cavity, a seal including a seal member disposed along the tubular body of the cannula, and a tubular pusher disposed in the tubular body and insertable through the seal to move the seal member from a normally closed position preventing fluid flow through the cannula to an open position allowing instruments of various sizes to be introduced through the tubular body via the tubular pusher without contacting the seal.