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Classification, in biology is the identification, naming, and grouping of organisms into a formal system based on similarities such as internal and external anatomy, physiological functions, genetic makeup, or evolutionary history.

Study Question 1

General Principles of Classification

Organisms that have similar and common features are grouped together while those that have different features are grouped separately.

Taxonomy is the study of grouping of organisms according to their relationship. There are seven major taxonomic units (taxa).

  • Kingdom
  • Phylum (phyla) or Division in plants
  • Class
  • Order
  • Family
  • Genus
  • Species

As you move from the kingdom to the species the differences decrease as the similarities increases.

Species is a group of organisms that can freely interbreed to give rise to viable/fertile offsprings.

Sometime members of different species may interbreed to give an offspring which is sterile. E.g. a donkey and a horse can interbreed to give rise to a mule which is infertile.

Binomial Nomenclature

This is the double naming system of organisms where organisms are assigned two names i.e. the generic name and the specific name.

Examples

In binomial nomenclature the following rules are observed.

  1. Generic name is written first followed by the specific name.
  2. First letter in the generic name is in capital and the rest are in small letters.
  3. Specific name is written in small letters.
  4. The two names are underlined separately when handwritten or italicised when printed.

Study Question 2

The Five Kingdoms of Classification

Carolus Linnaeus initially introduced the two kingdom system of classification. However many new life forms have been discovered which are neither animals nor plants. This has led to a more accepted classification system that adopts five kingdoms. These are;

  • Monera
  • Protoctista
  • Fungi
  • Plantae
  • Animalia.

Fig. 1.2

1. Kingdom Monera

The kingdom is made up of mainly the bacteria e.g. nitrobacter, azotobacter. Vibrio cholerae etc.

General characteristics

  1. They are unicellular and microscopic. Some are single cells while others are in colonies. They have different body shapes.

Fig. 1.4

  1. Most are heterotrophic, feeding either saprophytically or parasitically. Some are autotrophic.
  2. They are prokaryotic i.e. their nuclear material is not enclosed by a nuclear membrane.
  3. They have few organelles which are not membrane bound. They don’t have mitochondria.
  4. They have a cell wall though not made of cellulose.
  5. They reproduce asexually mainly through binary fission.
  6. Most of them respire an-aerobically but some respire aerobically.
  7. Most of them move by use of flagella.

Diagrams

Study question 3

2. Kingdom Protoctista

Examples include paramecium, amoeba, plasmodium, chlamydomonas, euglena, spirogyra, and trypanosome.

General characteristics

  1. They are eukaryotic whereby their nuclei is bound by a nuclear membrane.
  2. Some are heterotrophic while others are autotrophic.
  3. They have may organelles including mitochondria all of which are membrane bound.
  4.  They have different body forms; some are unicellular or colonial while others are multicellular.
  5. Reproduction is mainly asexual by fission, fragmentation or sporulation. Some reproduce sexually by conjugation.
  6. They are mobile and move by means of cilia, flagella or pseudopodia.
  7. Some may have specialised structures that perform specific functions such as contractile vacuole for osmoregulation.

Diagrams

Practical Activities 1 and 2

3. Kingdom Fungi

Examples

Saprophytic ones include mushrooms, toadstools, bread moulds, penicilia, yeast etc.

Parasitic ones cause plant diseases such as wheat rust, potato and tomato blight and animal diseases such as athlete’s foot and ringworm.

Practical Activities 3

General characteristics

  1. They are eukaryotic.
  2. Most have cell walls made of chitin but a few have cellulose cell walls.
  3. They store food particles in their cytoplasm in the form of glycogen or oil droplets but not starch.
  4. The basic unit is the hyphae. Hyphae are thin filaments and many of them make up structures called mycelium.
  5. Fungi have neither the chloroplasts nor the chlorophyll. They feed on already manufactured food. Hyphae act as the roots and are sent into the food material to obtain nutrients. In saprophytic fungi the hyphae are referred to as rhizoids and in parasitic ones as haustoria.
  6. They reproduce sexually (fusion of nuclei in hyphal branches) and asexually (spores and budding).

