The commission is authorized to appoint a general manager for the district, prescribe his duties and fix his salary, such general manager to be responsible to the commission and to serve at its pleasure. The general manager shall appoint all other agents and employees of the commission, except as otherwise provided in § 21-238, dismiss them, fix their salaries or remuneration within the limits authorized by the commission, assign their positions and titles, define their respective powers and duties, and require them, or any of them, to give bond payable to the Commonwealth of Virginia in such penalty as shall be fixed by the commission, conditioned upon the faithful discharge of their duties.