CLASSIFICATION OF LIVING ORGANISMS
MODERN CLASSIFICATION OF LIVING ORGANISMS
All organisms cannot be suitably classified as either plants or animals based on CarolusLinneaus classification. Therefore five kingdoms have been generally accepted for all living organisms, these include Monera, Protista, fungi, plantae and animalia.
KINGDOM MONERA (Prokaryotes)
This group consists of simplest living organisms (bacteria, blue-green anabaena).
- They are microscopic single-celled.
- The cell wall does not contain cellulose. It is made up of protein and fatty materials.
- They have no definite nucleus. Nucleus lack nuclear membrane and DNA are scattered in the cytoplasm.
- They lack most cell organelles except the
- Reproduction is asexual by binary fission.
- They are unicellular
- The organisms are all eukaryotes i. e. cell have definite
- Most protists are aquatic organisms.
- They move either by cilia, flagella or
- Some are free living while few are parasitic.
Protists can be broadly divided into two groups;
- Protozoa: animal-like protists e.g. amoeba, paramecium, plasmodium,
- Protophyta: Plant-like protists e. g. Diatoms, chlorella,
Note: Euglena is a protist with plant and animal like features.
- They are non-green organisms which do not photosynthesize (lack chlorophyll).
- All fungi except slime moulds are non-motile.
- They have rigid cell wall made up of chitin and
- They reproduce asexually by producing spores and sexually as well.
- Most of them are saprophytes while some are parasites.
- They lack true roots, stem and leaves.
- Few are unicellular (yeast) while most are multicellular (rhizopus, mushroom).
Multicellular fungi have filamentous bodies that are made up of a network of fine, branching filaments called hyphae (singular: hypha). This mass of hyphae is known as mycelium (plural: mycelia).
This consists of organisms with cellulose cell wall and chlorophyll for manufacturing their food. It consists of three major phyla i. e thallophyta, bryophyta and tracheophyta.
Examples include spirogyra, volvox, sargassum and kelp
- These are simple aquatic photosynthetic plants
- They are non vascular plants
- They lack true root, stem and leaves.
- They are either unicellular or multicellular and may be green, brown or red.
- They reproduce asexually (by cell division, fragmentation, spore) or sexually.
- They exhibit alternation of generation.
These include hornworts, liverworts and mosses
- Bryophytes are multicellular, non vascular plants.
- They also lack true roots, stems and leaves.
- They have chlorophyll as the only photosynthetic pigment.
- They are terrestrial but live in moist place.
- They produce spores asexually and also reproduce sexually. Hence, they also show a distinct alternation of generation.
- They are green multicelluar, terrestrial vascular plants i.e. they have tissues for conducting water and food.
- They have true roots, stem, and leaves.
It is the largest group of plants and can be subdivided into two i. e. pteridophytes and spermatophytes.
Examples include Dryopteris, platycerium, ferns etc
- They are spore bearing plants.
- They possess well developed vascular tissues and chlorophyll
- Asexual and sexual reproductive organs are referred to as sori and prothalusrespectively
- The stem grows horizontally and is referred to as rhizome
These are seed bearing plants. They can be grouped into two
Examples are cycads (whistling pine, fir), conifers and gingkos
- Their seeds are borne naked in special structures called They don’t have flowers e.g conifers, cycads.
- These are trees or shrubs with needle, broad or scale like leaves.
They form the largest group in the plants kingdom and are adapted to almost every kind of habitat. Examples are oil palm tree, water leaf, maize plant etc.
- They bear true flowers for sexual
- They have more abundant water conducting vessels than gymnosperms.
- They bear seeds enclosed in fruits.
- They show more specialized and complex reproductive mechanism involving pollination and
Angiosperms are grouped into two, monocotyledons and dicotyledons.
Examples include maize, guinea grass, rice, oil palm etc.
- They have seeds with one seed leaf (cotyledon).
- They are generally herbs with scattered vascular bundles in their stem.
- They have leaves with parallel venation and fibrous root system.
- They undergo hypogeal germination (i.e. their cotyledons remain below the ground at germination.)
- They do not undergo secondary
Examples include orange, hibiscus plants etc.
- They are more primitive angiosperms having seeds with two seed leaves
- They have tap root system
- They usually undergo secondary growth
- The leaves have veins arranged in branched network
- They undergo epigeal germination (i.e the cotyledons are borne above the soil).
This consists of multicellular organisms with no cell wall; they have no chloroplasts in their cell therefore they feed heterotrophically. Animals can be classified into two main groups:
- Invertebrates – animals without backbones.
- Vertebrates – animals with backbones.
These two groups can further be divided and subdivided based on body symmetry, body design and body cavity.
Phylum Porifera (sponges):
- They are primitive multicellular, aquatic animals (colonies of cells)
- They lack true tissues and nervous system
- Reproduce asexually and sexually (hermaphrodites
PhylumCoelenterata (Cnidaria: Hydra, jelly fish, sea anemones):
- They have two layered bodies surrounding a central hollow cavity called enteron.
- They have tentacles and most of them are marine
- Possession of stinging cells called nematocysts
- There is only one opening called mouth. No anus.
Phylum Platyhelminthes (Flatworms: Tapeworm, planaria, liver fluke)
- They have soft, flat, unsegmented elongated bodies
- They lack body cavity.
- They are bilaterally symmetrical and mainly parasites in man and other animals
- They possess alimentary canal. No anus
Phylum Nematoda (roundworm, hookworm, guinea worm)
- They have thread like, cylindrical, unsegmented body with no body cavity.
- They are bilaterally symmetrical.
- They have alimentary canal with mouth and anus
- Some are parasitic, while others are free living
Phylum Annelida (earthworm, leeches)
- They have internal and external segmented bodies which are long and cylindrical.
- They have true body cavity (ceoloma).
- The appendages (setae) are not jointed
Phylum Mollusca (snail, squid, octopus)
- They have soft unsegmented bodies
- Tentacles are present in most members
- Some have shells e.g snails, squid etc while others have no shell e. g. octopus, slug
Phylum Echinodermata (star fish, bristle star, sae lily)
- They possess tough, spiny and calcerous exoskeleton
- The head is not usually distinct
- They are all marine
- Most of them are stay shaped
Examples are britle star, star fish, sea urchin, sea lily etc
Phylum Arthropoda (the largest group in the animal kingdom)
- They have segmented bodies
- They have exoskeleton made up of chitin
- Their appendages are jointed.
- They have body divisions
- They are bilaterally symmetrical.
They can be subdivided into four classes
- g cockroach, housefly, grasshopper etc.
- Arachnida e.g. spider, scorpion, tick etc.
- Crustaceans e.g. crab, crayfish, prawns etc.
- Myriapoda e.g. centipedes and millipedes.
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