Phylum-Protozoa | Definition, Classification, Characteristics, Examples

Protozoans are microscopic, acellular animalcules that do not have tissue and organs, that possess one or more nuclei.

Phylum Protozoa includes all unicellular or acellular eukaryotes. They are the simplest and primitive organisms (Gr. Protos = first, zoon= animal).

They are capable of performing all the necessary life processes independently. Some 50,000 protozoan species are known, of which 30,000 species are living today.

Brief History

  • It was Leeuwenhoek (1677) who first studied protozoa.
  • Goldfuss (1817) coined the term, Protozoa.
  • The study of protozoa is done under a branch of Zoology called Protozoology. A number of scientists have made important contributions in this field.

Important Characteristics of Protozoa

  • Microscopic, Unicellular or acellular organisms, cell living eukaryotic organization.
  • Free-living or parasitic, some form colonies, otherwise solitary.
  • Symmetry: none or bilateral or radial.
  • Cell body naked or covered with a thin pellicle. Some have protective housing called test, shell or lorica.
  • No division of labour. Body organization of cytoplasmic level. Tissues absent.
  • Locomotory organelles are pseudopodia, flagella, cillia or none.
  • Nutrition is holozoic, or holophytic, or saprozoic, or parasitic. Myxotropic, i.e, combining more than one method. Digestion intracellular.
  • Respiration is done through the cell surface.
  • Osmoregulatory organelles, the contractile vacuole is present in free-living freshwater forms.
  • Reproduction is asexual by binary fission, multiple fission, or budding. Some reproduce sexually by fusion of gametes or by conjugation.
  • Encytment an adaptation for protecting against adverse conditions, distribution and sometimes for reproduction also.
  • Nucleus one or many. Some show nuclear dimorphism.
  • Examples of Protozoa are: Amoeba, Entamoeba, Euglena, Plasmodium and paramecium
Image of Examples of Protozoa | 1. Amoeba 2. Acinela 3. Euglena 4. Trypanosoma 5. Vorticella 6.Paramecium

Classification of Phylum Protozoa

Classification of Phylum Protozoa
Classification of Phylum Protozoa


Characteristics of Sarcomastigophora

  • Locomotory organelle – pseudopodia, or flagella or both
  • Nucleus – one or more, generally alike
  • Nutrition: autotrophic or heterotrophic type
  • Asexual reproduction: by multiple fission or binary fission
  • Sexual reproduction: by complete fission of gametes, i.e., Syngamy
  • A complicated life cycle exhibits an alternation of generations.

Super Class 1: Rhizopoda or Sarcodina

Characteristics of Sarcodina are followings:

  • Body without pellicle
  • Exoskeleton or endoskeleton present
  • Locomotion by pseudopodia
  • Nutrition holozoic or saprozoic type
  • Asexual reproduction by binary fission

Class 1: Actinopodia

Characteristics of Actinopodia:
  • Body naked or covered by the membrane-like test.
  • Axopodia with axial filament
  • Examples of Actinopodia are: Collozoum, Lithocircus, Actinophrys, Pseudospora
Labelled Diagram of Actinophrys

Class 2: Rhizopodia

Characteristics of Actinopodia:
  • Pseudopodia are lobopodia, filopodia or reticulopodia type without axial filament.
  • Examples of Actinopodia: Amoeba, Entamoeba, Euglypha
Euglypha. A. External Features B. Horizontal Section (Diagrammatic)
Entamoeba histolytica. (A) Trophozoite adult (B) Tetranucleate cyst.

