Phylum Protozoa

Protos—first, Zoan—animal, First animal phylum

Study of protozoans called Protozoology

Father of protozoology – Antony von Leeuwenhoek

Term Protozoa was assigned by Goldfuss

Protozoa belong to Kingdom Protista of Haeckel

Single-celled, solitary or colonial, eukaryotes

Important characters

  • Size ranges from 1µ to 5000µ
  • Aquatic – freshwater or marine forms, or endoparasites
  • Grade of organization– protoplasmic or subcellular
  • Locomotion by cilia, flagella or pseudopodia, or absent
  • Skeleton – may or may not be present; if present then either calcareous or siliceous. Foraminifers have external shell of CaCO3 ( ex. Polystomella), whereas Radiolarians have internal plates of silica
  • Osmoregulation by contractile vacuole in freshwater protozoans (isotonic to 0.8 % NaCl solution). C.V. absent in parasitic and most of the marine forms.
  • Symmetry—radial, bilateral or absent (asymmetrical)
  • Nutrition—Holozoic or holophytic.
  • Digestion intracellular within food vacuole. Reserve food material- Glycogen or Starchlike bodies
  • Respiration – exchange of gases occurs through plasma membrane
  • Excretion –excretory product NH3 (Ammonotelic). Excretion occurs through general body surface or plasmalemma.
  • Blood and nervous tissue absent. Sensory structure stigma (eyespot) present in Euglena
  • Reproduction – Asexual or Sexual. Asexual by budding, spore formation, binary fission or multiple fission. Sexual by Conjugation (Paramecium) or gamete formation (Plasmodium)


Basis of classification – Locomotory structures

1. Sub Phylum Sarcomastigophora

(i) Super Class Mastigophora (Flagellata) —

  • Locomotory structures- flagella
  • Most primitive class
  • Binary fission –longitudinal

Two subdivisions- Phytomastigophora and Zoomastigophora

Chloroplast presentChloroplast absent
Mostly free livingMostly parasites
Reserve food material –starch (paramylon
Reserve food material –Glycogen
Flagella- 1 to 2Flagella 1 to many
ex. Euglena, Volvox, Noctiluca and Ceratium
Noctiluca –shows bioluminescence, called
Burning of sea
ex. Trypanosoma , Leishmania ,
Giardia, Trichomonas, Trichonympha,
Proterospongia, Mastigamoeba


T. brucei—digenetic parasite, primary host-cattle and secondary host – a fly; Disease-Nagana

T. cruzi—digenetic parasite, primary host- man and secondary host – dipteran fly; Disease- Chagas

T.evansi—digenetic parasite, primary host –horse and secondary host – Horsefly (Tabanus); Disease- Surra (skin disease)

Trichomonas – T. tenax aggrevates pyrrhoea, T. vaginalis causes leucorrhoea.

Leishmania donovani— a digenetic parasite, primary host- man and secondary host – Sandfly(Phlebotomus); Disease- Kala-azar(Dumdum fever)
L. tropica—causes Oriental sore or Skin Leishmaniasis

Giardia— a bilaterally symmetrical form, commonly called ‘Grand old man of intestine’; causes Diarrhoea

Trichonympha – It is a symbiotic protozoan in the gut of termites, for cellulose digestion.

(ii) Super Class Sarcodina (Rhizopoda)-

  • Locomotory structures-Pseudopodia ( false legs)
  • Free living or parasitic forms
  • Includes Heliozoans, foraminifers, radiolarians and amoebids

ex. Amoeba, Entamoeba, Polystomella, Arcella, Pelomyxa and Babesia (an RBC parasite, transmitted through tick and causes Texas fever in cattles)

Entamoeba histolytia – a monogenetic parasite in colon (large intestine) of man; causes Amoebiasis or Amoebic dysentry. Infective stage – Tetranucleated cyst. Mode of infection – contaminated food and water.

E. gingivalis (or buccalis) – a monogenetic parasite in the pockets, between teeth and gums; aggrevates pyrrhoea. Infective stage – Trophozoite. Mode of infection – Mouth to mouth kissing.

E. coli – a commensal of colon

2. Subphylum Ciliophora (Class Ciliata) –

  • The biggest and the most advanced class of protozoa
  • Binary fission – Transverse
  • Nuclei dimorphic, Micronucleus- reproductive and Macronucleus- vegetative in function.

ex. Paramecium, Vorticella and Rectal ciliates. Rectal ciliates –found in the rectum of frog and toad, include Balantidium, Nyctotherus and Opalina. Opalina has multinucleated condition. (micro- and macronuclei not differentiated).

Though the locomotory structures in Opalina are cilia, due to multinucleated condition, it is sometimes included in superclass Opalinata.

3. Subphylum Sporozoa –

  • Locomotory structures absent
  • Members most specialized for parasitic mode of life.
  • Exclusively endoparasites (contractile vacuole absent)

ex. Plasmodium, Monocystis and Eimeria

Monocystis– endoparasite in the seminal vesicle of earthworms, feeding on sperms

Eimeria– causes coccidiosis in poultry birds

4. Subphylum Cnidospora –

  • Spore formation throughout life
  • Spores contain polar capsule and coiled filament for attachment
  • Trophozoite – multinucleated

ex. Nosema, Myxidium (Nosema bombysis causes ‘Pebrine’ disease in silkworm, Bombyx mori).

The group Metazoa is divided into Parazoa and Eumetazoa. Porifers belong to parazoa as ectoderm in these animals develops from vegetal pole and the endoderm develops from animal pole. In eumetazoans, it is just reverse
Buy me a coffeeBuy me a coffee


Leave a Reply