Characteristics of the Five-Kingdom classification
At present, as you know, the trend is to follow the five-kingdom classification. However, while accepting the Five-Kingdom Classification, zoologists still treat Protozoa which comes under Kingdom Protista as the most primitive group of non chordates. This is partly because of historical reasons and partly because most zoologists are also interested in Protozoa. We have also followed the five kingdom of classification in our course. Table shows the characteristics of the five kingdoms proposed by Whittaker and followed by us in the Animal Diversity Course.
The earliest organisms to inhabit earth. They are microscopic, single celled prokaryotes. They lack true nuclei, but have only nuclear regions or nucleoids. The single chromosome is formed of one circular mo.lecule of DNA not associated with any proteins. They also lack membranous organelles like mitochondria, chloroplasts etc. Mitosis or meiosis in cell division does not occur. Cells divide by binary fission. Cell walls are composed usually of peptidoglycan (a substance derived from amino acids and sugars); many secrete a capsule made of polysaccharide material. There is now a tendency to split Kingdom Monera into t\ko separate kingdoms viz. Kingdom Archaebacteria (ancient bacteria) and kingdom Eubacteria (true bacteria).
Representatives: archaebacteria, blue-green algae, actinomycetes, fruiting,bacteria, rickettsias, mycoplasmas, Bdello vibrios etc.
Protists are single celled eukaryotes; some form loose aggregations of cells called colonies. It is believed that the first eukaryotes were protists, some of which gave arise to higher eukaryotes - fungi, plants and animals that dominate the world today.
Representatives: protozoa and unicellular algae.
Diverse group of multicellular eukaryotes; plant-like but cannot carry out photosynthesis due to absence of chlorophyll. They obtain food through their surface from a living or a nonliving organic source by absorption, instead of ingesting it as animals do. In many cases they secrete digestive enzymes outside their body which break down food. The breakdown products are absorbed across the fungal wall. During reproduction fungi may produce both asexual and sexual spores.
Representatives: slime moulds, moulds or true fungi, yeasts, mildew, rust, mushrooms.
Multicellular, autotrophic eukaryotes with cell wall containing cellulose. All plants have reproductive tissues or organs and pass through distinct developmental stages and alternation of generations. Cells often have large central vacuole. Plants have indeterminate growth and often have no fixed body size nor exact shape.
Representatives: multicellular algae, mosses, horse tails, lycopods, ferns and seed plants.
Multicellular eukaryotic heterotrophs. Animals lack photosynthetic pigments and obtain nutrients by ingesting other organisms. Cells lack cell wall. Many of the members exhibit advanced tissue differentiation and complex organ systems and move about freely; characterised by quick response to stimuli, with specialised nervous tissue for coordination. , Growth is determinate, mostly with definite size and shape.
Representatives,: sponges, jelly fishes, flat worms, round worms, segmented worms, molluscs, arthropods, echinoderms, chordates (fishes, amphibians, reptiles, birds, mammals).
You may have observed that the Five-Kingdom-Classification of Whittaker does not include the viruses. They should probably have their own kingdom since they arc unlike any other group of organisms. But kingdoms are supposed to reveal evolutionary relationships, and since the evolutionary origins of viruses are unknown, we cannot yet indicate their exact position. They are supposed to have originated after cellular organisms came into existence, because viruses depend upon the latter for their existence. So they are not the most primitive organisms from which cellular organisms have evolved.