Carl von Linnaeus suggested 2-kingdom classification of organisms in his book, Systema Naturae. This was the first comprehensive classification of the living organisms.
These two kingdoms were,
1. Kingdom Plantae (non-motile organisms that possessed cell wall, chlorophyll and starch as stored material) and
2. Kingdom Animalia (motile organisms that did not possess cellulose and starch but stored glycogen as stored material).
But Bacteria, Fungi, Slime moulds (Mycetozoa), Phytoflagellates (Euglena), Cryptomonadina (Cryptomonas, Chilomonas), Dinoflagellates and diatoms could not be definetly placed into any of the two kingdoms. And hence this posed objections to two-kingdom classification.
In view of these objections, Ernst Haeckel proposed a third kingdom Protista for all non-multicellular animals.
These three kingdoms were,
1. Kingdom Plantae
2. Kingdom Animalia and
3. Kingdom Protista (Non multicellular animals)
But still bacteria, ray fungi, multicellular fungi and PPLO were still difficult to be classified in the above categories.
Finally, R.H. Whittaker suggested 5-kingdon classification for all organisms which is universally accepted.
1. Kingdom Monera which included prokaryotes such as bacteria, PPLO (pleuropneumonia-like organism), actinomycetes and blue-green algae.
2. Kingdom Protista which included single-celled eukaryotes with a definite nucleus but with or without chlorophyll and cell wall.
3. Kingdom Fungi which included multicellular fungi that had no chlorophyll and cell wall was made of N-acetyl glucosamine.
4. Kingdom Plantae (Metaphyta), which included plants with chlorophyll and cell wall made of cellulose.
5. Kingdom animalia (metazoa), which included all multicellular animals.
Thus the kingdom Protista now includes species of single-celled organisms that have the genetic material enclosed in a nucleus and have membrane bound organelles like Golgi body, mitochondria, lysosomes, centrosome etc.
The kingdom Protista includes both plant Protista (=Protophyta) and animal Protista (=Protozoa). Protists are the most nutritionally diverse of all eukaryotes. They use mitochondria for cellular respiration. Some which do not possess mitochondria either live anaerobically or contain mutualistic bacteria. Protists move by use of flagella or cilia during some part of their lives. They may reproduce sexually or asexually, sometimes both during certain stages of life. Some protists are involved in symbiotic relationships, living in body fluids, tissues or cells of hosts. These relationships may be mutualistic or parasitic.
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