Examples

Study Question 4

4. Kingdom Plantae

Study question 5

General Characteristics

  1. They are eukaryotic and multicellular.
  2. In most their body is differentiated into leaves, stem and roots.
  3. They reproduce both sexually and asexually.
  4. Their cells have cellulose cell walls
  5. They have photosynthetic pigment hence are autotrophic.
  6. Majority have a transport system
  7. They show alternation of generation.

The kingdom Plantae is divided into three main divisions.

  • Bryophyta.
  • Pteridophyta.
  • Spermatophyta.

A. Division Bryophyta

These are the mosses and the liverworts.

General Characteristics

  1. The lack the vascular system
  2. Contain chlorophyll and are therefore photosynthetic.
  3. They have rhizoids for anchorage and water and mineral salts absorption.
  4. They show alternation of generations.
  5. Fertilisation depends of availability of water. Male gametes are produced by the antheridia and female gametes by the archegonia.
  6. They grow on damp substratum such as walls, rocks and marshes.
  7. They are thalloid as in liverworts or differentiated into simple leaf like and stem like structures as in mosses.

Diagrams.

B. Division Pteridophyta

This includes ferns and horsetails.

They are more advanced compared to the bryophytes.

General Characteristics

  1. They have leaves, stems and roots but no flowers.
  2. They are photosynthetic.
  3. They have a clearly defined vascular system made of xylem and phloem.
  4. They have compound leaves with leaflets called pinna.
  5. On the lower side of mature leaves are the spores bearing structures (sporangia) which occur in groups called sori (sorus-singular). see diagram.
  6. They show alternation of generations where the sporophyte (fern plant) is the dominant one while the gametophyte is a heart shaped structure called Prothallus. See diagram.
  7. They have sexual reproduction which is dependent of water.

C. Division Spermatophyta

This comprises of all the seed bearing plants.

General Characteristics

  1. They contain chloroplasts hence are photosynthetic.
  2. The plant body is differentiated into roots, stems, leaves and seed bearing structures.
  3. Vascular system is highly developed with xylem tissue consisting of both xylem vessels and tracheids.
  4. Sexual reproduction is well defined.
  5. Seeds are produced after fertilisation.
  6. They show alternation of generation.

The division Spermatophyta is made up of two main subdivisions i.e.

  • Gymnospermaphyta
    • Angiospermaphyta

Gymnospermaphyta

General Characteristics

  • They bear male and female cones.
  • After fertilisation seeds are borne on the female cones and they are naked i.e. they are not enclosed in a fruit wall.
  • They show xerophytic characteristics such as needle like leaves, rolled leaves, thick waxy cuticle and sunken stomata.
  • Phloem doesn’t contain companion cells and xylem mainly consists of tracheids.

This subdivision has three main classes.

  • Coniferales
  • Cycadales
  • Ginkgoales

i) Class Coniferales

  • These include all the common gymnosperms.
  • They are found in areas of little water.
  • They have small needle-shaped leaves with waxy cuticle.
  • They have cones and most of them are ever green.
  • Male cones are in form of clusters at the base of the terminal bud.
  • Female cones are on lateral buds of young shoots and they contain naked seeds.

Diagrams.

ii) Class Cycadales

  • They resemble the palm trees by appearance.
  • They have long compound leaves which are clustered at the apex of a thick short un-branched stem.
  • They bear cones at the apex of the trunk.

iii) Class Ginkgoales

  • Members here are very rare.
  • They include the Ginkgo biloba of China.
  • They are deciduous with fan like leaves.

Angiospermaphyta

General characteristics

  • Are usually bisexual and flower bearing.
  • Seeds are enclosed in an ovary which develops into a fruit.
  • Xylem has tracheids and vessels while the phloem has companion cells.
  • They have double fertilisation.

This subdivision is divided into two classes.

