Chapter – 6

PHYLUM – PROTISTA (PROTOZOA)

Conventionally designated as phylum Protozoa, the term was coined by Goldfuss (1817).

Includes about 50,000 species (30,000 existing and 20,000 extinct).

These are unicellular or acellular animalcule without cell wall but, with well formed cell organelles.

Heterotrophic (holozoic or saprozoic) nutrition and single cell performs all life functions.

Both free living and parasitic, fresh water and marine.

Reproduction mostly asexual but in some also sexual.

Encystment commonly found during reproduction in unfavourable condition which also helps in

survival and dispersal.

Classification is based upon the type of locomotory organs which are protoplasmic structures.

In present 5 kingdom classification, Protozoan Protists are grouped into 4 phyla :

Zooflagellata (ii) Sarcodina (iii) Sporozoa (iv) Ciliata.

In 2-Kingdom system there are four classes of phylum protozoa.

Some authors also divide it in two subphyla as –

A. Plasmodroma B. Ciliata

Others divide it into 4 subphyla :

(i) Sarcomastigophora, (ii) Telosporidia, (iii) Cnidospora, (iv) Ciliophora.

Class: Flagellata/Mastigophora

These flagella bearing forms are mostly

parasitic.

Locomotory apparatus is whip like

‘flagellum’ generally single or double.

Sometimes regarded as non-photosynthetic

forms of originally photosynthetic flagellate

like Euglena.

Some euglenoids have been experimentally

converted into non-photosynthetic forms

permanently by using some chemicals.

Reproduce only asexually

Example – Trypanosoma, Leishmania,

Euglena etc.

Trypanosoma gambiense is a haemoflagelate of

human and other vertebrates. It shows

tetramorphism. It causes African sleeping

sickness and their spores are transmitted by tse-

tse fly (Glossina palpalis).

Euglena viridis

Trypanosoma genus is represented by following pathogenic species and their diseases:

T. brucie - Nagna fever

T. evnas - Surra disease

T. cruzi - Chagas disease

T. lewisi is the only non-pathogenic species but it is an endoparasite.

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Leishmania donovani causes kala-azar or visceral leishmaniasis. The spores of this disease are

transmitted by sand fly (Phlebotomus).

L.tropica - Oriental sore or Delhi boil or cutaneous leishmaniasis

L. brasiliensis - Espundia or bubos or naso-oral leishmaniasis

Trichomonas vaginalis causes leucorrhoea which is a vaginal disease in female, Trichomonas. humanis

causes an intestinal disorder in man.

Giardia found in human gut and it causes diarrhea.

Trichonympha digests cellulose in the gut of termites, cockroaches and other insects.

Class: Sarcodina (Rhizopoda)

Commonly called as amoebae.

The locomotory organ temporarily formed by protoplasmic extension. (pseudopodia) of

different types :

Some are monopodial or polypodial of different shapes.

If pseudopodia are pointed then they are called as axopodial or actinopodial (e.g. Actinophrys,

Actinosphareum).

If pseudopodia are filamentous and they are known as filopodia or reticulopodia (e.g. Polystomella

or Eliphidium)

If pseudopodia are blunt lobe like structures without any pointed end. Such pseudopodia are called

as lobosa.

Example - Amoeba, Pelomyxa (largest amoeba); Amoeba terricola (species inside the soil),

Entamoeba histolytica (causes amoebic dysentry or amoebiasis), E.coli (as commensals in the human large

intestile), E. gingivalis (as commensal in the gums of tooth).

Some sarcodines such as Mastigamoeba (amoeba with flagellum) have flagella or form flagella when

food becomes scarce.

Structure of Amoeba proteus

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Division

Metacystic Amoeba

Excystment

Cyst wall

Glycogen vacuole

Nucleus

Nucleolus

Quadrinucleate cyst

Binucleate cyst

Cyst

Chromatid bodies

Precystic form

Binary Division

Trophozoite

Amoebulae

Pseudopodia

Life cycle of Entamoeba histolytica

Sporozoite of Plasmodium

Class: Sporozoa These are all parasitic hence without locomotory apparatus.

nfective stage is sporozoites, it is a haploid reproductive stage.

Ex. Plasmodium (malaria parasite), Babesia (in RBC of cattle causing texas fever), Monocystis (a

parasite in the seminal vesicle of earthworm).

Some facts about Plasmodium

It is an intracellular parasite in RBC and liver cell, hence, the parasite of reticulo-endothelial system.

The asexual cycle is completed in man and is called as schizogony. The schizogony is completed in

three phases; exoerythrocytic phase (occurs in liver), erythrocytic cycle (takes place in RBC) and

exo-erythrocytic phase

(again inside the liver).

Sexual cycle is completed in the gut of female Anopheles. The sexual part of life-cycle is called

gametogony and in sub-peritoneal space it is sporogony.

The infective stage for man is sporozoite in which form the mosquito inoculates it in man.

From man to mosquito it is transmitted as gametocyte.

The phase of cycle in man from inoculation to the appearance of malaria fever is called incubation

period.

