Special tax districts are those units of local government, exclusive of the county and incorporated municipality, which have separate governing bodies, independent, in general, of other local governments, with power to provide some governmental or quasi-governmental service and to raise revenue by taxation, special assessment, or charges for services. Soil conservation districts are not properly classified as local government units, as they do not raise their own revenue. With the exception of school districts, historically most special districts have as their constitutional basis two amendments to the Constitution of 1876: (1) Article III, section 52 (1904), allowing the formation of special districts that could incur indebtedness up to one-fourth of the assessed property valuation, and (2) the conservation amendment, Article XVI, section 59 (1917), allowing the establishment of conservation and reclamation districts with no limit as to amount of debt or taxation. By 1994 a considerable number of special districts had been established for junior or community colleges. Forty-nine districts operated sixty-seven colleges in Texas, and the method had become the fastest growing style of post-secondary education. Not fully state supported, they are paid for in part by local taxes. About 17 percent of revenues come from local taxes, 61 percent from state appropriations, 15 percent from tuition and fees, and the remainder from federal aid and miscellaneous sources. Other special districts include over 900 water and utility districts, 326 housing authorities, 210 soil and water conservation districts, 86 hospital districts, 46 hospital authorities, 10 rural fire prevention districts, 8 mosquito control districts, 8 health districts, 5 noxious weed control districts, 3 three urban renewal agencies, 3 wind erosion conservation districts, one waste disposal authority, and one ground water subsidence district. Emergency medical and jail districts may also be established by counties. Municipal utility districts have been established by developers and have aroused criticism. Some claim that the structure manipulates the indebtedness so that unsuspecting home buyers end up with the financial burden.