Protozoans are motile, heterotrophic microorganisms that lack cell walls. They are generally colourless, but exhibit vast morphological diversity and are found in marine, freshwater, and terrestrial habitats. Some protozoa live as parasites, but most are free-living. Their motility and consumption of other organisms make them appear animal-like, hence their name (proto="first"; zoa="animal"). In arctic marine environments they occur nestled in sediments on the ocean floor, suspended in open waters, and even grow within sea ice.