Class 3: Piroplasmea

Characteristics of Piroplasmea:
  1. Small parasites in red blood cells of vertebrates.
  2. Do not produce spores.
  3. Example of Piroplasmea: Babesia
Diagrammatic Representation of Volvox Colony

Super Class 2: Mastigophora

Features of Maastigophora:

  • Locomotory organelles are flagella.
  • Primitive with a firm pellicle.
  • Asexual reproduction by longitudinal binary fission.
  • Nutrition autotrophic or heterotrophic

Class 1: Phytomastigophora

Characteristics of Phytomastigophora:
  • Chlorophyll bearing chromatophores present.
  • Nutrition generally holophytic type.
  • Free-living, freshwater or marine.
  • Reserve food paramylon or starch.
  • Flagella- one or two.
  • The sexual phase is present in the life cycle.
  • Examples are: Cryptomonas, Euglena, Noctiluca, Ceratium, Chrysamoeba, Coelomonas, Chilomonas, Chlamydomonas, Volvox
Labelled Diagram of Noctiluca
Diagrammatic Image of Ceratium

Class 2: Zoomostigiphorea

  • Chlorophyll bearing chromatoplast are absent.
  • Nutrition is saprozoic or holozoic
  • Flagella one or many.
  • Reserve food glycogen.
  • Sexual phase absent.
  • Examples are: Trypanosoma, Giardia, Trichomonas, Leishmania, Proterosporangia, Trichonympha, Mastigamoeba
leishmania (A) Leishmania Form (B) Leptomonad Form
Diagram of Trichomonas

Super Class 3 : Opalinata

Characteristics of Opalinata:

  • Parasitic in toads and frogs
  • Body covered by flagella
  • The nucleus is many and alike.
  • Example: Opalina


Characteristics of Sporozoa:

  • Exclusively endoparasitic
  • Locomotory organelles absent.
  • Nutrition saprozoic type
  • Pellicle present around the body
  • Asexual reproduction by Multiple Fission
  • Sexual reproduction by Syngamy
  • Both sexual and asexual phases present in the life cycle.

Class 1 : Telosporea

Characteristics of Telosporea:

  • Naked or encysted.
  • Sporozoites elongated
  • Polar capsules absent in spores
  • Examples: Monocystis, Plasmodium, Gregarina, Isospora, Eimeria
Monocystis – Trophozoite.

Class 2: Toxoplasmia

Characteristics of Toxoplasmia:

  • Spores are not formed.
  • Only asexual reproduction is found.
  • Examples: Toxoplasma, Sarcocysts

Class 3: Haplosporea

Characteristics of Haplosporea:

  • Spores cases present, but few in the cyst wall.
  • Only asexual reproduction.
  • Examples: Haplosporidium, Ichthysporidium
Plasmodium Sporozoite.
Giardia Labeled Diagram


Features of Cnidospora:

  • Spore with 1 to 4 polar filaments.
  • Many nuclei in adult trophozoite.

Class 1: Myxosporidia

Characteristics of Myxosporidia :

  • Spores enclosed within two or three valves.
  • Spores developed from several nuclei.
  • Examples: Myxidium, Triactinomyxon, Myxobolus

Class 2: Microsporidia

Characteristics of Microsporidia:

  • Parasitic in arthropods and fishes
  • Only one or two polar filaments present
  • Spores small with a univalved membrane
  • Examples: Nosema


Characteristics of Ciliophora:

  • Locomotory organelle cilia, present either throughout life or in young conditions.
  • Two types of nuclei present. One is mega and the other is micronuclei.
  • Asexual reproduction is done by budding or binary fission.
  • Sexual reproduction is carried out by conjugation.
  • Alternation of generations is absent.


Features of Ciliate:

  • Locomotor organelles numerous cilia.
  • Two kinds of nuclei are generally present. One is a larger macro-nucleus and the other is a smaller micro-nucleus.
  • One or more contractile vacuoles is present.
  • Nutrition is holozoic.
  • Definite mouth and gullet present.
  • Cytopyge (anal aperture) is permanent.
  • Examples are: Paramecium, Balantidium, Ephelota, Didinium, Podophyra, Colpidium, Halteria, Stentor, Nyctotherus
Ephelota (A) Adult (B) Budding form in Section (C) Budding form in lateral form
Labeled Diagram of Vorticelia

Examples of Protozoa

These are some of the main examples of protozoa and they are as follows:

  • Collozoum
  • Amoeba
  • Euglena
  • Trypanosoma
  • Opalina
  • Eimeria
  • Myxobolus
  • Nosema
  • Balantidium

Let us know them in detail one by one.


