At the head of each department there shall be a full time or part time director appointed by the city manager with the approval of the council. Each director shall be an administrative officer of the city. He shall have supervision and control of the department he heads subject to the direction of the city manager. Two or more departments may be headed by the same person and the city manager may serve as the director of one or more departments in addition to his duties as city manager, if the council approves.