Section 15. Oath and undertaking of commissioners

Before entering upon
  their duties such commissioners shall take the  constitutional  oath  of
  office,  and  make and file with the county clerk of their county, their
  joint and several undertaking, with two or more sureties to be  approved
  by  the  county  judge  of  their  county,  to the effect that they will
  faithfully discharge their duties as such commissioners, and truly keep,
  pay over and account for all moneys belonging to such corporation coming
  into their hands.