Classification of Collozoum:

ClassSarcodina or Rhizopoda

Distribution: Collozoums are found in the deep sea of tropical Pacific region.

Habits and Habitats: Collozoum is a marine colonial form and is usually found in deep seawater.

Characteristics features of Collozoum

  • The colony is naked without an external shell.
  • The cytoplasm is divided by a perforated chitinous membrane or the central capsule into an intracapsular or inner-medulla and an extracapsular or outer cortex.
  • The cortex or ectoplasm is vacuolated and pigmented.
  • It often contains yellow unicellular symbiotic algae.
  • The vacuoles are filled with fluids of light density.
  • The contractile vacuole is absent.
  • The cortex is common to the whole of the colony and may contain scattered siliceous spicules.
  • The central capsule undergoes repeated divisions, hence, the number of nuclei represents the number of individuals.
  • The increased individuals remain embedded in gelatinous mass to form colonies.
  • The skeleton is completely absent.
Labeled Diagram of Collozoum


Classification of Amoeba:

ClassSarcodina or Rhizopoda
Sub OrderGymnamobea or Nuda

Distribution: Amoeba are found all over the world.

Habits and Habitats: Amoeba is a minute fresh-water organism abundantly found in fresh-water ponds, pools, lakes, slow-running streams, springs, ditches etc. rich in decaying organic matter, vegetation, bacteria etc.

Characteristics features of Amoeba

  • In living condition, the organism appears as a whitish or colourless mass of gelatinous, irregular, transparent protoplasm slowly changing its shape. 
  • The size varies from 0.2 to 0.5 mm.
  • The body of the organism consists of a thin external layer, the plasmalemma; a non-granular region, just within, the ectoplasm and a granular inner region, the endoplasm, in which the nucleus lies. 
  • The plasmalemma is an elastic, semipermeable and tough membrane.
  • The ectoplasm is a clear, dense, transparent, firm and relatively thick layer.
  • The endoplasm is more fluid, semi-transparent central mass rich in neutral fats and carbohydrates. 
  • The endoplasm also contains several food vacuoles of different sizes and filled with particles of food in different stages of digestion and one or more round contractile vacuoles and other vacuoles.
  • A single biconvex disc-like and refractive body, the nucleus, is present in the endoplasm and is clearly visible in stained preparation. 
  • Amoeba has the capacity to form temporary protoplasmic extensions of the body, called pseudopodia (“false feet”).
  • The pseudopodia are blunt and finger-like with rounded tips and always undergo a continuous alteration of size and shape which results in locomotion of the organism. 
  • The nutrition is holozoic.
  • Respiration is affected by the diffusion of gases from the general surface of the body. 
  • The contractile vacuoles help in osmoregulation and excretion. 
  • Reproduction is by binary fission, encystment and sexual method.
Diagram of Amoeba


Classification of Euglena


Distribution: Cosmopolitan in distribution.

Habits and Habitat: Euglena is one of the commonest, largest fresh-water monoflagellate, and is abundant in stagnant ponds rich in organic matter, where their large number may impart green color to the pond.

Characteristics features of Euglena

  • The body is elongate and spindle-shaped, with a rounded anterior end, somewhat oval middle portion, and pointed posterior end.
  • The body is covered by a thin but double pellicle made of protein, with oblique but parallel striations. The pellicle is flexible enough to permit temporary changes in body shape during locomotion.
  • At the anterior end is a cytostome or cell mouth slightly to one side of the centre; it leads into a short cytopharynx or gullet which joins a large vesicle, the reservoir. The cytostome and cytopharynx are not used for ingestion of food but as a canal for the escape of fluid from the reservoir.
  • Near the reservoir is a contractile vacuole which is formed by flowing together of several vacuoles. The contractile vacuole discharges into the reservoir from where the fluid goes out through the cytostome.
  • The reservoir acts as a contractile vacuole and helps in osmoregulation. 
  • A flagellum arises from two roots in the reservoir. The roots of the flagellum arise from two basal granules.
  • Close to the reservoir lies a stigma or eyespot. 
  • The centre of the cell has a round nucleus with a clear nuclear membrane. 
  • Two delicate fibres called rhizoplates the basal join granules of the flagellum to the nucleus.
  • Radiating from the centre of the cell are several slender, elongated chromatophores containing chlorophyll.
  • The chromatophores manufacture their own organic paramylum. 
  • Asexual reproduction by longitudinal fission, encystment during adverse conditions and multiple fission in a cyst, producing four or more daughter cells enclosed within the cyst is called the palmella stage.
Diagram of Euglena


