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.
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