Mo. Rev. Stat. § 47.320
Whenever any new county is formed from territory taken from one or more older counties, or whenever any territory is taken from one county and attached to another, the new county so formed, or the territory so stricken off, shall be liable for its part of all the indebtedness or liabilities of the older county from which the same was detached, unless where, by contract between such new and old county, it is otherwise provided, and shall cause the same to be levied and collected as other taxes, and paid into the county treasury; and the county clerk of such county shall draw his warrant on the treasurer of his own county in favor of the treasurer of the old county from which the territory was taken for the amount so collected and paid in; and if any such detached territory shall fail to make provisions for paying the same, the county commission of the old county from which the same was taken shall cause the same to be levied and collected from the territory detached, in the same manner and to the same extent as if said detached territory had remained a part of the original county. In all cases where it is desired to strike territory from one county and attach the same to another, the county to which any such territory is to be attached may assume the payment of that portion of the indebtedness due to the older county, and which would otherwise have to be paid by the territory detached; but such fact shall be specified in the petition to the county commission and published in the notice of the election for the proposed change, and if not so stated in the notice, said county shall not be deemed to have assumed the payment of such liabilities, but the same shall be paid by the territory so detached as herein specified.
§ 47.320, RSMo