Mo. Const. art. III, § 11

The first election of senators and representatives under this constitution, shall be held at the general election in the year one thousand nine hundred and forty-six when the whole number of representatives and the senators from the districts having even numbers, who shall compose the first class, shall be elected, and two years thereafter the whole number of representatives and the senators from districts having odd numbers, who shall compose the second class, shall be elected, and so on at each succeeding general election. [Effective - 27 Feb 1945 , see footnote]