The kingdom Protista (and not Ptotista), which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy. The protists do not have much in common besides a relatively simple organization, either they are unicellular, or they are multicellular without specialized tissues. This simple cellular organization distinguishes the protists from other eukaryotes, such as fungi, animals and plants.
Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems, particularly in the ocean as part of the plankton. Other protists, such as the Kinetoplastids and Apicomplexa, are responsible for a range of serious human diseases, such as malaria and sleeping sickness.
