Mo. Rev. Stat. § 521.470
521.470 Defendant may plead to merits, when.
521.470. Defendant may plead to merits, when. — Every defendant not served with a summons may, at any time before final judgment against him, appear and plead to the merits of the action on such terms as the court may direct.
(RSMo 1939 § 1484) Prior revisions: 1929 § 1320; 1919 § 1771; 1909 § 2340