General characteristics and classification of phylum Protozoa

  • Term Protozoa (From Greek, protos meaning first, zoon meaning animals) was given by Goldfass.
  • According to five-kingdom classification system, protozoans belong to the phylum Protozoa of kingdom Protista.

General characteristics:

  • The protozoans are minute, generally microscopic and eukaryotic organisms.
  • They are the simplest and primitive of all the animals with very simple body organization, i.e. Protoplasmic grade of organization.
  • They are unicellular organisms without tissues and organs.
  1. Habit and habitat:
  • They may either be free-living (inhabiting fresh water, salt water or damp places) or parasitic (living as ecto- or endoparasites). Some are commensals in habit.
  • Body is either naked or covered by a pellicle (plasmalemma or theca or lorica).
  • Protozoans are either solitary or colonial; in colonial forms, the individuals are alike and independent.
  1. Cell structure:
  • Body shape is variable; it may be spherical, oval, elongated or flattened.
  • They are usually asymmetrical but Giardia is bilaterally symmetrical.
  • The protoplasm is differentiated into outer ectoplasm and inner endoplasm.
  • They may have one or more nuclei. Nucleus may be monomorphic or dimorphic, vesicular (e.g. Entamoeba) or massive (e.g. Amoeba).
  • Vesicular nucleus is commonly spherical, oval or biconvex.
  • Dimorphic nuclei are found in Ciliata, one larger macronucleus (with trophochromatin) and other small micronucleus (with idiochromatin).
  • Locomotory organelles are either pseudopodia, flagella, cilia or none.
  1. Life processes:
  • There is no physiological division of labor and all the vital activities of life are performed by a single cell.
  • Nutrition may be holozoic (animal like), holophytic (plant like) sporozoic or parasitic.

*In Euglena, the mode of nutrition is mixotrophic (both holozoic and holophytic).

  • Digestion takes place inside the food vacuoles, i.e. intracellular.
  • Respiration and occurs by diffusion through general body surface.
  • Excretion occurs through general body surface like respiration. They are ammonotelic (excrete nitrogenous waste product in the form of ammonia).
  • In some forms, egestion occurs through a temporary opening in the ectoplasm or through permanent opening called Cytopyge.
  • Contractile vacuoles perform osmoregulation in fresh water forms and also help in removing excretory products.

*Contractile vacuole is absent in marine and parasitic forms.

  1. Reproduction:
  • Reproduction is either sexual or asexual; asexual binary reproduction occurs by fission, multiple fission, budding or sporulation and sexual reproduction occurs by gamete formation or conjugation.
  • Binary fission may be simple or transverse or longitudinal or oblique.
  • Life cycle often exhibits alternation of generation, i.e. it includes asexual and sexual phases.
  • Encystment usually occurs to protect the cell from the unfavorable conditions and it also helps in dispersal.


On the basis of locomotory organelles, phylum Protozoa has been divided into the following four classes.

Class: Flagellata or Mastigophora

  • Usually free living but few are parasitic forms.
  • One or more flalgella usually present for locomotion or food capturing or attachment or protection.
  • Body is covered with a pellicle which provides a definite shape.
  • Some forms are green due to the presence of chloroplasts (e.g. Euglena).
  • Asexual reproduction occurs by longitudinal binary fission.
  • Single nucleus present in a cell.
  • e.g. Volvox, Noctiluca, Trichomonas, Trypanosoma, Giardia, Leishmania etc.

Class: Rhizopoda or Sarcodina

  • Free living or endoparasite.
  • Contractile vacuole may be present or absent.
  • Locomotory organelles are pseudopodia which also help in food capturing.
  • Body is without a pellicle, and has no fixed shape.
  • Protoplasm is differentiated into ectoplasm and endoplasm.
  • Single nucleus is found in the endoplasm.
  • Nutrition is holozoic and parasitic in few forms.
  • Reproduction takes place by fission
  • e.g. Amoeba, Entamoeba etc.

Class: Spoprozoa

  • They are exclusively endoparasites.
  • Locomotory organelles are absent.
  • Body is covered by a thick pellicle.
  • Nutrition is saprophytic and contractile vacuole is absent.
  • Sexual reproduction takes place by gamete or spore formation.
  • e.g. Monocystis, Plasmodium etc.

Class: Ciliata

  • They may be either free living or endoparasite.
  • Nucleus may be one, two or many in number.
  • Body organization is complex.
  • Body shape and size is definite and is covered with a pellicle.
  • Cilia are the locomotory organelles.
  • They have a holozoic mode of nutrition.
  • Small micronucleus is reproductive in function whereas large macronucleus is vegetative in function.
  • Asexual reproduction occurs by transverse fission and sexual by conjugation.
  • e.g. Opalina, Nyctotherus, Balantidium, Paramecium etc.


General characteristics and classification of phylum Protozoa