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Symptoms of malaria appear after the release of haemozoin when RBC raptures and schizozoites

freed out.

There are following four species of Plasmodium:

P. vivax (causes benign tertian fever), P. falciparum (causes malignant tertian fever), P. ovale

(causes benign tertian fever), P. malariae (causes quartan malaria).

Tertian fever appears every third day, the quartan fever appears every fourth day.

Malignant tertian fever is also known as cerebral malaria or pernicious malaria or aestivo-autumnal

malaria or black-water fever.

Comparison between the life-cycle of different species of Plasmodium

S. No. Characters P. vivax P. malariae P. ovale P.falciparum

1. Geographical

Distribution

Worldwide, in

tropical

Worldwide, in

tropical and

subtropical regions

Tropical Africa

Worldwide, in tropical

subtropical and warmer

temperate regions

2. Type and effects of

Malaria fever

Benign tertian.

Death rate low Quartan Severe

Ovale or mild tertian

Severe

Malignant tertian

Death rate high.

3.

Duration of pre-

erythrocytic cycle (pre-

patent period)

8 days 7-12 days 9 days 5-6 days

4. No. of metacryptozoites

formed per schizont 10,000 2,000 15,000 40,000

5. Duration of erythrocytic

cycle 48 hours 72 hours 48 hours 36-48 hours

6. No. of merozoites

formed per schizont 12-24 6-10 6-12 18-24

7. Incubation period

(average) 14 days 18-24 days 14 days 12 days

8. Signet ring form in

R.B.C.

About 1/3 diameter

of R.B.C.

About 1/3 diameter

of R.B.C.

About 1/3 diameter

of R.B.C.

Small ring situated at

edge of R.B.C.

9. Late trophozoite in

R.B.C.

Amoeboid and

large

Compact and often

band-shaped, small.

Compact and small

slightly amoeboid

Compact and medium

sized. Rarely seen in

peripheral blood.

10. Schizont in R.B.C.

Large, 10

diameter, with 2-

24 merozoites

Medium sized, 7

diameter. With 6-12

merozoites.

Medium sized, 7

diameter. With 6-12

merozoites.

Small, 5 diameter.

Not seen in peripheral

blood.

11. Gametocytes in R.B.C.

Round. Fills

R.B.C. Male 9,

Female 10-11.

Pigment granules

evenly distributed

Round or ovoid, Fills

R.B.C., 7. Pigment

granules at centre and

periphery.

Round or ovoid. Fill

¾ of R.B.C., 9 .

Pigment granules,

evenly distributed.

Crescentric. Male 9-11

. Female 12-14 .

Pigment granules

around nucleus.

12. Host R.B. C.

Enlarged, Red

Schuffner’s dots

usually seen

Not enlarged.

Ziemann’s dots seen.

Slightly enlarged,

Schuffner’s dots

seen.

Not enlarged Greenish

Maurer’s dots or clefts

seen.

13. Pigment (haemozoin)

Yellowish-brown.

Fine granules and

rodlets

Dark brown to black.

Abundant as coarse

granules.

Dark brown. Less

abundant coarse

granules

Dark brown or black.

Coarse granules in a

compact mass.

14. Microgametes formed 4 to 8 2 to 5 - - - 4 to 8

15. Duration of sexual

cycle at 25ºC 10 days 25-38 days 16 days 10-12 days

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Life cycle of Plasmodium

Class: Ciliata (Infossuria)

This is the most advanced protozoa with many cilia at the surface.

Achieved highest speed (2 mm/sec) among protozoans.

These have higher cellular organisation with many specialised organelles.

Two or three nuclei present, macronucleus and micronucleus.

Macronucleus controls the somatic activities of cell; micronucleus controls the reproduction.

Besides asexual reproduction there is also sexual reproduction, the reunion of two haploid individuals gives rise to young ones.

One of the most important achievements in the Protist allows the variation of recombination in the offspring.

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Ex. Paramecium, Vorticella (bell animalcule), Balantidium, Tetrahymena.

Paramecium (the slipper animalcule) Cell is slipper shaped with broader

anterior end and tapering posterior end. Fresh water form, so presence of

contractile vacuole. Cytostome as distinct part on particular

site of cell surface for entry of food. Cytopharynx following cytostome is a

tube like passage comparable to gullet/esophagus.

Cytoproct (cell anus) isa site on the posterior end through which egestion takes place.

Cyclosis is a special movement of cytoplasm within the cell which moves food vacuole in a definite pathway.

Trichocysts are the small, dart-like specialized organelles just below the cell membrane; when ejected with force it is used for sticking to any surface; also help in locomotion; earlier thought to be defence organelle.

Reproduction commonly asexual, by transverse binary fission also sexually by conjugation. But the sexes are not distinct. Two different strains of same species, fuse together at oral sides, exchange their nuclei and get separated. After union of two haploid nuclei, the diploid nucleus is formed which then further divide to give rise to many individuals. This genetic exchange is necessary otherwise the Paramecium dies out slowly if continues to reproduce only asexually.

Paramecium caudatum