Protozoa: General Characters and Classification

Protozoa: General Characters and Classification 

Protozoa may be defined as microscopic and acellular animalcules, without tissues and organs, having one or more nuclei, but no nucleus ever incharge of a specialized part of cytoplasm. 

They exist either single or in colonies which differ from a metazoan in having all the individuals alike except when engaged in reproductive activities.

General Characters:

1. Small, usually microscopic animalcules, ordinarily not visible without a microscope.

2. Simplest and most primitive of all animals, with protoplasmic grade of organization.

3. Body unicellular, containing one or more nuclei which are monomorphic or dimorphic. 

4. Solitary or forming loose colonies in which individuals remain alike and independent. 

5. Body symmetry none, bilateral, radial or spherical.

6. Body naked or bounded by a pellicle and often provided with simple to elaborate shells or exoskeletons.

7. Body form usually constant, varied in some, while changing with environment or age in many.

8. The single cell body performs all the essential and vital activities, which characterize the animal body; hence only subcellular physiological division of labour. 9. Locomotor organelles are finger-like pseudopodia or whip-like flagella or hairlike cilia or absent.

10. Nutrition holozoic (animal-like), holophytic (plant-like), saprozoic or parasitic. With or without definite oral and anal apertures. Digestion occurs intracellularly inside food vacuoles.

11. Respiration and excretion through general surface or through contractile vacuoles, which serve mainly for osmoregulation.

12. Reproduction asemal by binary or multiple fission and budding, and sexual by conjugation of adults (hologamy) or by fusion of gametes (syngamy).

13. Life history often complicated with alternation of asexual and sexual phases. 14. Encystment commonly occurs to help in dispersal as well as to resist unfavourable conditions of food, temperature and moisture. 

15. Free-living Protozoa mostly aquatic, inhabiting fresh and sea waters and damp places. Parasitic and commensal Protozoa live over or inside the bodies of animals and plants. Sufficient moisture is essential in their environment.

16. The single-celled individual not differentiated into somatoplasm and germplasm; therefore, exempt from natural death which is the price paid for the body. 17. About 50,000 known species.

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Classification:

Phylum Protozoa is a large and varied group and poses a number of problems in its classification. The conventional scheme followed by Hyman (1940), Hickman (1961) and Storer (1965) etc. recognizes 2 subphyla on the basis of organs of locomotion and 5 classes, briefly outlined as follows:

Subphylum A. Plasmodroma

Locomotory organelles are flagella, pseudopodia, or none. Nuclei of one kind.

Class 1. Mastigophora

Move by one to many flagella. Ex. Euglena. 

Class 2. Sarcodina

 Move and capture food by pseudopodia.

Ex. Amoeba 

Class 3. Sporozoa

 No locomotory organs. All parasitic. Spore- formation common. Ex. Plasmodium

Subphylum: B. Ciliophora

Cilia or sucking tentacles throughout or at certain stages.Nuclei of 2 kinds. 

Class: 4. Ciliata

Move by cilia throughout life. Ex. Paramecium

Class: 5. Suctoria

Move by cilia as young and by tentacles as adult. Ex. Podophyra 

The following classification of Protozoa is based on the scheme given by the Committee on Taxonomy and Taxonomic Problems of the Society of Protozoologists, and mainly proposed by B.M. Honigberg and others (1964). 

It divides Protozoa first into 4 subphyla 

(1) Sarcomastigophora, 

(2) Sporozoa, 

(3) Cnidospora, and 

(4) Ciliophora. 

Only important orders have been mentioned here.

SUBPHYLUM I. SARCOMASTIGOPHORA

Locomotor organelles pseudopodia or flagella or both. Nuclei of one kind (monomorphic).

Superclass A. Mastigophora (Flagellata)

1. Simple, primitive, with firm pellicle.

2. Locomotor organelles flagella. 

3. Nutrition autotrophic or heterotrophic or both.

Class 1. Phytomastigophorea 

(= Phytoflagellata)

1. Chlorophyll-bearing chromatophores present.

2. Nutrition mainly holophytic by phototrophy.

3.Reserve food starch or paramylon.

4.Flagella 1 or 2, sometimes more.

Order 1. Chrysomonadida 

1. Small, with thin pellicle, often amoeboid. Flagella 1 to 3.

2. Gullet absent. Stigma often present. 

3. Chromatophores 1 or 2, yellow or brown, and discoidal. 4. Starch absent. Leucosin and fats may be present.

Examples: Chrysamoeba, Synura, Ochromonas, Dinobryon. 

