Protozoans were first viewed under a microscope by Leeuwenhoek, who described them as animalcules.
"Protozoa" literally mean the first primitive animalcules.
(Protos – first, zoon – animal).
The term protozoa were first coined by Goldfuss.
They are unicellular.
According to Whittaker's five kingdom classification, protozoans are separated from animal phyla and included in the kingdom Protista.
deals with the study of protozoa.

VVI Examples to remember of Phylum Protozoa for Entrance Exams of Nepal:
  • Amoeba
  • Entamoeba
  • Leishmania donovani
  • Leishmania tropica
  • Trypanosoma gambiens
  • Trypanosoma cruzi
  • Giardia lambia (Grand old man of Intestine)
  • Trichonympha
  • Paramecium
  • Plasomodium
  • Monocystic
  • Euglena

Characteristics of Phylum Protozoa

· They are simple, primitive, and microscopic animalcules.
· They are unicellular or acellular eukaryotes, with one or more nuclei which may be monomorphic or dimorphic.
· They may be solitary or colonial and unicellular body performs all life activities.
· Protoplasmic grade of organization.
· They may be free-living, found in fresh and salt waters and in damp places or parasites or commensals.
· Nutrition may be holozoic (animal-like), holophytic (plant-like), saprophytic, parasitic, or mixotrophic.
· Freshwater protozoans have one or two contractile vacuoles which are not present in marine and parasitic protozoans.
· Reproduction may be asexual or sexual.
Asexual reproduction occurs by binary fission (Paramecium), multiple fission (Plasmodium), or budding (Vorticella), and sexual reproduction is performed by gamete formation or conjugation.
· Respiration and excretion through the general body surface.
· Locomotary organelles may be pseudopodia (Amoeba), whip-like flagella (Euglena), or hair-like cilia (Paramecium) or locomotory organelles may be absent (as in sporozoans e.g. Plasmodium).
· Body is naked or bounded by a definite layer called a pellicle.
· Protoplasm is differentiated into an outer clear and transparent ectoplasm and an inner granular and semi-fluid endoplasm.
· Cytoplasm shows the cyclosis.
· Digestion is intracellular.
· Mixotrophic mode of nutrition occurs in Euglena.
· Nitrogenous waste material in protozoa is ammonia so ammonotelic.
· Encystment occurs to overcome unfavourable conditions. Encystment helps in dispersal as well as to resist unfavourable conditions.

Classification of Phylum Protozoa

On the basis of locomotory organelles, protozoans are divided into four classes.
  • Rhizopoda
  • Ciliata
  • Flagellata
  • Sporozoa

i) Rhizopoda or Sarcodina
· The pseudopodia are present in the naked body which is without definite shape which helps in locomotion and feeding.
· Usually one nucleus is present.
· Holozoic type of nutrition, parasitic type in some forms.
· Reproduction takes place asexually by binary and multiple fission.
· Amoeba takes the solid food by the process of phagocytosis.
· Amoeba can't digest fat.
· Food vacuole of Amoeba is analogous to the alimentary canal of animal or gastrovascular cavity of Hydra.
· Different types of taxis in Amoeba are – Thermotaxis (response to temperature), Phototaxis (response to light), Thigmotaxis (response to touch), Rheotaxis (response to water current), Galvanotaxis (electric current), Geotaxis (gravity).

Example: Amoeba, Entamoeba, etc.

ii) Ciliata

· Free-living or endoparasites
· Reproduction by binary fission or external budding or conjugation.
· Have two types of nuclei – micronucleus and macronucleus
· Micronucleus performs a reproductive function and Macronucleus performs vegetative function i.e. Heterokaryotic.
· Have numerous cilia all over the body wall and they help in locomotion.
· Body covered with a pellicle.
· Holozoic type of nutrition. One or more contractile vacuole

Example: Paramecium, Vorticella, Balantidium, Nyctotherus, Opalina, etc.

iii) Flagellata or Mastigophora
· Locomotary organelles are one or more flagella.
· Body is covered by pellicle and has a definite shape.
· They are free-living but some are parasitic.
· Asexual reproduction by longitudinal binary fission.
· The nutrition is either holozoic or holophytic or saprophytic.
· Ceratium and Noctiluca are bioluminescent protozoans.

Examples: Euglena, Trypanosoma, Leishmania, Giardia (Grand old man of the intestine), Volvox, Trichomonas, etc.

iv) Sporozoa
· Exclusively parasitic class i.e. endoparasites.
· They have no locomotory organelles.
· Body covered with a pellicle.
· Nutrition may be saprophytic or parasitic.
· Absence of contractile vacuole.
· They reproduce by gametic fusion and by spore formation.

Examples: Plasmodium (Malarial parasite), Monocystis, Eimeria, etc.

Also, Read our Other Notes Important for Entrance Exams:

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