Mo. Rev. Stat. § 84.200
It shall be the duty of the sheriff of the county or city in which any of the cities to which sections 84.010 to 84.340 may apply shall be located, whenever called on for that purpose by said boards, to act under their control for the preservation of the public peace and quiet; and, if ordered by them to do so, he shall summon the posse comitatus for that purpose, and hold and employ such posse subject to their direction; whenever the exigency or circumstances may, in their judgment, warrant it, the said boards shall have the power to assume the control and command of all conservators of the peace of the county or city in which any of the cities to which sections 84.010 to 84.340 may apply, whether sheriff, constable, policemen or others, and they shall act under the orders of the said boards and not otherwise.
§ 84.200, RSMo