Order 2. Cryptomonadida

1. Small, with a rigid pellicle. Flagella 2.

2. Anterior gullet reaches up to middle of body. 

3. Chromatophores 2, yellow, brown Or colourless.

4. Reserve foodstuff starch, sometimes oils. Examples: Chilomonas, Cryptomonas

Order 3. Euglenida 

1. Large, pellicle thick and firm. Flagella 1 or 2. 

2. Anterior end, with a gullet leading into a reservoir.

3. Chromatophores, numerous, green or colourless.

4.Reserve foodstuff paramylon and oils.

Examples Euglena, Peranema, Phacus, Copromonas

Order 4. Volvocida 

(=Phytomonadida) 

1. Small, with rigid cellulose covering (theca).

2. No gullet. Flagella 2 to 4.

3. Chromatophore green, usually cup-shaped. 

4. Reserve foodstuff starch and oils,

Examples: Chlamydomonas, Volvox.

Order 5. Chloromonadida 

1. Small, dorso-ventrally flat. Pellicle delicate.

2.Gullet present. 

3. Chromatophores green and numerous. 

4. Reserve foodstuff oils,

Examples Vacularia, Coelomonas, Gonyostomum.

Order 6. Dinoflagellida 

1. Small, planktonic. Naked, amoeboid or with a thick cellulose theca.

2. Gullet present or absent. Flagella 2.

3. Chromatophores yellow or brown.

4. Reserve foodstuff starch or oils or both.

5. Some are bioluminescent.

Examples: Noctiluca, Ceratium.

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Class 2. Zoomastigopohrea 

(=Zooflagellata)

1. Chlorophyll or Mostly parasitic. chromatophores absent.

2. Nutrition holozoic or saprozoic.

3. Reserve food glycogen.

4. Flagella one to many. 

Order 1. Rhizomastigida

1. Small, amoeboid, chiefly freshwater. 

2. Locomotion by 1-4 flagella and pseudopodia.

Examples: Mastigamoeba, Dimorpha.

Order 2. Kinetoplastida 

1. No gullet. Kinetoplast present.

2. Flagella 1 to 4. No definite pellicle.

3. Mostly parasitic forms living in blood.

Examples: Bodo, Leishmania, Trypanosoma.

Order 3. Choanoflagellida

1. A collar round the base of a single flagellum.

2. Free-living, solitary or colonial. Example: Proterospongia.

Order 4. Diplomonadida

1. Bilaterally symmetrical, binucleate, with delicate pellicle and often with a cytostome.

2. Flagella 3 to 8. One often trailing or forming border of an undulating membrane. 3. Mostly intestinal parasites.ge

Examples: Hexamita, Giardia.

Order 5. Hypermastigida

1. Highly specialized, numerous flagella. 

2. Kinetosomes arranged in a circle, plate or longitudinal or spiral rows. 

3. Mouth absent. Food ingested pseudopodia.

4. Gut parasites of termites and cockroaches.

Examples: Lophomonas, Trychonympha

Order 6. Trichomonadida

1. Flagella 4 to 6. One flagellum trailing 

2. Parasites of vertebrates.

Example: Trichomonas.

Superclass B. Opalinata

1. Entire body covered by cilia-like flagella. 2. Nuclei 2 to many, monomorphic.

3. Reproduction by symmetrogenic fission or by syngamy of anisogametes. 

4. Parasitic mainly in frogs and toads.

Examples: Opalina, Zelleriella.

Superclass C. Sarcodina (Rhizopoda)

1. Body mostly amoeboid without definite pellicle. Some with a skeleton of some kind.

2. Locomotion by pseudopodia. 

3. Nutrition holozoic or saprozoic.

Class 1. Rhizopodea

Pseudopodia lobopodia, filopodia as reticulopodia, without axial filaments .

Subclass (a) Lobosia

Pseudopodia as lobopodia. 

Order 1. Amoebida

1. Body amoeboid, naked, without skeleton.

2. Nucleus with honeycomb lattice. 

3. Largely freshwater and free-living. Many parasitic. Examples: Amoeba, Entamoeba, Pelomyxa

Order 2. Arcellinida 

(=Testacida)

1. Body enclosed in one-chambered shell of pseudochitin, with a single opening through which lobopodia protrude. 

2. Free-living, mostly freshwater.

Examples: Arcella, Difflugia, Euglypha.

Subclass (b) Filosia

Pseudopodia as filopodia. Naked or with a shell with single aperture.

Examples: Allogromia, Penardia (naked).

Subclass (c) Granuloreticulosia 

Pseudopodia delicate granular reticulopodia.

Order: Foraminiferida

Large sized with uni- or multichambered calcareous shell with one or more openings through which reticulopodia emerge. Examples: Globigerina, Elphidium

Class 2. Actinopodea

Pseudopodia mainly axopodia with axial filaments, radiating from a spherical body.

