A cannula coupled to a displacement limiting coupling. The cannula includes at least one rigidizable segment and means for rigidizing the rigidizable segment. The displacement limiting coupling couples to an insertable device and aligns the axes of the insertable device and the cannula and establishes limits on the relative axial displacement between the cannula and the insertable device. The insertable device may have a steerable tip that maintains an overlap with the rigidizable segment due to the axial displacement limits.
