The chapter discusses the classification and evolutionary relationships of the Kingdom Protozoa, which includes all eukaryotic microorganisms. The author, T. Cavalier-Smith, argues for a more conservative approach to classifying these organisms, treating them as a single kingdom with subkingdoms, infrakingdoms, parvkingdoms, and superphyla. He critiques the broader kingdom Protista, which includes not only protists but also bacteria and fungi, as being too diverse and poorly defined. Cavalier-Smith emphasizes the importance of phylogenetic evidence from ultrastructure and molecular sequences in defining the boundaries between different kingdoms. Key points include: - The distinction between Protozoa and Plantae is based on the presence or absence of chloroplasts and the ability to feed by phagocytosis. - The distinction between Protozoa and Fungi is based on the presence of chitinous cell walls and the mode of nutrition. - The distinction between Protozoa and Chromista is based on the location of chloroplasts and the presence of a periplast membrane. - The distinction between Protozoa and Animalia is based on the presence of collagenous connective tissue and a double-stranded ciliary necklace. - The distinction between Protozoa and Archezoa is based on the presence of mitochondria, peroxisomes, and dictyosomes. Cavalier-Smith proposes a revised classification that includes eight kingdoms: Eukaryota, Archaeobacteria, Protista, Animalia, Fungi, Plantae, Chromista, and Archezoa. He emphasizes the need to recognize the fundamental differences between these kingdoms and to avoid polyphyletic groupings. The chapter also discusses the evolution of eukaryotes, suggesting that the transition from a primitive archezoan to a well-developed, aerobically respiring protozoan involved significant changes in cell and macromolecular structure.The chapter discusses the classification and evolutionary relationships of the Kingdom Protozoa, which includes all eukaryotic microorganisms. The author, T. Cavalier-Smith, argues for a more conservative approach to classifying these organisms, treating them as a single kingdom with subkingdoms, infrakingdoms, parvkingdoms, and superphyla. He critiques the broader kingdom Protista, which includes not only protists but also bacteria and fungi, as being too diverse and poorly defined. Cavalier-Smith emphasizes the importance of phylogenetic evidence from ultrastructure and molecular sequences in defining the boundaries between different kingdoms. Key points include: - The distinction between Protozoa and Plantae is based on the presence or absence of chloroplasts and the ability to feed by phagocytosis. - The distinction between Protozoa and Fungi is based on the presence of chitinous cell walls and the mode of nutrition. - The distinction between Protozoa and Chromista is based on the location of chloroplasts and the presence of a periplast membrane. - The distinction between Protozoa and Animalia is based on the presence of collagenous connective tissue and a double-stranded ciliary necklace. - The distinction between Protozoa and Archezoa is based on the presence of mitochondria, peroxisomes, and dictyosomes. Cavalier-Smith proposes a revised classification that includes eight kingdoms: Eukaryota, Archaeobacteria, Protista, Animalia, Fungi, Plantae, Chromista, and Archezoa. He emphasizes the need to recognize the fundamental differences between these kingdoms and to avoid polyphyletic groupings. The chapter also discusses the evolution of eukaryotes, suggesting that the transition from a primitive archezoan to a well-developed, aerobically respiring protozoan involved significant changes in cell and macromolecular structure.