NEW SYSTEM OF CLASSIFICATION

The modern classification system based on physiology, biochemistry and embryology have revealed distinct similarities between living organisms which external appearances of organisms will not show or discover. Not all biologists have agreed on this new classification of five kingdoms instead of two kingdoms of Plantae and Animalia of Linnaeus.

The new system of classification was introduced by Margulis and Schwartz in 1982. All cellular organisms fall into groups namely Prokaryote and Eukaryotae. These two groups have their DNA located in different areas of their cells. The DNA of prokaryote (Pro=before; karyon=nucleus) is not enclosed by nuclear membranes and float freely in the cytoplasm. The cells of prokaryote have no true nuclei. The DNA of eukaryotes (eu=true) are enclosed with nuclear membranes and located in nucleus. Based on the cellular organization and structural similarities, Schwartz recognised five kingdoms namely: Prokaryote (prokaryotes), Protoctista (protoctists), Fungi, Plantae and Animalia. Of the five kingdoms, four kingdoms are classified as follows: Protoctista, Fungi, Plantae and Animalia and placed under Eucaryotae because their DNA is located in nucleus and enclosed with nuclear membranes. This classification places Prokaryote as a king-dom having no regard to the fact that all organisms fall into two distinct groups namely Prokaryote and Eukaryotae.

 

Kingdom: Prokaryote (Prokaryotes)

Characteristic features

1. Lack organized DNA. That is DNA is scattered in the cytoplasm and not in nucleus.
2. They have no mitochondria in their cytoplasm.
3. The cell walls lack cellulose. The cell walls are made of protein and fatty materials.
4. They are microscopic single-celled organisms.
5. They are motile or non-motile organisms.
6. They feed autotrophically or heterotrophically.
7. They reproduce only asexually.

Examples are bacteria and photosynthetic blue-green bacterium (Anabaena) formally known as blue-green alga.

 

Kingdom Protoctista (Protoctists)

Characteristic features

1. Have well organized DNA in the nucleus bounded by nuclear membrane hence they are called eukaryotic organisms.
2. They are single-celled (unicellular) organisms.
3. They are motile and non-motile organisms.
4. Protozoans (Amoeba, Euglena, Chlamydomonas, Spirogyra Plasmodium, Chlorella and Fucus are examples of Protoctistists.

 

Kingdom: Fungi

Characteristic features

1. Lack chlorophyll. Nutrition is therefore heterotrophic. Food is digested outside the body and digested soluble food is absorbed into the body by diffusion.
2. They are simple multicellular organisms.
3. Their bodies are not differentiated into root, stem and leaves.
4. They are mostly non-motile organisms composed of thread-like multinucleate hyphae collectively known as mycelium.
5. Reproduction is by means of spores.
6. Carbohydrates are stored as glycogen and not starch.
7. The cell wall is made of chitin and not cellulose.

Examples are Mucor, Rhizopus, Mushroom and slime mould.

 

Kingdom: Plantae

Characteristic features

1. They are multicellular and non-motile organisms.
2. Their cells are bounded by rigid cellulose cell walls external to the cell membrane.
3. They contain chlorophyll that enables them to manufacture their own food by photosynthesis.
4. Most of the plants store carbohydrates as starch or sucrose. The Kingdom Plantae is divided main into four phyla namely Bryophyta (Bryophytes), Pteridophyta (Ferns), Gynospermophyta or Coniferophyta (Conifers), and Angiospermophyta (Angiosperms – flowering plants).

 

Phylum: Bryophyta

Characteristic features

1. They are non-vascular multicellular organisms.
2. They have chlorophyll as the only photosynthetic pigment.
3. They are terrestrial but live in moist environment.
4. The body is differentiated into stem-like and leaf-like structures. They lack true roots, stem and leaves. They lack vascular tissues and therefore unable to transport food and material round the body.
5. The leaves of some are spirally arranged on the stem.
6. Asexual reproduction is by spores while sexual Reproduction is by gametes. Water is needed in sexual reproduction hence they are regarded as the amphibians of the plant kingdom. They show alternation of generations.
7. Some are thalloid in form e.g. Liverworts. Examples are Marchantia sp. (liverwort) and Funaria hygrometrica (a moss).

 

Phylum: Pteridophyta (Ferns)

Characteristic features

1. The body is differentiated into true roots, stem and leaves with well developed vascular bundles comprising xylem and phloem. The young leaves are coiled.
2. The stem is a rhizome.
3. Reproduction is by means of spores.
4. Asexual reproductive organ consists of sori while the sexual reproductive organ is a heart- shaped prothallus.
5. Water is needed for sexual reproduction (prothallus).
6. They are mainly terrestrial, with a few aquatic members. Examples are Dryopteris, Cyclo-sorus, Platycerium, Phymatodes are terrestrial while Nephrolepis, Salvinia oblongata and Azolla are aquatic members.

