Protozoa- Definition, Characteristics, Classification, Examples

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Protozoa Definition

Protozoa may be defined as  “microscopic acellular animalcules existing singly or in colonies, without tissue and organs, having one or more nuclei”.

General Characters of Phylum Protozoa

Some of the characteristics are:

  1. There are about 50,000 known species of Phylum Protozoa.
  2. Protozoans exhibit mainly two forms of life; free-living (aquatic, freshwater, seawater) and parasitic (ectoparasites or endoparasites). They are also commensal in habitat.
  3. They are small, usually microscopic, not visualize without a microscope.
  4. They are the simplest and primitive of all animals.
  5. They have a simple body organization. i.e. with a protoplasmic grade of organization.
  6. The body is unicellular (without tissue and organs).
  7. They have one or more nuclei which are monomorphic or dimorphic.
  8. Body naked or bounded by a pellicle, but in some forms may be covered with shells and often provided with an internal skeleton.
  9. They are solitary (existing alone/single) or colonial (individuals are alike and independent).
  10. Body shape variables may be spherical, oval, elongated or flattened.
  11. Body symmetry either none or bilateral or radial or spherical.
  12. Body form usually constant, varied in some, while changing with environment or age in many.
  13. Body protoplasm is differentiated into an outer ectoplasm and inner endoplasm.
  14. The single-cell body performs all the essential and vital activities, which characterize the animal body; hence only subcellular physiological division of labor.
  15. Locomotory organs are fingers like pseudopodia, whip-like flagella, hair-like cilia or none.
  16. Nutrition may be holozoic (animal-like), holophytic (plant-like), saprozoic or parasitic.
  17. Digestion occurs intracellularly which takes place inside the food vacuoles.
  18. Respiration occurs by diffusion through the general body surface.
  19. Excretion occurs through the general body surface, but in some forms through a temporary opening in the ectoplasm or through a permanent pore called cytopyge.
  20. Contractile vacuoles perform osmoregulation in freshwater forms and also help in removing excretory products.
  21. Reproduction asexual (binary or multiple fission, budding, sporulation) or sexual (conjugation (hologamy), game formation (syngamy)).
  22. The life cycle often complicated with alternation of asexual and sexual phases (alternation of generation).
  23. Encystment commonly occurs to resist unfavorable conditions of food, temperature, and moisture, and also helps in dispersal.
  24. The single-celled individual not differentiated into somatoplasm and germplasm; therefore, exempt from natural death which is the price paid for the body.
  25. Protozoans exhibit mainly two forms of life; free-living (aquatic, freshwater, seawater) and parasitic (ectoparasites or endoparasites). They are also commensal in habitat.
  26. Examples: Euglena, Amoeba, Plasmodium, Paramecium, Podophyra, etc.

Classification of Phylum Protozoa

Phylum protozoa is a large and varied group and possess a complication in its classification.

The conventional scheme followed by Hyman (1940), Hickman (1961) and Storer (1965), etc. recognizes two subphyla on the basis of organs of locomotion and 5 classes as follows:

Sub Phylum A: Plasmodroma

  • Locomotory organelles are flagella, pseudopodia, or none.
  • Nuclei is of one kind.

Class 1: Mastigophora

  • Move by one to many flagella.
  • Example: Euglena.

Class 2: Sarcodina

  • Move and capture food by pseudopodia.
  • Example: Amoeba.

Class 3: Sporozoa

  • No locomotory organs.
  • All parasitic.
  • Spore-formation is common.
  • Example: Plasmodium.

Sub Phylum B: Plasmodroma

  • Locomotory organelles are cilia or sucking tentacles.
  • Nuclei of two kinds.

Class 4: Ciliate

  • Move by cilia.
  • Example: Paramecium.

Class 5: Suctoria

  • Move by cilia as young and by tentacles as an adult.
  • Example: Podophyra.

Another classification is based on the scheme given by the Committee on Taxonomy and Taxonomic Problems of the Society of Protozoologists, and mainly proposed by BM Honigberg and others (1964).

It divides protozoa into four subphyla.

Subphylum I: Sarcomastigophora

Subphylum II: Sporozoa

Subphylum III: Cnidospora

Subphylum IV: Ciliophora

Subphylum I: Sarcomastigophora

  • Locomotor organelles are pseudopodia or flagella.
  • The nucleus is of a single type (monomorphic).
  • There is no spore formation.
  • Syngamy occurs in reproduction.

