Most are unicellular, but some may be colonial or multicellular. The kingdom includes protozoa, algae, chytrids, water molds, and slime molds. Many protista form plankton, which consists of microscopic organisms that form the bases of food webs. Protozoa are unicellular protista that move by using cilia, pseudopods, flagella, or amoeboid motion. Holozoic protozoa take in food by phagocytosis, and saprozoic protozoa do so by diffusion, facilitated diffusion, active transport, or pinocytosis.
Contractile vacuoles pump out water, and secretory granules contain enzymes.
A cyst form enables certain protozoa to survive harsh environmental conditions. All protozoa reproduce asexually, by binary fission, budding, or multiple fission, and may reproduce sexually. Some protozoa form zooplankton. The protozoa include seven phyla. Phylum Sarcomastigophora includes the flagellated Mastigophora and the Sarcodina, whose amoebae have pseudopods.