 

Phylum: Gymnospermonphyta (Coniferophyta)

Characteristic features

1. Large plants with well developed vascular bundles with true roots, stem and leaves.
2. The leaves are green in colour, small, scaly and needle-like.
3. They have cones where naked seeds are formed because there is no ovary. Fruit is not formed.
4. They are terrestrial in habitat. Examples are whistling pine (Casuarina sp.) and Cycas.

 

Phylum: Angiospermophyta

Characteristic features

1. They possess true flowers for sexual reproduction.
2. Well developed true roots, stem and leaves are present.
3. Well developed vascular bundles are present.
4. Seeds and fruits are produced after fertilization. Seeds are enclosed in an ovary.
5. They are terrestrial but few are aquatic.

Examples are oil palm tree, goatweed, mango tree and maize. This Phylum is very large and it has been divided into two classes namely Monocotyledoneae (monocots) and Dicotyledoneae (Dicots).

 

Class: Monocotyledoneae

Characteristic features

1. Leaves have parallel veins.
2. Flowers are generally inconspicuous.
3. Embryo has one cotyledon (seed leaf).
4. Roots are fibrous.
5. Pith is absent.
6. Size of cortex is narrow.
7. Cambium is absent.
8. Vascular bundles scattered all over the ground tissue.

Examples are maize, palm tree, grasses (e.g. Eleusine indica)

 

Class: Dicotyledoneae

Characteristic features

1. Leaves have net-like veins.
2. Flowers are generally conspicuous.
3. Embryo has two cotyledons.
4. The size of the cortex is wide.
5. Vascular bundles arranged in a ring of cambium.
6. Cambium is present.
7. Pith is present.

Examples are mango, orange, iroko and trees.

 

Kingdom: Animalia

Characteristic features

1. All are multicellular organisms.
2. DNA is located in nucleus enclosed with nuclear membranes.
3. Cells have no cell walls.
4. All possess nervous systems except the sponges.
5. All possess true tissues except sponges.
6. Feeding is heterotrophic.

The Kingdom Animalia is divided into two divisions namely non-vertebrate and vertebrate chordates. The non-vertebrates are animals without back-bones while vertebrates are animals with backbones. Examples of invertebrates are Coelenterata, Platyhelminthes, Nematoda, Annelida, Echinodermata, Arthropoda and Mollusca.

The vertebrates include Pisces, Amphibia, Reptilia, Aves and Mammalia.

The kingdom is divided into two sub kingdom namely – Invertebrata and Vertebrata.

Sub-Kingdom: Invertebrata

This is a group of animals without backbones (vertebral column).

 

Sub-Kingdom: Vertebrata

This is a group of animals with backbones (vertebral column).

 

Sub-Phylum: Invertebrata

Characteristic feature

No vertebral column.

 

Phylum: Cnidaria (Coelenterata)

Characteristic features

1. They are multicellular organisms.
2. They have two distinct body layers (diplo-blastic), ectoderm (outer layer) and endo-derm (inner layer), separated by a featureless middle layer called mesogloea.
3. They have only one body cavity called enteron.
4. There is only one opening called mouth to the outside world. No anus.
5. Mouth is surrounded with tentacles.
6. They possess of stinging cells called nematocysts.
7. They have specialized cells such as glandular, nerve, sensory and absorptive cells.
8. Asexual reproduction is by budding. Hydra, Obelia, sea anemone, jelly fish, coral and Physalia are examples.

 

Phylum: Platyhelminthes

Characteristic features

1. Possess soft, flat and unsegmented body.
2. They are bilaterally symmetrical.
3. They have no body cavity (acoelomate).
4. They possess alimentary canal with highly branched diverticula. No anus.
5. Some are free living e.g Planaria, while others are parasitic e.g liver flukes and tapeworms.

 

Phylum: Nematoda

Characteristic features

1. They have soft, cylindrical and slender bodies pointed at both ends.
2. Body is not segmented.
3. They have alimentary canal with both mouth and anus.
4. They are bilaterally symmetrical.
5. They have false body cavity (Pseudocoelomates).
6. They are mostly parasitic but few are free living. Examples are Ascaris (roundworms), hookworms, threadworms, guinea worms and vinegar eel (free living).

