A motor driven agitator providing slow rotational movement of a specimen holder rack with momentary periodic motion in the direction of the rotational axis induces maximum penetration of a treating fluid into specimens held in porous capsules on the rack. A self-aligning, automatic engaging clutch arrangement permits the agitator cover with motor, to be conveniently removed and replaced without complex mechanical components or annoying delays caused by unsuccessful efforts to align separated driving and driven members.
