All single-celled eukaryotes are placed under protista. Phylogenetically the kingdom protista acts as a connecting link between the prokaryotic kingdom- Monera and the complex multicellular kingdoms- Fungi, Plantae and Animalia.
Being eukaryotes, the protistan cell body contains a well defined nucleus and other membrane-bound organelles. It is surrounded by plasmalemma (cell membrane). Cilia and flagella occur in a number of forms. Nucleus has typical structure-porous nuclear envelope, chromatin, nucleolus and nucleoplasm.
Monerans lack nuclear membrane.