The Kingdom Protista is a highly diverse group of unicellular eukaryotic organisms. Because it serves as a "catch-all" category for organisms that do not fit into the plants, animals, or fungi kingdoms, its members exhibit significant variation in cellular structure.

Members with cell walls: Includes plant-like protists such as diatoms and dinoflagellates, which possess cell walls often made of cellulose or silica.

Members without cell walls: Includes animal-like protists such as protozoans (e.g., Amoeba and Paramecium) and euglenoids, which lack a cell wall and instead may have a flexible protein-rich layer called a pellicle.

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