Mo. Rev. Stat. § 79.020

The mayor and board of aldermen of such city, whether the same shall have been incorporated before becoming a city of the fourth class or not, with the consent of a majority of the voters of such city voting on the question, shall have power to extend the limits of the city over territory adjacent thereto, and to diminish the limits of the city by excluding territory therefrom, and shall, in every case, have power, with the consent of the voters as aforesaid, to extend or diminish the city limits in such manner as in their judgment and discretion may redound to the benefit of the city.