Classification of Living Things I – SS1 Biology Lesson Note (First Term)

Classification is the arrangement of organisms into groups, with each group split into smaller groups whose members share certain features that distinguish them from other groups. The largest group is the kingdom, and the groups run down through phylum, class, order, family and genus to species. Each organism is named using binomial nomenclature, which gives its genus and species names.

Behavioural objectives

By the end of the lesson, students should be able to:

• define classification and taxonomy;
• list the taxonomic groups from kingdom to species;
• explain binomial nomenclature and write a scientific name correctly;
• name the five kingdoms of living things and state the features of each;
• state the main features of the plant divisions and of the animal phyla, with examples;
• distinguish between monocotyledonous and dicotyledonous plants.

What is classification?

Nature comprises millions of organisms, hence the need for identifying, naming and classifying them in a meaningful manner. The present and widely accepted method of classification of organisms was introduced by Carolus Linnaeus. The study of the general principles of classification is called taxonomy or systematics.

Classification is the arrangement of organisms into groups. Each group is then split into smaller groups, and the members of each group have certain features in common which distinguish them from other groups. The largest group of organisms is the kingdom.

The arrangement of living organisms from the highest to the lowest (with a decreasing variety of organisms) is as follows:

1. Kingdom
2. Phylum or division
3. Class
4. Order
5. Family
6. Genus
7. Species

What is binomial nomenclature?

Binomial nomenclature is the standard system of naming living organisms. Each type of organism is given two names:

• The name of the genus (generic name) to which the organism belongs.
• The name of the species (specific name) to which it belongs.

The generic name is always written first, with an initial capital letter. The name is underlined or italicised, e.g. Homo sapiens is man’s scientific name.

How are living organisms classified in modern times?

Not all organisms can be suitably classified as either plants or animals under the classification of Carolus Linnaeus. Therefore five kingdoms have been generally accepted for all living organisms. These are Monera, Protista, Fungi, Plantae and Animalia.

What are the features of Kingdom Monera (prokaryotes)?

This group consists of the simplest living organisms (bacteria and blue-green algae such as Anabaena).

• They are microscopic and single-celled.
• The cell wall does not contain cellulose. It is made up of protein and fatty materials.
• They have no definite nucleus. The nucleus lacks a nuclear membrane and the DNA is scattered in the cytoplasm.
• They lack most cell organelles except the ribosome.
• Reproduction is asexual, by binary fission.

What are the features of Kingdom Protista?

• They are unicellular organisms.
• The organisms are all eukaryotes, i.e. their cells have definite nuclei.
• Most protists are aquatic organisms.
• They move by cilia, flagella or pseudopodia.
• Some are free-living while a few are parasitic.

Protists can be broadly divided into two groups:

1. Protozoa: animal-like protists, e.g. amoeba, paramecium, plasmodium, trypanosome.
2. Protophyta: plant-like protists, e.g. diatoms, chlorella, chlamydomonas.

Note: Euglena is a protist with plant-like and animal-like features.

What are the features of Kingdom Fungi?

• They are non-green organisms which do not photosynthesise (they lack chlorophyll).
• All fungi except slime moulds are non-motile.
• They have rigid cell walls made up of chitin and polysaccharides.
• They reproduce asexually by producing spores, and sexually as well.
• Most of them are saprophytes while some are parasites.
• They lack true roots, stems and leaves.
• A few are unicellular (yeast) while most are multicellular (rhizopus, mushroom).
• Multicellular fungi have filamentous bodies made up of a network of fine, branching filaments called hyphae (singular: hypha). This mass of hyphae is known as mycelium (plural: mycelia).

What are the features of Kingdom Plantae?

This kingdom consists of organisms with cellulose cell walls and chlorophyll for manufacturing their food. It has three major phyla: Thallophyta, Bryophyta and Tracheophyta.

Thallophyta (algae)

Examples include spirogyra, volvox, sargassum and kelp.

1. These are simple aquatic photosynthetic plants.
2. They are non-vascular plants.
3. They lack true roots, stems and leaves.
4. They are either unicellular or multicellular and may be green, brown or red.
5. They reproduce asexually (by cell division, fragmentation or spores) or sexually.
6. They exhibit alternation of generation.

Bryophyta

These include hornworts, liverworts and mosses.

1. Bryophytes are multicellular, non-vascular plants.
2. They also lack true roots, stems and leaves.
3. They have chlorophyll as the only photosynthetic pigment.
4. They are terrestrial but live in moist places.
5. They produce spores asexually and also reproduce sexually. Hence, they also show a distinct alternation of generation.

Tracheophyta

1. They are green, multicellular, terrestrial vascular plants, i.e. they have tissues for conducting water and food.
2. They have true roots, stems and leaves.
3. It is the largest group of plants and can be subdivided into two: pteridophytes and spermatophytes.

Pteridophyta (ferns)

Examples include Dryopteris, Platycerium and other ferns.

• They are spore-bearing plants.
• They possess well-developed vascular tissues and chlorophyll.
• Asexual and sexual reproductive organs are referred to as sori and prothallus respectively.
• The stem grows horizontally and is referred to as a rhizome.