Superclass A: Mastigophora

  • They are commonly called flagellates.
  • Locomotory organelles are flagella in adults.
  • The body is covered by a pellicle.
  • Binary fission is longitudinal.
  • They are mostly free-living though some are parasitic.
  • Nutrition is autotrophic or heterotrophic or both.

Class 1: Phytomastigophorea

  • Chlorophyll-bearing chromatophores present.
  • Nutrition mainly holophytic by phototrophy.
  • Reserve food is starch or paramylon.
  • They have usually only one or two flagella.
  • The nucleus is vesicular.

Order 1: Chrysomonadina.

  • Examples: Chromulina, Ochromonas, Dinobryon, Synura, Chrysamoeba, etc.

Order 2: Coccolithophorida.

  • Examples: Coccolithus, Rhabdosphaera, etc.

Order 3: Heterochloride.

  • Examples: Heterochloris, Myxochloris, etc.

Order 4: Cryptomonadida.

  • Examples: Chilomonas, Cryptomonas, etc.

Order 5: Dinoflagellida.

  • Examples: Noctiluca, Ceratium, etc.

Order 6: Euglenida.

  • Examples: Euglena, Phacus, Copromonas, Peranema, etc.

Order 7: Volvocida (Phytomonadida).

  • Examples: Volvox, Chlamydomonas, Eudorina, etc.

Order 8: Chloromonadida.

  • Examples: Vacularia, Coelomonas, Gonyostomum, etc.

Class 2: Zoomastigophorea

  • Chlorophyll or chromatophores absent.
  • Mostly parasitic.
  • Reserve food as glycogen.
  • Flagella one to many.
  • There is an undulating membrane.

Order 1: Choanoflagellida.

  • Example: Proterospongia.

Order 2: Rhizomastigida.

  • Examples: Mastigoamoeba, Dimorpha, etc.

Order 3: Hypermastigida.

  • Examples: Trichonympha, Lophomonas, Leptomonas, etc.

Order 4: Diplomonadida.

  • Examples: Giardia, Hexamita, etc.

Order 5: Kinetoplastida.

Suborder 1: Bodonina.

  • Examples: Bodo.

Suborder 2: Trypanosomatina.

  • Examples: Trypanosoma, Leishmania, etc.

Order 6: Bicosoecida

  • Examples: Salpingoeca, Poteriodendron, etc.

Order 7: Retortamonadida.

  • Example: Chilomonas.

Order 8: Oxymonadida.

  • Example: Oxymonas, Pyrsonympha, etc.

Order 9: Trichomonadida.

  • Example: Trichomonas.

Superclass B: Opalinata

  • They have numerous cilia like organelles in oblique rows over the entire body surface.
  • There is no cytostome.
  • Two or more monomorphic nuclei are present.
  • Binary fission is interkinetal.
  • There is syngamy with flagellated anisogametes.
  • All are parasitic, mainly in frogs and toads.
  • Examples: Opalina, Protoopalina, Zelleriella, Protozelleriella, and Cepedea.

Superclass C: Sarcodina

  • Locomotory organelles are pseudopodia.
  • The amoeboid form is predominant.
  • Some have a hard shell.
  • They generally do not form spores.
  • The formation of gametes and flagellated young ones are common.
  • Nutrition holozoic or saprozoic.

Class 1: Rhizopodea

  • Locomotory organelles are pseudopodia (lobopodian or filopodia but never axopodia).
  • They are generally creeping forms.

Subclass a: Lobosia

  • Pseudopodia as lobopodian.

Order 1: Amoebida.

  • Examples: Amoeba, Entamoeba, Pelomyxa, etc.

Order 2: Arcellinida.

  • Examples: Arcella, Diffugia, Euglypha, etc.

Subclass b: Filosia

  • They have tapering and branching filopodia.
  • Examples: Gromia, Allogromia, Penardia (naked).

Subclass c: Granuloreticulosia

  • They have finely granular reticulose rhizopodia (reticulopodia).

Order 1: Foraminiferida

  • Examples: Globigerina, Elphidium, etc.

Subclass d: Mycetozoia

  • The amoeboid trophic stage develops either into a multicellular aggregation or into a true multinucleate plasmodium.
  • The life cycle is complex and has sexual reproduction.
  • Usually, sporangia are formed which liberate spores.
  • Nutrition is phagocytic.
  • Example: Plasmodiophora.