  • Monocotyledonae. – examples
  • Dicotyledonae. – examples
Class MonocotyledonaeClass Dicotyledonae.  
They have seeds with one cotyledon.Have two cotyledons.
They have narrow-long leaves with parallel venation.Broad leaves with reticulate venation.
Most of their leaves have a modified petiole to form a leaf sheath.Leaves have distinct petioles.
Their stems have scattered vascular bundle.Vascular bundles are arranged to form a concentric ring.
Pith is usually absent.Pith is present.
Vascular cambium is usually absent hence no secondary growth.Vascular cambium is present hence there is secondary growth.
They have a fibrous root systemThey have a tap root system
Floral parts are in threes or in multiples of three.Floral parts are in fours, fives or their multiples.
In the root vascular bundles are arranged in a ring with phloem and xylem alternating.In roots, the xylem is centrally placed and star shaped with the phloem alternating with the arms of the xylem.

Study question 8

Practical activity 6

  • Kingdom Animalia

Study Question 9

General characteristics

  1. Most show locomotion but a few are sessile
  2. Most reproduce sexually and a few asexually
  3. They are eukaryotic and multicellular
  4. All are heterotrophic
  5. Their cells have no cell walls

Kingdom Animalia has nine phyla but only two will be discussed i.e. Arthropoda and chordata.

Phylum Arthropoda

Practical Activity 7

General Characteristics

  1. They are segmented.
  2. They are bilaterally symmetrical.
  3. They have open circulatory system where blood flows in open cavities called haemocoel.
  4. Head is well developed with eyes, sensory structures and a fairly developed brain.
  5. Gaseous exchange is through the tracheal system which opens through the spiracles to the outside. Some aquatic ones use gills.
  6. Reproduction is mostly sexual with internal fertilization. They have different sexes.
  7. They have jointed appendages hence the name arthropoda.
  8. They have a body covered with exoskeleton made of chitin. This provides a surface for muscle attachment. It is shed periodically to allow growth through a process called moulting.
  9. Most have their body divided into head, thorax and abdomen. In some, the head and the thorax are fused to form Cephalothorax. The thorax and the abdomen are all segmented.

The phylum arthropoda is divided into five classes.

  • Crustacea
  • Chilopoda
  • Diplopoda
  • Arachnida.
  • Insecta.

Different members of the phylum are placed to their respective classes based on;

  • Number of limbs
  • Presence and number of antennae
  • Number of body parts.

1. Class Crustacea

Examples.  Daphnia, crayfish. Crab and prawn.

General Characteristics

  1. Head and thorax are fused to form cephalothorax.
  2. They have two pairs of antennae.
  3. They have between five and twenty pairs of limbs modified for different functions e.g. locomotion defence and feeding.
  4. They have a pair of compound eyes.
  5. Gaseous exchange is through the gills.
  6. They have three pairs of mouth parts made of one pair of mandibles (lower) and two pairs of maxillae (upper).

2. Class Chilopoda

These are the centipedes.

Diagram

General Characteristics

  1. Body is divided into two parts, the head and the trunk.
  2. The body is dorsa-ventrally flattened.
  3. Body is made up of 15 or more segments.
  4. Head has a pair of simple eyes.
  5. Each segment has a pair of walking legs.
  6. Head has a pair of antennae.
  7. Have poison claws n the head and are therefore carnivorous.
  8. Have a tracheal system for gaseous exchange.
  9. Have separate sexes.

3. Class Diplopoda

These are the millipedes.

General Characteristics

  1. They have cylindrical body.
  2. Have three body parts, head, and thorax and body trunk.
  3. They have two clumps of many simple eyes.
  4. They have no poison claws and are therefore herbivorous.
  5. Heads has a pair of short antennae and mandibles.
  6. Each body segment has a pair of spiracles for breathing.
  7. Body has between 9-100 segments.
  8. Each segment has two pairs of walking legs except the first thoracic segment.

4. Class Arachnida

These include the scorpions, spiders, ticks and mites.

Diagrams

General Characteristics

  1. Body has two parts, cephalothorax and abdomen.
  2. Cephalothorax has two chelicerae which produce poison to paralyse the prey.
  3. Cephalothorax has four pairs of walking legs each having seven joints.
  4. At the end of each leg are two toothed claws.
  5. Cephalothorax has eight simple eyes.
  6. Most have lung books for gaseous exchange, some use gill books or tracheal system.
  7. They have no antennae but have a pair of pedipalps which are sensitive to touch.

