Mo. Rev. Stat. ยง 509.010
Shall be petition and answer โ reply if answer contains counterclaim.
509.010. Shall be petition and answer โ reply if answer contains counterclaim. โ 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.
(L. 1943 p. 353 ยง 32)