Class 2: Actinopodea

  • Pseudopodia mainly axopodia with axial filaments, radiating from a spherical body.
  • They are primarily sessile or floating forms.
  • Gametes are usually flagellated.
  • Reproduction is both sexual and asexual.

Subclass a: Radiolaria

  • The central capsule is perforated by one to many pores.
  • They have spicules or siliceous skeleton.
  • Filopodia or axopodia are present.
  • The capsule separates the protoplasm into ectoplasm and endoplasm.
  • All are marine.
  • Examples: Thalassicola, Collozoum, Lithocircus, etc.

Subclass b: Acantharia

  • Imperforate, non-chitinoid central capsule without pores.
  • The anisotropic skeleton of strontium sulfate.
  • Axopodia present.
  • Marine
  • Example: Acanthometra.

Subclass c: Heliozoia

  • There is no central capsule.
  • Rounded body with radiating axopodia.
  • Usually naked, if a skeleton is present it is made of siliceous scales and spines.
  • They have axopodia or filopodia.
  • There may be more than one nucleus, mostly in freshwater.
  • Examples: Actinophrys, Actinosphaerium, Clathrulina, etc.

Subclass d: Proteomyxidia

  • Largely marine and freshwater parasites of algae and higher plants.
  • Filopodia and reticulopodia in some species.
  • Examples: Vampyrella, Pseudospora, etc.

Class 3: Piroplasmea

  • Small, round-shaped or amoeboid parasites in vertebrate red blood cells.
  • Example: Babesia.

Subphylum II: Sporozoa

  • Locomotory organelles absent.
  • Spores usually present.
  • Exclusively endoparasites.
  • Cilia or flagella may be present in gametes.
  • Syngamy takes place after which many spores are formed.
  • The spores are simple and contain one to many sporozoites.
  • Sporozoites are the infective phase.
  • The nucleus is of the single type.

Class 1: Telosporea

  • Pseudopodia are generally absent.
  • Locomotion by gliding or body flexion.
  • Spores are formed and there are flagellated microgametes in some.
  • Spores are without polar capsules and filaments, naked or encysted.
  • Reproduction by both sexual and asexual methods.

Subclass a: Gregarinia

  • Mature trophozoites are large and extracellular.
  • Reproduction is entirely sexual with sporogony.
  • The spores contain eight sporozoites.
  • They are parasites of the digestive tract and body cavity of invertebrates.
  • Examples: Gregarina, Monocystis, Nematocystis, etc.

Subclass b: Coccidia

  • Mature trophozoites are small and typically intracellular.
  • Each oocyst produces many sporozoites.
  • They are parasites of the digestive tract or blood of vertebrates.
  • Gametocytes are dimorphic.
  • Sporozoites multiply by schizogony in tissue cells.
  • Examples: Eimeria, Isospora, Plasmodium, etc.

Order 1: Eucoccida

  • Schizogony takes place.
  • Both sexual and asexual phases take place.
  • They are parasitic in epithelial and blood cells of invertebrates and vertebrates.

Suborder 1: Eimeriina

  • Macrogamete and microgametocyte develop independently.
  • There is no syzygy.
  • Macrogametocyte produces many microgametes.
  • The zygote is non-motile.
  • Oocyst does not increase the size during sporogony.
  • Sporozoites are encased in sporocyst.
  • Example: Eimeria.

Suborder 2: Haemosporina

  • Macrogamete and microgametocyte develop independently.
  • There is no syzygy.
  • Microgametocyte produces only a few microgametes.
  • Zygote of often motile.
  • Oocyst increases size during sporogony.
  • Sporozoites are naked.
  • Schizogony takes place in vertebrates and sporogony in an invertebrate host.
  • Hemoglobin of host cells forms pigment.
  • Example: Plasmodium.

Class 2: Toxoplasmea

  • Spores are absent.
  • There are no flagella or pseudopodia at any stage.
  • Reproduction by Asexual Reproduction (binary fission).
  • Cysts are formed which have many naked sporozoites.
  • Examples: Sarcocystis, Toxoplasma, etc.

Class 3: Haplosporea

  • Spores are present.
  • Pseudopodia may be present but flagella are absent.
  • Reproduction only by an asexual method.
  • Schizogony takes place.
  • Examples: Caelosporidium, Ichthyosporidium, etc.