Spermatophyta

These are seed-bearing plants. They are grouped into two: gymnosperms and angiosperms.

Gymnosperms

Examples are cycads (whistling pine, fir), conifers and ginkgos.

1. Their seeds are borne naked in special structures called cones. They do not have flowers, e.g. conifers, cycads.
2. These are trees or shrubs with needle-like, broad or scale-like leaves.

Angiosperms

1. They form the largest group in the plant kingdom and are adapted to almost every kind of habitat. Examples are oil palm trees, water leaves, maize plants etc.
2. They bear true flowers for sexual reproduction.
3. They have more abundant water-conducting vessels than gymnosperms.
4. They bear seeds enclosed in fruits.
5. They show more specialised and complex reproductive mechanisms involving pollination and fertilisation.
6. Angiosperms are grouped into two: monocotyledons and dicotyledons.

Monocotyledonous plants

1. Examples include maize, guinea grass, rice, oil palm etc.
2. They have seeds with one seed leaf (cotyledon).
3. They are generally herbs with scattered vascular bundles in their stem.
4. They have leaves with parallel venation and fibrous root systems.
5. They undergo hypogeal germination (i.e. their cotyledons remain below the ground at germination).
6. They do not undergo secondary growth.

Dicotyledonous plants

1. Examples include orange, hibiscus plants etc.
2. They are more primitive angiosperms, having seeds with two seed leaves.
3. They have a tap root system.
4. They usually undergo secondary growth.
5. The leaves have veins arranged in a branched network.
6. They undergo epigeal germination (i.e. the cotyledons are borne above the soil).

What are the features of Kingdom Animalia?

This kingdom consists of multicellular organisms with no cell wall. They have no chloroplasts in their cells, therefore they feed heterotrophically. Animals can be classified into two main groups:

1. Invertebrates: animals without backbones.
2. Vertebrates: animals with backbones.

These two groups can further be divided and subdivided based on body symmetry, body design and body cavity.

Invertebrates

Phylum Porifera (sponges)

• They are primitive multicellular, aquatic animals (colonies of cells).
• They lack true tissues and a nervous system.
• They reproduce asexually and sexually (they are hermaphrodites).

Phylum Coelenterata (Cnidaria: hydra, jellyfish, sea anemones)

• They have two-layered bodies surrounding a central hollow cavity called an enteron.
• They have tentacles and most of them are marine.
• They possess stinging cells called nematocysts.
• There is only one opening, the mouth. There is no anus.

Phylum Platyhelminthes (flatworms: tapeworm, planaria, liver fluke)

• They have soft, flat, unsegmented, elongated bodies.
• They lack body cavities.
• They are bilaterally symmetrical and mainly parasites in man and other animals.
• They possess an alimentary canal. There is no anus.

Phylum Nematoda (roundworm, hookworm, guinea worm)

• They have a thread-like, cylindrical, unsegmented body with no body cavity.
• They are bilaterally symmetrical.
• They have an alimentary canal with a mouth and anus.
• Some are parasitic, while others are free-living.

Phylum Annelida (earthworm, leeches)

• They have internally and externally segmented bodies which are long and cylindrical.
• They have true body cavities (coelom).
• 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, while others have no shells, e.g. octopus, slug.

Phylum Echinodermata (starfish, brittle star, sea lily)

• They possess tough, spiny and calcareous exoskeletons.
• The head is not usually distinct.
• They are all marine.
• Most of them are star-shaped.
• Examples are brittle stars, starfish, sea urchins, sea lilies etc.

Phylum Arthropoda (the largest group in the animal kingdom)

• They have segmented bodies.
• They have exoskeletons made up of chitin.
• Their appendages are jointed.
• They have body divisions.
• They are bilaterally symmetrical.

They can be subdivided into four classes:

• Insecta, e.g. cockroach, housefly, grasshopper etc.
• Arachnida, e.g. spider, scorpion, tick etc.
• Crustacea, e.g. crab, crayfish, prawns etc.
• Myriapoda, e.g. centipedes and millipedes.

Evaluation

1. What is binomial nomenclature? Write the scientific name of man correctly.
2. List the seven taxonomic groups from the highest to the lowest.
3. Name the five kingdoms of living organisms.
4. State three features of Kingdom Fungi.
5. Name the two groups of angiosperms and give one example of each.

Assignment

1. Angiosperms and gymnosperms belong to the plant group known as
   • A. Schizophyta
   • B. Bryophyta
   • C. Pteridophyta
   • D. Spermatophyte
2. Sting cells are normally found in
   • A. Flatworms
   • B. Hydra
   • C. Snails
   • D. Paramecium
3. Which of the following are differentiated into true roots, stems and leaves?
   • A. Schizophyta
   • B. Bryophyta
   • C. Pteridophyta
   • D. Algae
4. Each of the following is an arthropod EXCEPT
   • A. crab
   • B. millipede
   • C. spider
   • D. snail
5. In which of the following groups of animals are flagella and cilia found?
   • A. Flatworms
   • B. Protozoa
   • C. Nematodes
   • D. Coelenterates
6. State two distinguishing features of each of the four classes of the phylum Arthropoda.
7. In a tabular form, state four differences between monocot and dicot plants.