A cannula comprising a monolithic, flexible body having a central passageway bounded by a first flange and a second flange, the central passageway having an irregular shape and a diaphragm positioned therein, the first diaphragm having the same irregular shape and including at least one first diaphragm slit, and a second diaphragm positioned adjacent to or at one of the first or second flanges and having at least one second diaphragm slit.