 

Phylum: Annelida

Characteristic features

1. The body is segmented with each segment separated from other by septa.
2. Body is cylindrical in shape.
3. They have alimentary canal with mouth and anus.
4. They possess true body cavity (coeloma) with a distinct head.
5. The appendages (setae) are not jointed Examples are earthworms, tubeworms and leeches.

 

Phylum: Mollusca

Characteristic features

1. They possess soft and unsegmented bodies.
2. Tentacles are present in most members.
3. The bodies of most of them are covered with shells (e.g. land snail, (Archatina) while others are not (e.g. octopus and squid).

 

Phylum: Echinodermata

Characteristic features

1. They possess tough spiny and calcareous exoskeleton.
2. They are radially symmetrical in adults but bilaterally symmetrical in larvae.
3. The head is not usually distinct.
4. Most of them are star shaped.
5. They are all marine organisms.

Examples are bristle star, starfish, sea urchin, sea cucumber and sea lily.

 

Phylum: Arthropoda

Characteristic features

1. Their bodies are covered with hard exoskeleton of chitin.
2. Their appendages are jointed.
3. Often the bodies are divided into three regions head, thorax and abdomen. In some, the head and thorax are fused together to form a cephalothorax as in crustaceans and arachnids.
4. They have bilateral symmetry.

Examples are insects (e.g. mosquito and cockroach), crustaceans (e.g. crab and lobster), diplopods (e.g millipede), chilopods (e.g centipede) and arachnids (e.g spider and tick).

 

Sub-Kingdom: Vertebrata

Characteristic feature

1. They possess vertebral column (backbones).
2. The sub-phylum Vertebrata is represented by five classes namely: Pisces, Amphibia, Reptilia, Aves and Mammalia.

 

Class: Pisces (Fish)

Characteristic features

1. They are all aquatic.
2. They possess gills for respiration throughout life.
3. They possess fins for locomotion.
4. Their bodies are streamlined in shape.
5. They are cold blooded (poikilothermic).
6. Fertilization is external.
7. Most of them have scales (e.g. Tilapia) while others are scaleless (e.g.Clarias).

Examples are Tilapia, Hemichromis, Clarias, electric cat fish, Gymnarchus and shark.

 

Class: Amphibia

Characteristic features

1. Possess moist, soft and scaleless skin.
2. They are cold blooded (poikilothermic).
3. Adults possess paired limbs with webbed toes.
4. The larval stage is aquatic while the adult is either aquatic or terrestrial. Tadpole larva is vegetarian while adult is usually carnivorous.
5. The heart is two chambered in larval stage and three chambered in adult.
6. Fertilization is external.
7. Breathing is by gills in the larval stage and by lungs, skin and mouth in the adult stage. Examples are toad, frog and newt.

 

Class: Reptilia

Characteristic features

1. They possess dry and scaly skin.
2. Most possess four limbs which end in claws while others (snakes) have no limbs.
3. Dentition is homodont (the same type of teeth).
4. They are cold blooded (poikilothermic).
5. Breathing is by lungs.
6. Fertilization is internal with laying of soft and leathery shelled eggs. Most members are oviparous while a few others retain their eggs in their bodies until hatching (ovoviviparous).
7. They are mostly terrestrial but few such as turtles and crocodiles are aquatic. Examples are lizard, snake, crocodile, tortoise and chameleon.
8. The heart is three chambered except crocodile with four chambered heart.

 

Class: Aves

Characteristic features

1. The body is covered with feathers except the legs and the feet, which are covered with scales.
2. Their forelimbs are modified as wings.
3. The mouth is modified into horny toothless beak or bill.
4. Their bones are porous, light and in some cases filled with air.
5. Their bodies are supported by two hind limbs, (bipedal stance).
6. They are warm blooded (homeothermic). They are very active animals.
7. Breathing is by lungs.
8. They lay hard shelled eggs (oviparous).
9. Fertilization is internal.
10. They are mostly terrestrial.
11. The heart is four chambered.

Examples are domestic fowl, duck, eagle and pigeon.

 

Class: Mammalia

Characteristic features

1. They possess hair.
2. They possess mammary glands. The mother’s mammary glands secrete milk used in nourishing the young after birth.
3. They possess external ears.
4. They are viviparous; that is the young are born at an advanced stage of development. However, the monotremes lay eggs which hatch into young nourished by milk secreted by mammary glands of the mother.
5. Breathing is by lungs.
6. They have diaphragm (breathing muscle) separating the thoracic cavity from the abdominal cavity.
7. The heart is four chambered.
8. Most possess four limbs for locomotion.
9. They are warm blooded (homeothermic) hence they are very active animals.

Examples are duck-billed platypus, kangaroo, cat, pangolin, elephant, giraffe, monkey, lion, gorilla and man.