Mo. Rev. Stat. § 84.150: Total number of officers and number in each rank.

The total number of officers and the number of officers at each rank of the police force in each such city shall be determined by the board of police commissioners. The officers of the police force shall have commissions issued to them by the boards of police commissioners, and those heretofore and those hereafter commissioned shall serve so long as they shall faithfully perform their duties and possess the necessary mental and physical ability, and be subject to removal only for cause after a hearing by the board, who are hereby invested with exclusive jurisdiction in the premises.