Last reviewed 30 Sept 2026 · 8 min read
Why classify?
There are millions of species. Classification (taxonomy) arranges them in groups based on similarities, making study easier and showing evolutionary relationships.
- Aristotle first classified organisms (plants and animals). Carolus Linnaeus (the "Father of Taxonomy") gave the binomial nomenclature and the hierarchy of groups.
- Binomial nomenclature: every organism has a two-part scientific name — genus (capitalised) and species (lower case), written in italics or underlined: Homo sapiens (human), Mangifera indica (mango), Panthera leo (lion), Oryza sativa (rice).
- Hierarchy of ranks: Kingdom → Phylum (Division in plants) → Class → Order → Family → Genus → Species. (King Philip Came Over For Good Soup.) The species is the basic unit: a group of organisms that can interbreed and produce fertile offspring.
- Ernst Haeckel and R. H. Whittaker (1969) proposed the five-kingdom classification.
The five kingdoms
| Kingdom | Cell type | Body | Nutrition | Examples |
|---|---|---|---|---|
| Monera | prokaryotic | unicellular | autotrophic or heterotrophic | bacteria, cyanobacteria (blue-green algae), archaebacteria |
| Protista | eukaryotic | mostly unicellular | autotrophic (diatoms, dinoflagellates) or heterotrophic (amoeba, paramecium, Plasmodium, Euglena) | protozoa, unicellular algae |
| Fungi | eukaryotic; cell wall of chitin | multicellular (yeast is unicellular) | heterotrophic (saprophytic or parasitic); absorb food | mushrooms, yeast, Penicillium, Aspergillus, mould, rust |
| Plantae | eukaryotic; cell wall of cellulose | multicellular | autotrophic (photosynthesis) | all plants |
| Animalia | eukaryotic; no cell wall | multicellular | heterotrophic (ingest food) | all animals |
Viruses are not placed in the five kingdoms — they are acellular, made of a nucleic acid (DNA or RNA) inside a protein coat, and reproduce only inside a host cell. Lichens are a symbiotic association of an alga (or cyanobacterium) and a fungus (indicators of pollution). Mycorrhiza is an association between a fungus and plant roots.
Classification of plants (Plantae)
| Group | Features | Examples |
|---|---|---|
| Thallophyta (Algae) | simple, no root/stem/leaf; mostly aquatic; have chlorophyll | Spirogyra, Chlamydomonas, Ulothrix, kelp, seaweed |
| Bryophyta ("amphibians of the plant kingdom") | small, non-vascular (no xylem/phloem), need water for reproduction; live in moist places | mosses (Funaria), liverworts (Marchantia) |
| Pteridophyta | vascular tissue (xylem, phloem), seedless; reproduce by spores | ferns (Pteris, Adiantum), Selaginella, horsetails |
| Gymnosperms | vascular, naked seeds (no fruit); often cones | pine, cycas, deodar, fir, spruce, Ginkgo |
| Angiosperms (flowering plants) | vascular; seeds enclosed in fruits; flowers | mango, rice, wheat, rose, sunflower |
- Cryptogams (no seeds — algae, bryophytes, pteridophytes) and Phanerogams (seed plants — gymnosperms and angiosperms).
- Angiosperms are divided into monocots and dicots:
| Feature | Monocotyledons | Dicotyledons |
|---|---|---|
| Cotyledons in the seed | one | two |
| Leaf venation | parallel | reticulate (net-like) |
| Root system | fibrous | taproot |
| Flower parts | in threes | in fours or fives |
| Vascular bundles | scattered | in a ring |
| Examples | wheat, rice, maize, grass, palm, onion, banana | pea, gram, mango, mustard, sunflower, rose |
Classification of animals (Animalia)
Animals are divided into invertebrates (no backbone) and vertebrates (backbone).
Invertebrates
| Phylum | Key features | Examples |
|---|---|---|
| Porifera (sponges) | pores over the body, canal system; sessile, aquatic | Sycon, Spongilla, bath sponge |
| Coelenterata (Cnidaria) | tentacles with stinging cells (cnidoblasts); radial symmetry | Hydra, jellyfish, sea anemone, corals |
| Platyhelminthes (flatworms) | flat body, no body cavity; many parasites | tapeworm, liver fluke, planaria |
| Nematoda (roundworms) | round, unsegmented; pseudocoelom | Ascaris, hookworm, filarial worm |
| Annelida | segmented body; a true coelom | earthworm, leech, Nereis |
| Arthropoda (largest phylum) | jointed legs, exoskeleton of chitin, segmented | insects (cockroach, butterfly, bee, housefly), prawn, crab, spider, scorpion, centipede |
| Mollusca (second largest phylum) | soft body, often a shell; muscular foot | snail, octopus, squid, pearl oyster, mussel |
| Echinodermata | spiny skin, radial symmetry, tube feet; marine | starfish, sea urchin, sea cucumber |
- Insects have 3 pairs of legs (6), 3 body parts (head, thorax, abdomen), usually 2 pairs of wings; spiders have 4 pairs of legs and 2 body parts. The body of a cockroach is covered by chitin. Butterfly life cycle: egg → larva (caterpillar) → pupa → adult; a metamorphosis. Silk is made by the silkworm (Bombyx mori); sericulture = rearing silkworms; apiculture = beekeeping (honey); lac is from lac insects.
Vertebrates (Phylum Chordata)
| Class | Features | Examples |
|---|---|---|
| Pisces (fishes) | aquatic; gills; fins; scales; cold-blooded; two-chambered heart | rohu, catla, shark (cartilaginous), seahorse, eel |
| Amphibia | live on land and in water; moist skin; gills in larvae, lungs in adults; three-chambered heart; lay eggs in water; cold-blooded | frog, toad, salamander |
| Reptilia | dry scaly skin; lungs; lay eggs on land; mostly three-chambered heart (four in crocodile); cold-blooded | snake, lizard, turtle, tortoise, crocodile |
| Aves (birds) | feathers, beak, wings, hollow bones; four-chambered heart; warm-blooded; lay eggs | pigeon, sparrow, ostrich (largest bird), kiwi, penguin, peacock (national bird) |
| Mammalia | mammary glands; hair or fur; four-chambered heart; warm-blooded; most are viviparous (give birth to young ones); a diaphragm | human, dog, cow, whale (aquatic mammal), bat (flying mammal) |
- Cold-blooded (poikilothermic) animals change body temperature with the surroundings (fish, amphibians, reptiles); warm-blooded (homoeothermic) maintain constant temperature (birds and mammals).
- Egg-laying mammals (monotremes): platypus and echidna. Marsupials carry young in a pouch: kangaroo, koala.
- Largest animal: blue whale; largest land animal: African elephant; tallest: giraffe; fastest land animal: cheetah; largest bird: ostrich; largest reptile: saltwater crocodile; smallest bird: hummingbird.
- Amphibians are the first vertebrates to have moved to land; lungfish breathe air.