GENERAL CHARACTERISTICS OF PROTOZOA

• Proto=Primitive/First/early; Zoon=Animals
• Earlier protozoans were included under animal kingdom, but now a days they are included in kingdom protista. This is because, protozoa are unicellular eukaryotes.
• They live in wide varieties of moist habitats such as fresh water, marine environment and may be in moist soil.
• They may be parasitic or commensal, symbiotic or free living, sessile or motile and mostly solitary and some are colonial.
• Protoplasmic grade of body organization is seen.
• Germ layers are absent.
• Body is naked or covered by pellicle or may be covered by an exoskeleton made up of shell/test/cyst/theca.
• Almost all types of symmetry are found, i.e-bilateral, radial, spherical and may be no symmetry (asymmetry).

• Locomotory organelles may be finger like pseudopodia, hair like cilia or whip like flagella. Some species do not have any locomotory organelles (eg., Sporozoans).
• Division of labour is restricted to organelles.
• Dimorphic nuclei are found.
• Nutrition may be holozoic, saprozoic or parasitic.
• Respiration takes place through general body surface.
• Digestion occurs inside the food vacuole. Mouth and anus are absent.
• Excretion by general body surface or by the help of contractile vacuole. In some forms temporary or permanent opening called cytopyge are found in the body surface.
• Intracellular circulation or cyclosis occurs.
• Nervous system is absent but a neuromotor system may be found.
• Asexual reproduction occurs by simple fission, budding and cyst formation. Sexual reproduction if occur then by conjugation or by syngamy.