Phylum Protozoa: Classification, Characteristics, Examples & Diseases | BSc Zoology Notes

 Introduction — What is Phylum Protozoa?

Phylum Protozoa includes unicellular eukaryotic organisms that perform all vital life processes within a single cell.. The word Protozoa comes from two Greek words:

• Proto = First / Primitive
• Zoa = Animal

Protozoans were first observed by Antonie van Leeuwenhoek in 1674 using a microscope. The term “Protozoa” was coined by Goldfuss in 1817, and the phylum was later established by Von Siebold in 1845.

Distribution of Protozoa

Protozoa are found almost everywhere on Earth:

• They are Fresh water — lakes, ponds, rivers
• Some of them are Marine water — oceans and seas
• They found in moist soil and leaf litter
• Inside bodies of other animals as parasites
• Also found in hot springs and polar ice regions

 Protozoa are cosmopolitan in distribution.

🔬 General Characters of Phylum Protozoa

Body Organization

• Body is unicellular — all life activities performed by a single cell
• Cell is eukaryotic — membrane-bound nucleus present
• Body is microscopic in size (mostly 1 micron to 1 mm)
• Body is naked or covered by a pellicle (flexible protein layer)
• Some are covered by a shell or test (e.g., Foraminifera)

Symmetry

• Most are asymmetrical
• Some show bilateral symmetry (e.g., Giardia)
• Some show radial symmetry (e.g., Actinophrys)

Locomotion

Protozoa move by different locomotory organelles:

Organelle	Example
Pseudopodia (false feet)	Amoeba
Flagella (whip-like)	Euglena, Trypanosoma
Cilia (hair-like)	Paramecium
Absent (no movement)	Plasmodium (sporozoites move by gliding)

 Nutrition

Protozoa show varied modes of nutrition:

• Holozoic — engulf solid food particles (e.g., Amoeba)
• Holophytic / Autotrophic — photosynthesis (e.g., Euglena)
• Saprozoic / Absorptive — absorb dissolved organic matter
• Parasitic — live inside host (e.g., Plasmodium, Entamoeba)
• Mixotrophic — both autotrophic and heterotrophic (e.g., Euglena)

Reproduction

A. Asexual Reproduction

• Binary fission — most common; cell divides into two cells
• Multiple fission / Schizogony — nucleus divides many times, then cytoplasm divides (e.g., Plasmodium)
• Budding — small outgrowth separates
• Plasmotomy — cytoplasm divides without nuclear division

B. Sexual Reproduction

• Syngamy — fusion of two gametes
  • Isogamy — fusion of similar gametes
  • Anisogamy — fusion of dissimilar gametes
• Conjugation — exchange of micronuclei between two individuals (e.g., Paramecium)

Other Key Characters

• Osmoregulation done by contractile vacuole
• Digestion done inside food vacuole
• Respiration by simple diffusion through cell membrane
• Many can form cysts — resistant stage to survive unfavorable conditions
• Some are colonial (e.g., Volvox)
• Cytoplasm divided into:
  • Ectoplasm — outer, clear, firm layer
  • Endoplasm — inner, granular, fluid layer

 Classification of Phylum Protozoa

Phylum Protozoa is classified into 4 main classes based on locomotory organelles:

Class	Locomotion	Example
Rhizopoda (Sarcodina)	Pseudopodia	Amoeba
Mastigophora (Flagellata)	Flagella	Euglena, Trypanosoma
Ciliata (Ciliophora)	Cilia	Paramecium
Sporozoa (Apicomplexa)	No locomotory organelle	Plasmodium

CLASS 1 — RHIZOPODA (SARCODINA)

Key Characters

• Locomotion by pseudopodia
• Body naked or with shell/test
• Mostly free-living; some parasitic
• Asexual reproduction by binary fission
• No mouth — food engulfed by pseudopodia (phagocytosis)
• Mostly freshwater; some marine

Types of Pseudopodia

Type	Shape	Example
Lobopodia	Broad, blunt	Amoeba
Filopodia	Thin, pointed	Euglypha
Rhizopodia	Branched and anastomosing	Foraminifera
Axopodia	Stiff, with axial rod	Actinophrys

Important Examples

1. Amoeba proteus

• Free-living, found in freshwater ponds
• Moves and feeds by lobopodia
• Reproduces by binary fission
• Has contractile vacuole for osmoregulation
• Shows cyclosis — circular flow of cytoplasm

2. Entamoeba histolytica

• Parasitic protozoan — causes Amoebiasis (Amoebic dysentery)
• Lives in large intestine of humans
• No contractile vacuole
• Transmitted through contaminated food and water
• Cysts are infective stage

3. Foraminifera

• Marine protozoa with calcium carbonate shells
• Shells have pores through which rhizopodia extend
• Dead shells form Globigerina ooze on ocean floor
• Used in oil exploration as index fossils
• Ex: Globigerina, Nummulites

4. Radiolaria

• Marine protozoa with silica-based skeleton
• Beautiful, intricate glass-like structure
• Dead skeletons form Radiolarian ooze
• Ex: Spumellaria, Acanthometra

CLASS 2 — MASTIGOPHORA (FLAGELLATA)

