Mo. Rev. Stat. § 521.350: Practice in civil cases to govern in absence of special rules.

In all cases not specially provided for by sections 521.010 to 521.650, all pleadings and other proceedings in attachment causes shall conform to and be governed, as near as may be, by the laws regulating the practice in civil cases; and the respective courts may prescribe by rule the time and manner of interpleading, of exhibiting and filing papers, or taking any other needful steps therein, when the same are not prescribed by law; but nothing herein contained shall be construed to prevent the defendant from pleading to the merits of any action instituted upon a demand not due, at any time before the maturity thereof.