Classification of Trypanosoma

Phylum Protozoa
Order Protomonadina

Distribution: It is commonly found in Africa, Congo, Nigeria, Uganda and America.

Habits and Habitat: Trypanosoma is a parasitic and polymorphic form found in the blood and lymphatic system of man and other vertebrates.

Characteristics features of Trypanosoma

  • It is unicellular, microscopic, colourless, slender, flattened, leaf-like protozoan of about 10 to 40 u in length. 
  • The spindle-shaped body remains spirally twisted and is usually pointed at both ends.
  • The body is covered by a strong pellicle. 
  • The cytoplasm is differentiated into ectoplasm and endoplasm. 
  • The anterior end is more pointed and it originates along with the filamentous structure, the flagellum which is continued throughout the length of the body and is attached with the cytoplasm of the body.
  • At the base of the flagellum is situated a dark stained granule, the blepharoplast and a rounded structure of the parabasal body or kinetonucleus.
  • Blepharoplast and parabasal body is together known as kinetoplast. 
  • When the flagellum beats, the pellicle attached to it is drawn out into a membranous fold, the undulating membrane. 
  • There is a single large, rounded nucleus with a well-defined nucleolus.
  • The endoplasm contains several scattered greenish refractive volutin granules.
  • The cytostome and cytopharynx are absent. 
  • Asexual reproduction takes place by binary fission.
  • Sexual reproduction is not known. 
  • Tryanosomes are polymorphic i.e., in different stages of life history their forms of body change. 
  • They are transmitted by insects in terrestrial forms and by leeches in aquatic form.
  • Trypanosoma causes sleeping sickness in man.
Labeled Diagram of Trypanosoma


Classification of Opalina


Distribution: Cosmopolitan in distribution

Habits and Habitat: Opalina is found in the digestive tract of the Amphibia and in a few species from fishes and snakes.

Characteristics features of Trypanosoma

  • The entocommensal Opalina is generally flattened with an oval body having an anterior end slightly narrower than the posterior one. 
  • The body is covered with a tough pellicle having longitudinal spiral grooves bearing cilia.
  • The distribution of cilia is uniform and the ciliary pattern rather simple.
  • The cytoplasm is differentiated into ectoplasm and endoplasm.
  • The ectoplasm contains contractile fibrils, the myonemes, which help in flexion and extension of the body. 
  • The endoplasm typically contains numerous nuclei and granules. 
  • The cytostome, peristome and contractile vacuoles are absent.
  • The food is absorbed from the general surface of the body. 
  • Asexual reproduction is by binary fission.
  • Sexual reproduction by gamete formation.
  • The fusion of isogamous gametes results in a zygote followed by encystment.
  • The cysts are eliminated from the digestive tract and are ingested by other individuals in which these excyst and develop into multinucleate Opalina.
Diagram of Opalina


Classification of Eimeria


Distribution: Cosmopolitan in distribution.

Habits and Habitat: Eimeria is an intracellular parasite in the digestive epithelium of arthropods and vertebrates, especially domestic animals and also in the bile ducts, kidneys. testes, blood vessels and coelom.