Key Characters

• Locomotion by one or more flagella
• Body covered by a pellicle
• Nutrition — holozoic, holophytic, saprozoic, or parasitic
• Reproduce by longitudinal binary fission
• Both free-living and parasitic forms
• Some have chromatophores (chloroplasts) for photosynthesis

Sub-classes

Sub-class	Characters	Example
Phytomastigina	Plant-like; have chromatophores	Euglena, Volvox
Zoomastigina	Animal-like; no chromatophores	Trypanosoma, Leishmania

Important Examples

Euglena viridis

• Mixotrophic — both autotrophic and heterotrophic
• Has chloroplasts for photosynthesis
• Has stigma (eyespot) for detecting light
• Has contractile vacuole
• Reproduces by longitudinal binary fission
• Shows euglenoid movement (metaboly)

Trypanosoma gambiense

• Causes African Sleeping Sickness
• Transmitted by Tsetse fly (Glossina)
• Lives in blood and lymph of humans
• Has a kinetoplast — modified mitochondrion
• Shows undulating membrane

 Leishmania donovani

• Causes Kala-azar (Visceral Leishmaniasis)
• Transmitted by sandfly (Phlebotomus)
• Affects liver, spleen, and bone marrow
• Has amastigote (inside host) and promastigote (in sandfly) stages

Trichomonas vaginalis

• Causes Trichomoniasis — sexually transmitted infection
• Lives in urogenital tract
• Has 4 anterior flagella and 1 recurrent flagellum
• No cyst stage

Volvox

• Colonial flagellate — thousands of cells in a hollow sphere
• Shows division of labour among cells
• Reproduces both asexually (daughter colonies) and sexually
• Considered a link between unicellular and multicellular organisms

CLASS 3 — CILIATA (CILIOPHORA)

Key Characters

• Locomotion by cilia present all over body or in specific regions
• Most complex and highly evolved protozoa
• Have two nuclei:
  • Macronucleus — controls vegetative functions
  • Micronucleus — controls reproductive functions
• Reproduce asexually by transverse binary fission
• Sexual reproduction by conjugation
• Have a cytostome (cell mouth) and cytopyge (cell anus)
• Contractile vacuole present for osmoregulation
• Mostly free-living in freshwater

Important Examples

Paramecium caudatum

• Called "Slipper animalcule" due to slipper shape
• Covered by thousands of cilia
• Has oral groove → vestibule → cytostome → cytopharynx for feeding
• Food vacuoles circulate in cytoplasm (cyclosis)
• Reproduces asexually by transverse binary fission
• Sexual reproduction by conjugation
• Has trichocysts — defense organelles that discharge threads

Vorticella

• Called "Bell animalcule"
• Bell-shaped body attached by a contractile stalk
• Cilia present only around oral disc
• Lives attached to aquatic plants and debris
• Reproduces by budding and binary fission

 Balantidium coli

• Only ciliate parasitic to humans
• Causes Balantidiasis (intestinal infection)
• Lives in large intestine of pigs and humans
• Transmitted through contaminated water

Stentor

• Trumpet-shaped ciliate
• One of the largest protozoa
• Has macronucleus shaped like a string of beads
• Shows regeneration

CLASS 4 — SPOROZOA (APICOMPLEXA)

Key Characters

• No locomotory organelle in adult stage
• All members are obligate intracellular parasites
• Body covered by pellicle
• No contractile vacuole (live in isotonic environment)
• Reproduce by multiple fission (schizogony)
• Form spores for transmission
• Life cycle involves alternation of hosts (definitive + intermediate)
• Have apical complex — organelles for penetrating host cells

Important Examples

Types of Malaria

Species	Type of Malaria	Fever Cycle
P. vivax	Benign Tertian Malaria	Every 48 hours
P. falciparum	Malignant Tertian / Cerebral Malaria	Every 36–48 hours
P. malariae	Quartan Malaria	Every 72 hours
P. ovale	Mild Tertian Malaria	Every 48 hours

   P. falciparum causes the most dangerous and fatal form of malaria.

1. Monocystis lumbrici

• Parasite of earthworm (Pheretima)
• Lives in seminal vesicles of earthworm
• Life cycle involves schizogony and sporogony
• No intermediate host required

2. Eimeria

• Causes Coccidiosis in rabbits and poultry
• Transmitted through contaminated feed and water
• Affects intestinal epithelium

Economic and Medical Importance of Protozoa

Harmful Protozoa — Disease Table

Protozoan	Disease	Transmission
Plasmodium vivax	Malaria	Female Anopheles mosquito
Entamoeba histolytica	Amoebiasis / Dysentery	Contaminated food/water
Trypanosoma gambiense	Sleeping Sickness	Tsetse fly
Leishmania donovani	Kala-azar	Sandfly (Phlebotomus)
Trichomonas vaginalis	Trichomoniasis	Sexual contact
Giardia lamblia	Giardiasis	Contaminated water
Balantidium coli	Balantidiasis	Contaminated water
Toxoplasma gondii	Toxoplasmosis	Undercooked meat / cats

 Beneficial Protozoa

• Foraminifera and Radiolaria → form ocean sediments used in oil exploration
• Free-living protozoa → decompose organic matter → nutrient recycling
• Protozoa in gut of termites → help in cellulose digestion
• Used as bio-indicators of water pollution
• Used in scientific research — Amoeba and Paramecium are model organisms