Mo. Rev. Stat. § 1.100

1.100. Population, how determined — effective date of census — change in population, effect of. — 1. The population of any political subdivision of the state for the purpose of representation or other matters including the ascertainment of the salary of any county officer for any year or for the amount of fees he may retain or the amount he is allowed to pay for deputies and assistants is determined on the basis of the last previous decennial census of the United States. For the purposes of this section the effective date of the 1960 decennial census of the United States is July 1, 1961, and the effective date of each succeeding decennial census of the United States is July first of each tenth year after 1961; except that for the purposes of ascertaining the salary of any county officer for any year or for the amount of fees he may retain or the amount he is allowed to pay for deputies and assistants the effective date of the 1960 decennial census of the United States is January 1, 1961, and the effective date of each succeeding decennial census is January first of each tenth year after 1961.

2. Any law which is limited in its operation to counties, cities or other political subdivisions having a specified population or a specified assessed valuation shall be deemed to include all counties, cities or political subdivisions which thereafter acquire such population or assessed valuation as well as those in that category at the time the law passed. Once a city, county, or political subdivision has come under the operation of such a law a subsequent change in population shall not remove that city, county, or political subdivision from the operation of that law regardless of whether the city, county, or political subdivision comes under the operation of the law after the law was passed. No person whose compensation is set by a statutory formula, which is based in part on a population factor, shall have his compensation reduced due solely to an increase in the population factor.

(RSMo 1939 §§ 654, 13430, A.L. 1945 p. 1550, A. 1949 S.B. 1001, A.L. 1957 p. 587, A.L. 1959 H.B. 304, A.L. 1971 H.B. 154, A.L. 2017 H.B. 451) Prior revisions: 1929 §§ 654, 11808; 1919 §§ 7057, 11016; 1909 §§ 856, 10719 (1975) Held that this section clearly prevented St. Louis City from coming under law applicable to cities of 300,000 to 700,000 population. State ex rel. McNeal v. Roach (Mo.), 520 S.W.2d 69. (1975) This section is applicable to population for the purpose of convening grand juries. State ex. rel. Woods v. Connett (Mo.), 525 S.W.2d 326. (1981) Legislation enacted to address the class of which the City of St. Louis is the only member is not special legislation within the meaning of Article III, Section 40 , of the Missouri Constitution. Boyd-Richardson Co. v. Leachman (Mo.), 615 S.W.2d 46.