5. Class Insecta

They include grasshoppers, bees, houseflies, butterflies, termites, beetles etc.

Insects form half the population of animals on earth. They occupy all habitats i.e. air, water, and land. Their food is varied such as plant tissues, animal fluids, dead animals and excretions of animals making them to be found almost everywhere on earth.

General Characteristics

  1. Body is divided into three parts, head, thorax and abdomen.
  2. Thorax is made up of three segments with three pairs of legs. Some have one or two pairs of wings on the thorax.
  3. Head has one pair of antennae.
  4. They undergo complete or incomplete metamorphosis.
  5. Excretion is through the malpighian tubules which remove uric acid.
  6. Gaseous exchange is through the tracheal system but they breathe through the spiracles.
  7. The head a pair of compound eyes and several simple eyes.
  8. Abdomen is made up of 11 or fewer segments. The terminal segments are modified for reproduction.
  9. Mouth parts consist of the mandibles, maxillae and labium. The mouth parts are modified according to their feeding habits such as sucking, biting, chewing etc.

Assignment

Discuss the economic importance of arthropods.

Study Question 10

Practical Activity

Phylum Chordata

Chordate, common name for animals of the phylum Chordata, which includes vertebrates as well as some invertebrates that possess, at least for some time in their lives, a stiff rod called a notochord lying above the gut. About 43,700 living species are known, making the chordates the third largest animal phylum.

In animals such as the Amphioxus the notochord persists but in others it is replaced at later stages of development by the vertebral column.

Members in this phylum inhabit both aquatic (marine and fresh water) and terrestrial (burrowers and arboreal) environments.

General Characteristics

  1. Members have a notochord at some stage of their development.
  2. They are bilaterally symmetrical.
  3. Heart is ventrally placed. Blood flows from the heart through the arteries and gets back to the heart through the veins.
  4. They have a post anal tail although it is greatly reduced in some.
  5. They have an endoskeleton.
  6. They have a closed circulatory system.
  7. They have visceral clefts where in fish they become the gills in higher chordates they are only present in the embryo.
  8. They have a tubular dorsal nerve cord. It develops anteriorly into brain and posteriorly as the spinal cord. Spinal cord is enclosed by the vertebral column.
  9. They have segmented muscle blocks called myotomes on either side of the body.

The main classes of the phylum chordata are;

  • Pisces
  • Amphibia
  • Reptilia.
  • Aves.
  • Mammalia

Pisces

Diagram

These are the fishes. They include those with a skeleton made of cartilage e.g. shark and those with a bony skeleton such as the tilapia, Nile perch, lung fish, dog fish, and cat fish etc.

General Characteristics

  1. The move by fins
  2. Bodies are covered with scales
  3. Have gills for gaseous exchange in water.
  4. They don’t have a middle or inner ear.
  5. They have streamlined bodies.
  6. They have a lateral line for sensitivity.
  7. Their heart has two main chambers i.e. the auricle and the ventricle.
  8. They are poikilothermic/ectothermic.
  9. Eyes are covered by a nictating membrane 

Amphibia

They include the toads, newts, salamanders and frogs.

The toad is the most advanced amphibian. Its skin is less moist and therefore uses the lungs more for gaseous exchange. They therefore stay mostly on land and only return to the ponds during reproduction.

Diagrams

General Characteristics

  1. They have a double circulatory system.
  2. They have a three chambered heart with two atria and one ventricle.
  3. Fertilisation is external and they breed in water.
  4. Gaseous exchange is through the skin, lungs and gills.
  5. They have two eyes and an eardrum behind the eyes.
  6. They are ectothermic.
  7. They have 4 well developed limbs. The hind limbs are more muscular than the forelimbs.
  8. Reptilia

Examples include tortoise, turtles, snakes, crocodiles, lizards and chameleons.