Characteristics features of Eimeria

  • Eimeria is monogenetic.
  • If the food contaminated with oocytes is swallowed, the host’s digestive juices dissolve the cysts and release the sporozoites.
  • The sporozoites enter epithelial cells of the digestive tract and enlarge as feeding trophozoites.
  • A sporozoite has an elongated, slightly curved body with only one end pointed. Its cytoplasm has a nucleus, mitochondria, Golgi bodies, endoplasmic reticulum, ribosome, lysosomes, vacuoles with reserve food material etc.
  • The trophozoite is more or less pear-shaped and is with granular cytoplasm containing a nucleus with nucleolus and a cosmophil granule at its blunt end. 
  • Trophozoites undergo rapid schizogony to produce many merozoites. 
  • The merozoites invade other cells to repeat cycles.
  • A merozoite is very small with one-pointed and other rounded ends. The nucleus occupies the centre of the granular cytoplasm while terminal granule and cytoplasmic granules are as pointed and rounded ends respectively.
  • Some merozoites develop into gametocytes; also within epithelial cells, some become large macrogametes and others divide to yield many microgametes.
  • Macrogametes and microgametes form a zygote that secretes an elliptical covering forming the
  • In this stage, the coccidia passes out of the host along with the faeces. 
  • The zygote within divides into 4 spores, each enclosing 2 sporozoites and this is the infective stage for a new host.
Life cycle of Eimeria


Classification of Myxobolus


Habits and Habitat: Myxobolus is a parasite in the cavities and tissues of fresh-water fishes.

Characteristics features of Myxobolus

  • The spore of Myxobolus is somewhat oval with two hollow polar capsules at the anterior end, each enclosing an invaginated spirally twisted polar thread.
  • In the large cavity behind are two uninucleate amoeboid gametes that fuse to form a zygote. 
  • On reaching the fish host, the threads are everted and thrust into the
  • host tissues. The amoeboid zygote escapes into the body which soon becomes the multinucleate agamont. 
  • The agamont nuclei come together to form groups or bodies called pansporoblasts.
  • Pansporoblasts has two envelope cells and one or more cells called sporoblast. 
  • From the division of the sporoblast nucleus, the structures of the spores are derived.
Diagram of Myxobolus


Classification of Nosema


Habits and Habitat: Nosema bombycis causes the so-called “Pebrine” disease in Silkworm.

Characteristics features of Nosema

  • The spore gets into the gut of the silkmoth larva. 
  • The released amoebulae attack cells of the gut and undergo binary and multiple fission.
  • These parasites then invade the bloodstream and many organs of the body where they undergo schizogony. 
  • The shizonts divide and the large number of sporonts are formed. 
  • The sporonts from many spores which set new infection in the same bost
  • N. B. Nosema apis is a parasite on the eggs of honey bees and destroys a large number of honey bees.
Nosema bombycis Diagram


Classification of Balantidium


Distribution: Cosmopolitan in distribution.

Habits and Habitat: Balantidium is a parasite of the digestive tract of amphibian and mammals including man.

Characteristics features of Balantidium

  • The body is oval with uniform cilia arranged in longitudinal spiral rows, and measures 10 to 36 μ in length.
  • The anterior end is somewhat pointed and the posterior end rounded. 
  • The peristome begins at or near the anterior end.
  • The cytostome leads into the cytopharynx which is very poorly developed. 
  • The cytoplasm is differentiated into ectoplasm and endoplasm. 
  • The endoplasm contains 2 nuclei-the macronucleus is large and conspicuous and the small spherical micronucleus lies in its contact.
  • There are usually two (1 to 4) well-developed and functional contractile vacuoles. 
  • The nutrition is holozoic.
  • Food vacuoles may contain bacteria and other material from the colon or sometimes red corpuscles and other tissue elements.
  • At the extreme posterior end of the body is an anal spot or cytopyge leading into short non-ciliated dilation.
  • Asexual reproduction by transverse binary fission.
  • Sexual reproduction by conjugation followed by encystment. 
  • The double-layered cysts which reach a diameter of 60-85 μ are the largest ones encountered in human stools.
  • The cysts are passed out with the faeces of the host which can infect a new host.
  • This parasite in extreme cases may cause recurring attacks of diarrhoea and in severe infections, chronic dysentery.
Labeled Diagram of Balantidium

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