Mo. Rev. Stat. § 236.060: Writ of ad quod damnum.

Upon filing the petition, it shall be the duty of the court to cause a writ of ad quod damnum to be issued, under the seal of the court, directed to the sheriff, commanding him to summon twelve fit persons of his county to meet at the place where it is proposed to erect a dam, on a day to be named in the writ, then and there to inquire, by the said jury, touching the matters contained in the petition a copy of which shall accompany the writ.