General Characteristics

  1. They are ectothermic.
  2. They have a well developed lung for gaseous exchange.
  3. They have double circulatory system with the heart having three chambers i.e. two atria and a partially divided ventricle. Crocodiles however have a four chambered heart.
  4. The body is covered with a dry scaly skin reducing desiccation.
  5. Some have four limbs while others don’t have any limbs such as the snakes.
  6. Fertilisation is internal. They lay eggs with a leathery shell to avoid desiccation. Some species of chameleons give birth to young ones.
  7. Aves

Examples include doves, chicken, hawks, eagles and turkeys.

They are terrestrial and arboreal while some have been adapted for aquatic life.

General Characteristics

  1. Bodies are covered with feathers for in insulation.
  2. They have beaks.
  3. They internal auditory canal/ meatus
  4. Fertilisation is internal and they lay hard calcareous eggs.
  5. They have lungs for gaseous exchange.
  6. They have air sacs which store air in them reducing their body density for flight.
  7. They are endothermic.
  8. They have hollow bones.
  9. They have scales on their hind limbs.
  10. They have double circulatory system with a four chambered heart.
  11. The sternum is enlarged to form keel for attachment of flight muscles.
  12. Mammalia

Study Question 11

  • Some are arboreal such as the tree squirrels, and some monkeys.
  • Some are terrestrial either on the surface of the earth or in tunnels.
  • Some are aquatic such as the dolphins and whales.

General Characteristics

  1. They have double circulatory system
  2. They have mammary glands hence the name Mammalia.
  3. Their body is usually covered with fur or hair.
  4. They have two eternal ears (pinna)
  5. They have sweat glands.
  6. They have lungs for gaseous exchange.
  7. They have four limbs.
  8. They have a diaphragm which separates the body cavity into thoracic and abdominal cavities..
  9. The brain is highly developed.
  10. They have seven cervical vertebrae at their neck.
  11. They are endothermic.
  12. They have heterodont type of dentition where the teeth are differentiated into four types, incisors, canines, pre-molars and molars. The number varies in relation to feeding habits.
  13. Although most mammals give birth to live young ones, some are egg laying such as the duck billed platypus. After hatching, the young ones are fed on milk.
  14. Practical Activity 9
  15. Practical Activity 10.

The Dichotomous Key

 The word dichotomous means separating into two. I.e. Separation of different or contradictory things: a separation into two divisions that differ widely from or contradict each other. As you move down the key you progress from general characteristics to more specific characteristics. The last single choice reveals the identity of the unknown organism.

Rules Used in Constructing a Dichotomous Key

  1. Use morphological features as far as possible.
  2. Start with the major characteristics and proceed to lesser variations that separate the organisms into smaller groups. E.g. in leaves start with type of leaf i.e. simple or compound.
  3. Select a single characteristic at a time and identify it by a number such as.
    1. Type of leaf
    1. Type of venation
  4. Use identical forms of words for the two contrasting statements e.g.

1.     a) leaf simple.

b) Leaf compound

2.     a) Leaf net veined.

        b) Leaf parallel veined.

  • The statements should always be written in positive form. Where a negative statement cannot be avoided, the first statement must be in the positive form e.g.
    • Animal with wings
    • Animal without wings
  • Avoid overlapping statements or generalisations such as
  • Short plants
  • Tall plants

Be very specific in your description such as

  • Plant I metre tall and above.
  • Plant 15cm to 60cm tall.

Some common Features Used For Identification.

  • In animals
    • Locomotory structures (legs, wings and fins)
    • Antennae, presence and number
    • Presence and type of eyes
    • Number of body parts
    • Body segmentation
    • Type of skeleton present
    • Feeding structures
    • Presence of hair, fur, scales or feathers on the body
  • In plants
Part of plantSome characteristics.  
LeafPhylotaxyLeaf typeLeaf venationMarginLaminaColour
 FlowerInflorescenceFlower shapeNumber of floral
StemType of stem( woody, herbaceous or fleshy)Shape (rectangular or cylindrical)Texture of the stem (smooth or spiny/thorny)
RootsRoot system (taproot or fibrous)Storage roots.