Mo. Rev. Stat. § 509.010: Shall be petition and answer — reply if answer contains counterclaim.

There shall be a petition and an answer; and there shall be a reply if the answer contains a counterclaim denominated as such; an answer to a cross-claim, if the answer contains a cross-claim; a third-party petition if leave is given to summon a person who was not an original party; and there shall be a third-party answer, if a third-party petition is served. No other pleading shall be required except that the court may order a reply to an answer or a third-party answer.