The city of St. Louis, as now existing, is recognized both as a city and as a county unless otherwise changed in accordance with the provisions of this constitution. As a city it shall continue for city purposes with its present charter, subject to changes and amendments provided by the constitution or by law, and with the powers, organization, rights and privileges permitted by this constitution or by law. As a county, it shall not be required to adopt a county charter but may, except for the office of circuit attorney, amend or revise its present charter to provide for the number, kinds, manner of selection, terms of office and salaries of its county officers, and for the exercise of all powers and duties of counties and county officers prescribed by the constitution and laws of the state.