The suction tube comprises a clamping body (14 15) which engages a locking element (11) in a positive fit manner in an intermediate space (16) above the oblique sliding planes (123). The moveable slider (6) has guides (19) in a V-formation for two projections (17 18) forming part of the locking element (11). The clamping bodies are movable from the locking position along the oblique sliding planes. The locking element has a T-shaped cross section.
