The Swedish naturalist Carl von Linne, who gave us the first comprehensive classification of organisms and also the binominal nomenclature, suggested the two kingdom classification of organisms in his book, Systema Naturae, published in 1758. The two kingdoms were PLANTAE that contained organisms that possessed cell wall, chlorophyll and starch as stored material, and ANIMALIA which were motile and did not possess cellulose and starch but stored glycogen as stored material.
Objections to 2-kingdom classification
1) Bacteria can be placed with neither animals nor plants because they have a cell wall made of murein (mucopeptide), have free DNA and no membrane bound organelles.
2) Fungi are plant-like with a cell wall made of N-acetyl glucosamine but have no chlorophyll and are saprozoic in obtaining nutrition.
3) Slime moulds (Mycetozoa) have trophozoid stage Amoeba-like that feeds by endophagy but they reproduce like plants by producing cell wall containing sporangia.
4) Phytoflagellates such as Euglena have chlorophyll but no cell wall, while Paranema has not chlorophyll and feeds by endophagy. Their stored material is starch.
5) Among Cryptomonadina (brown algae) Cryptomonas possesses chlorophyll but Chilomonas does not have chlorophyll and is holozoic.
6) Dinoflagellates and diatoms, although do not have chlorophyll but have brown colour due to xanthophyll. Like plants they have cellulose covering around body but are holozoic in nutrition.
In view of the above mentioned objections, Ernst Haeckel (1866) proposed a third kingdom PROTISTA for all non-multicellular animals, in which gametes are formed from a single cell and not in gonads. So Haeckel’s classification had three kingdoms, namely, Plantae, Animalia and Protista. But bacteria, ray fungi, multicellular fungi and PPLO were still difficult to be classified in the above categories.
In 1968, an American zoologist, R.H. Whittaker suggested 5-kingdon classification for all organisms which is still universally accepted.
1) Kingdom MONERA, which included prokaryotes such as bacteria, PPLO, actinomycetes and blue-green algae.
2) Kingdom PROTISTA, included single-celled eukaryotes with a definite nucleus but with or without chlorophyll and cell wall.
3) Kingdom FUNGI, included multicellular fungi that had no chlorophyll and cell wall was made of N-acetyl glucosamine.
4) Kingdom PLANTAE (=METAPHYTA), with chlorophyll and cell wall made of cellulose.
5) Kingdom ANIMALIA (=METAZOA), included all multicellular animals.
Thus the kingdom Protista or Protoctista now includes about 80,000 species of single-celled organisms that have the genetic material enclosed in a nucleus and have membrane bound organelles, namely, golgi body, mitochondria, lysosomes, centrosome etc. The kingdom includes both plant Protista (=Protophyta) and animal Protista (=Protozoa)
CLASSIFICATION OF PROTOZOA
In modern classifications PROTOZOA is considered as a convenient name for unicellular animals and is not given any rank or status.
SLEIGH’S SYSTEM OF CLASSIFICATION
(Proposed by M.A. Sleigh, 1973 and given in Rupert & Barnes, 1994)
FLAGELLATED PROTOZOA (11phyla)
1. Phylum DINOPHYTA (Dinoglagellates, Noctiluca, Ceatium). With equatorial and longitudinal flagella in grooves. Body covered by cellulose plates.
2. Phylum PARABASALIA (Zooflagellates, Trichomnas, Trichonympha). Many flagella. A parabasal body is made by golgi body, basal body and filament.
3. Phylum METAMONADA (Multiflagellates). One flagellum out of 1-4 flagella is turned backward.
Class Anaxostylea (Giardia)
Class Axostylea (Oxymonas)
4. Phylum KINETOPLASTIDA (Leishmania; typanosoma; Bodo). DNA containing body, kinetoplast present. Flagella emerge from a pit.
5. Phylum EUGLENOPHYTA (Euglena). Colourless or green with 2 flagella arising from an anterior recess.
6. Phylum CRYPTOPHYTA (Chilomonas). Biflagellates with chromoplastids.
7. Phylum OPALINATA (Opalina; Zelleriella; Cepedia). Rows of cilia all over the body. Divide by plasmotomy. Frog parasites.
8. Phylum HETEROKONTA (Chromulina). With 2 dissimilar flagella. Chlorophyll present.
9. Phylum CHLOROPHYTA (Chlamydomonas; Volvox; Polytomella). With chlorophyll. Many multicellular with flagellate reproductive stage.
10. Phylum HAPTOPHYTA (Coccolithus). Phytoflagellates with flagella-like organelle called haptomnema.
11. Phylum CHOANOFLAGELLIDA (Proterospongia). Solitary of colonial with a single flagellum surrounded by collar.