The kingdom Protista is composed of Eukaryotic organisms that cannot be classified as animals, plants, or fungi. Of the six kingdoms, Protista is the least satisfying classification, because its members display the greatest variety. Most protists are single cell organisms, but some, such as the multicellular algae, are not. Some protists are photosynthetic, while others are heterotrophic. Some share characteristics with plants, others with fungi, and still others with animals.