Which Kingdom do prokaryotic organisms belong to?

All prokaryotic organisms were groupedtogether under Kingdom Monera and the unicellular eukaryotic organismswere placed in Kingdom Protista. Kingdom Protista has brought togetherChlamydomonas, Chlorella (earlier placed in Algae within Plants and bothhaving cell walls) with Paramoecium and Amoeba (which were earlier placedin the animal kingdom which lack cell wall). It has put together organismswhich, in earlier classifications, were placed in different kingdoms. Thishappened because the criteria for classification changed.