The kingdom is diverse, including both simple and fairly complex forms. Both uni- and multicellular forms are found within the group. Many biologists favor dividing this group into several kingdoms. Protista are virtually ubiquitous in water and moist terrestrial habitats. There are diverse symbioses between protists and hosts, with relationships grading from mutualism to parasitism.
Most protists have cilia or flagella at some time in life.
Almost all protists are unicellular, but several groups of algae are multicellular. Subsequently, fusion of gametes completes the cycle. Classification of Protista Autotrophic protists are called algae. In algae, the body is called a thallus since it lacks roots, stems and leaves. Differences between groups are due to diversity in photosynthetic pigments, carbohydrate food reserves, number and position of flagella, life cycle patterns and cell wall characteristics.