Protozoa: salient features, classification and economic importance

Protozoa is a main group of kingdom Protista. The present article will provide you with details of the salient features of protozoa. You will also find the classification and characteristics of different phyla of protozoa.


Protozoans were first studied by Leeuwenhoek in 1677. The name protozoa were coined by Goldfuss in 1817. Protozoans were treated as first animals before the discoveries of bacteria and other monerans. Therefore they are named as Protozoa (Gk. Protos means first, zoion means animal). It is the largest group which includes about 50,000 species.

Characteristics of Protozoa

The main characteristics of Protozoa are as below:

  • Protozoa are microscopic hence they can be seen only using microscope.

  • They are unicellular and the animal is made up of only one cell. All the functions are performed by the cell.

  • They are heterotrophs as they cannot prepare their own food.

  • They are solitary but sometimes become colonial.

  • They are generally aquatic or found in moist places.

  • Locomotion takes place by pseudopodia, flagella or cilia.

  • Protozoans show holozoic nutrition but some are parasites.

  • Their shape may be oval, round, elongated or irregular.

  • Protoplasm is found in two parts ectoplasm ( outside) and endoplasm (inside).

  • Respiration takes place by diffusion.

  • Reproduce by sexual or asexual methods,. Sexual reproduction occurs by fusion of gametes. Binary fission, multiple fission, budding or sporulation are the types of asexual reproduction found in protozoans.

  • In unfavourable condition, protozoa reproduce by cyst formation.

  • Body is naked or enclosed in a pellicle.

  • Mostly protozoa have one nucleus but some have more than one e.g. Paramoecium has two nuclei.

  • Excretion takes place through body surface.

  • Classification of protozoa

    In five kingdom classification , the protozoan protists have been classified into four phyla on the basis of their locomotion and presence of locomotory organs. The four phyla are:

    1. Rhizopoda or Sarcodina

    2. Mastigophora or Zooflagellata

    3. Sporpozoa

    4. Ciliata

    Characteristics of Rhizopoda or Sarcodina

    The salient features of Rhizopoda are:

    1. They are irregular in shape, e.g. Amoeba.

    2. Locomotory organ is pseudopodia.

    3. Pellicle is not found.

    4. Nutrition is holozoic.

    5. Found in fresh water or seawater.

    6. Mostly free living but some are parasites. E.g. Entamoeba histolytica, which is found in the intestine of human beings, Whereas Entamoeba gingivalis is found in gums of teeth and cause dysentery and pyorrhea.

    7. Some Sarcodines have a shell which is made up of silica. These Sarcodines are called as helizioans. They produce filamentous outgrowth from the holes of their shell. These outgrowths are called filopodia. E.g. Radiolaria.

    8. Binary fission, multiple fission, budding and spores formation are the main types of asexual reproduction.

    Characteristics of Mastigophora or Zooflagellata

    The important feature of phyla Mastigophora or Zooflagellata are given below:

    1. One or two flagella are found on body for locomotion.

    2. Body is covered by pellicle.

    3. The cell body has definite shape.

    4. They are free living but some are parasites. e.g. Trypanosoma, Leishmania, Giardia , etc. Some flagellates may be symbiotic. e.g. Trichonympha . Trichonympha is found in alimentary canal of some insects which produces cellulose enzyme and helps in the idgestion in insects.

    5. They show heterotrophic nutrition by absorption or phagocytosis.

    6. Asexual reproduction takes place through by binary fission and cyst formation.

    7. Reserve food is glycogen.

    8. Sexual reproduction is recorded only in some forms. E.g. Trypanosoma, Volvox, Leishmania, mastigina, trychonympha, Giardia, Lophomonays, etc.

    Salient features of Sporozoa

    The Salient features of phyla sporozoa are as below:

    1. All are parasites. They are endoparasites.

    2. Pellicle is found surrounding the body.

    3. Locomotory organs are not found.

    4. Contractile vacuoles are not found.

    5. Alternation of generation is found.

    6. Nutrition is saprozoic. i.e. the food is absorbed through the general surface.

    7. Asexual reproduction takes place by multiple fission or schizogony.

    8. Sexual reproduction takes place through syngamy. It is followed by spore formation, hence called sporozoans.

    9. They may be passed in one (monogenetic) or two (digenetic) different hosts.

    10. They cause severe diseases in the birds, human beings, etc. e.g. Plasmodium, Monocystis, Eimeria, Gregarina etc.

    Characteristics of Phyla Ciliata

    (Note: Image courtesy

    The salient feature of phyla Ciliata are given below:

    1. Body is covered by pellicle.

    2. Cilia are present all over the surface of the body throughout the whole life and help for locomotion and to catch the food.

    3. Contractile vacuoles are not found.

    4. Nutrition is holozoic.

    5. Two nuclei are found in them. A large macronucleus and a small micronucleus.

    6. Asexual reproduction takes place through binary fission and sexual reproduction takes place by conjugation.

    7. Micronucleus helps in reproduction whereas macronucleus help in regulation for cellular activities.

    8. Protoplasm is divisible into ectoplasm and endoplasm.

    Some examples of phyla Ciliata are paramecium, Vorticella, balantidium, Ephelota, etc.

    Economic Importance of Protozoans

    Protozoans are useful as well as harmful to us. Though they cause many diseases in the human body and in useful animals but on the other hand they use many disease-causing microorganisms as their food thus protect us from diseases. They do clean water by eating harmful aquatic microorganisms.


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