Subclass (a) Heliozoia

1. Spherical protozoans, called sun-animalcules.

2. Pseudopodia (axopodia) radiating. 

3. Naked or skeleton of siliceous scales or spines.

Examples: Actinophrys, Actinosphaeriun.

Subclass (b) Radiolaria

1. Body naked or with perforated chitinoid central capsule separating ectoplasm from endoplasm.

2. Reticulopodia, axopodia or filopodia. 

3. Skeleton mostly of siliceous spicules or of strontium sulphate.

Examples: Collozoum, Thalassicola.

Subclass (c) Acantharia 

1. Imperforate non-chitinoid central capsule without pores.

2. Skeleton of strontium sulphate. 

3. Pseudopodia are axopodia.

Example: Acanthometra.

Subclass (d) Proteomyxidia

1. Pseudopodia are filopodia.

2. Mostly parasites on algae. Examples: Vampyrella, Pseudospora...

Class 3. Piroplasmea

Small in red blood cells of vertebrates. Example: Babesia (formerly included with Sporozoa, but its species do not produce spores).

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SUBPHYLUM II. SPOROZOA

Locomotor organelles absent. Spores present. Exclusively endoparasites.

Class 1. Telosporea 

Spores without polar capsules and filaments naked or encysted.

Subclass (a) Gregarinia

1. Mature trophozoites large, extracellular in host's gut and body cavities. 

2. Each spore produces 8 sporozoites. 3. Parasites in invertebrates.

Examples: Monocystis, Gregarina.

Subclass (b) Coccidia /APICOMPLEXA

1. Mature trophozoites small and intracellular. 

2. Each oocyst produces many sporozoites. 3. Blood or gut parasites of vertebrates.

Examples: Eimeria, Isospora, Plasmodium.

Class 2. Toxoplasmea

Spores absent. Only asexual reproduction. Example: Toxoplasma.

Class 3. Haplosporea

Spore cases present. Only asexual reproduction. Example: Ichthyosporidium.

SUBPHYLUM III. CNIDOSPORA

Spores with polar filaments present.

Class 1. Myxosporidea 

1. Spores large, developed from several nuclei.

2. Spores with two or three valves.

3. Parasites mostly in fishes. 

Examples: Ceratomyxa,Myxidium,

Class 2. Microsporidea

1. Spores small, developed from one nucleus. 

2. Spores with a univalved membrane.

3. Intracellular parasites in arthropods and fishes. Example: Nosema.

SUBPHYLUM IV. CILIOPHORA

Presence of cilia as locomotor and feeding organelles at some stage in the life cycle. Nuclei of 2 kinds (dimorphic).

Class: Ciliata

 (=Infusoria) 

1. Locomotor organelles numerous hair-like cilia,present throughout life.

2. Definite mouth (cytostome) and gullet present except in a few parasitic forms. Anal aperture (cytopyge) permanent.

3. One or more contractile vacuoles present even in marine and parasitic types. 

4. Mostly two kinds of nuclei, large macronucleus and smaller micronucleus.

Subclass (a) Holotricha

1. Body cilia simple and uniform.

2. Buccal cilia mostly absent.

Order 1. Gymnostomatida

Large ciliates without oral ciliature. Cytostome opens directly. No vestibule.

Examples Dileptus. Coleps,Didinium, Prorodon, 

Order 2. Trichostomatida

With vestibular but no buccal ciliature.

Examples: Balantidium, Colpoda.

Order 3. Chonotrichida

No body ciliature. A spirally coiled apical funnel contains vestibular cilia. 

Examples: Spirochona, Lobochona.

Order 4. Apostomatida

Spirally arranged body cilia. Cytostome midventral. 

Example: Hyalophysa

Order 5. Astomatida

Body ciliation uniform. Cytostome absent.

Example: Anoplophyrya, Maupasella.

Order 6. Hymenostomatida

Body ciliation uniform. Buccal cavity with ciliary membranes. 

Examples: Colpidium, Paramecium. ventral

Subclass (b) Peritricha

1. Adult without body cilia.

2. Apical end with buccal cilia.

Order: Peritrichida 

Examples: Vorticella, Carchesium.

Subclass (c) Suctoria

1. Sessile and stalked body. 

2. Young with cilia, adult with suctorial tentacles

Order: Suctorida 

Examples: Acineta, Ephelota, Podophyra.

Subclass (d) Spirotrichia

1. Reduced body cilia. 

2. Buccal cilia well marked.

Order 1. Heterotrichida

Body cilia short. Uniform or absent. Examples: Stentor, Bursaria, Spirostomum.

Order 2. Oligotrichida

Body cilia reduced absent. or Buccal membranes conspicuous. 

Examples: Strombidium, Halteria.

Order 3. Hypotrichida

Dorso-ventrally flattened. Fused cilia forming .

Examples: Euplotes, Stylonchia.

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