The assistant district attorney for the Twenty-First Judicial District shall receive as compensation for his services, in addition to the annual salary paid by the state, an annual salary of four thousand six hundred fifty dollars, payable monthly on his own warrant. The additional salary shall be paid by the parishes composing the Twenty-First Judicial District, apportioned as follows:
(1) By the parish of Livingston, the sum of one thousand six hundred thirty-five dollars per annum.
(2) By the parish of St. Helena, the sum of nine hundred fifteen dollars per annum.
(3) By the parish of Tangipahoa, the sum of two thousand one hundred dollars per annum.