Subphylum III: Cnidospora

  • Spores have several cells having one or more polar filaments which are coiled threads and can be shot out, and one or more sarcoplasms or sporoplasms (analogous to sporozoites).
  • All are parasitic.
  • Zygote gives rise to one or more trophozoites without sporogony.

Class 1: Myxosporidea

  • Spores are of multicellular origin and large.
  • There are one or more sporoplasms with two or three valves.
  • They are parasites of fish.
  • Examples: Myxobolus, Myxidium, Ceratomyxa, etc.

Class 2: Microsporidea

  • Spores are of unicellular origin and small.
  • There is one long tubular polar filament through which the sporoplasms emerges one valve only.
  • They are cytozoic (intracellular parasites) in arthropods and vertebrates.
  • Example: Nosema.

Subphylum IV: Ciliophora

  • They possess simple ciliary organelles for locomotion, infraciliature is subpeculiar.
  • They have two nuclei, a trophic macronucleus, and a reproductive micronucleus.
  • Binary fission is perkinetal.
  • Conjugation takes place with the fusion of nuclei, autogamy and cytogamy also occur.
  • There are never any free gametes.
  • Nutrition is mixotrophic or heterotrophic.
  • They usually have a cytostome.

Class 1: Ciliata

  • They possess cilia or compound ciliary structure as locomotory or food acquiring organelles.
  • There is the presence of an infraciliary system, composed of basal granules below the cell surface and interconnected by longitudinal fibrils.
  • Most ciliates possess a cell mouth or cytostome.
  • Anal aperture (cytopyge) permanent.
  • Two types of nuclei, one vegetative (macronucleus) and the other reproductive (micronucleus).
  • Fission is transverse.
  • Sexual reproduction never involves the formation of free gametes.
  • One or more contractile vacuoles present even in marine and parasitic types.

Subclass 1: Holotricha

  • Body cilia simple and uniform.
  • Buccal cilia mostly absent.

Order 1: Gymnostomatida.

  • Examples: Coleps, Dileptus, Didinium, Prorodon, Nassula, etc.

Order 2: Trichostomatida.

  • Examples: Colpoda, Balantidium, etc.

Order 3: Chonotrichida.

  • Examples: Spirochona, Lobochona, Chilodochona, etc.

Order 4: Apostomatida.

  • Example: Hyalophysa.

Order 5: Astomatida.

  • Examples: Anoplophyra, Maupasella, Hoplitophyra, etc

Order 6: Hymenostomatida.

  • Examples: Colpidium, Tetrahymena, Paramecium, etc.

Order 7: Thigmotrichida.

  • Examples: Thigmophyra, Boveria, etc.

Subclass 2: Peritricha

  • Adults without body cilia.
  • Apical end with buccal cilia.

Order 1: Peritrichida.

  • Examples: Vorticella, Carchesium, Trichodina, etc.

Subclass 3: Suctoria

  • Sessile and stalked body.
  • Young with cilia, and adult with suctorial tentacles.

Order 1: Suctorida.

  • Examples: Acineta, Ephelota, Podophyra, etc.

Subclass 4: Spirotrichia

  • Reduced body cilia.
  • Buccal cilia are well marked.

Order 1: Heterotrichida.

  • Examples: Stentor, Bursaria, Spirostomum, Nyctotherus, etc.

Order 2: Oligotrichida.

  • Examples: Halteria, Strombidium.

Order 3: Tintinnida.

  • Examples: Codonella, Favella, etc.

Order 4: Entodinomorphida.

  • Examples: Entodinium, Cycloposthium, etc.

Order 5: Odontostomatida.

  • Example: Saprodinium.

Order 6: Hypotrichida.

  • Examples: Euplotes, Stylonychia, Urostyla, Oxytricha, etc.


  1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
  2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.
  3. Trivedi P.C., Pandey S, and Bhadauria S. (2010). Textbook of Microbiology. Pointer Publishers; First edition
  4. Sastry A.S. & Bhat S.K. (2016). Essentials of Medical Microbiology. New Delhi : Jaypee Brothers Medical Publishers.

About Author

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Laxmi Neupane

Laxmi Neupane is doing her Ph.D. in Pharmaceutical Biotechnology at Helmholtz Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. She did her Master’s degree (M.Sc.) in Medical Microbiology from the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal, and her bachelor’s degree (B.Sc.) in General Microbiology from Pinnacle Academy, Kathmandu, Nepal. Her research interest is in isolating antimicrobial myxobacteria from the soil sample.

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