The third kingdom, next to the kingdoms of plants and animals, in the classification of living organisms. The Protista include all organisms that do not have any extensive development of tissues, such as algae, fungi, protozoa and bacteria.
The taxonomic Kingdom from which the other three eukaryotic kingdoms ( Fungi, Animalia and Plantae) are thought to have evolved. The earliest eukaryotes were single-celled organisms that would today be placed in this admittedly not monophyletic group. The endosymbiosis theory suggests that eukaryotes may have evolved independently several times.
Protista, a Greek word meaning "primitive" or "first," is the third kingdom in the classification of life forms. Protista consists of single-celled microbes -- such as algae, bacteria, fungi, and protozoa -- and multicellular organisms that are not differentiated (organized) into distinct tissues and organs, as are higher animals and plants.