BIOLOGY O LEVEL(FORM FOUR) NOTES – CLASSIFICATION OF LIVING THINGS

Classification of Living Things

Kingdom Animalia

The kingdom Animalia is vast and composed of a wide variety of animals that differ greatly in structure, morphology, and body functions. This kingdom includes organisms ranging from simple sponges to complex mammals, all sharing certain fundamental characteristics.

The Distinguishing Characters

• Animals are multicellular, with each cell enclosed by a plasma membrane.
• Animal bodies are differentiated into tissues, allowing specialized functions.
• Animals are heterotrophic, obtaining food by consuming other organisms.
• Animals are capable of locomotion, enabling movement from place to place.
• Animals possess a nervous system that coordinates responses to stimuli.

Animals can be classified into two major groups based on the presence or absence of a notochord.

• A notochord is a supportive rod-like structure running longitudinally along the dorsal side of the animal. It may persist throughout life or be replaced by a backbone.

• Chordates are animals possessing a notochord.
• Non-chordates are animals lacking a notochord.

Chordates include only one phylum, Chordata, while non-chordates comprise many distinct phyla.

1. Phylum Platyhelminthes

Also known as flatworms, this phylum is characterized by dorsoventrally flattened bodies.

The phylum is divided into three classes:

1. Trematoda
2. Cestoda
3. Tubellaria

(a) Trematoda (Flukes)

• All members are parasitic.
• Some live in the bloodstream, ducts, gills, or skin of fish.
• They have flat bodies with suckers at the anterior end for attachment.

Note: Parasites depend on other organisms for food and survival.

(b) Cestoda (Tapeworms)

• Flat and segmented bodies.
• All are parasitic.
• Some species have suckers and hooks (e.g., pork tapeworm) for attachment.
• The adult tapeworm lives in the human intestine, attaching to the intestinal wall and absorbing nutrients through its skin.
• Tapeworm infestation can cause malnutrition, anemia, and intestinal irritation.
• Prevention includes inspecting meat for bladderworms, thorough cooking, and maintaining proper sanitation to avoid contamination.

(c) Tubellaria

• Flat bodies with cilia on the ventral side for movement.
• Some are parasitic, while others are free-living (e.g., planaria).

2. Phylum Nematoda (Roundworms)

Nematodes are abundant, often found in large populations, especially in soil. They have cylindrical, unsegmented bodies with pointed ends and a protein cuticle.

• Unsegmented cylindrical body with pointed ends.
• Cuticle composed of protein.
• No cilia or flagella.
• Some species are free-living, while others are parasitic.

Ascaris is a giant roundworm infecting the small intestine. Its eggs are passed in feces and transmitted through contaminated food. Infection often causes no symptoms and may go untreated.

Threadworms are small roundworms infesting the intestine in large numbers. Females lay eggs around the anus, causing itching and irritation. Transmission occurs through direct contact or contaminated food.

Filariasis, including Elephantiasis, is common in mosquito-infested areas and caused by filarial worms.

Wuchereria bancrofti infects lymph vessels and skin tissues. Microscopic larvae are transmitted by mosquitoes, causing inflammation, especially in the legs.

Ancylostomiasis is caused by hookworms (Ancylostoma duodenale) that enter through the skin, migrate to the lungs, and then attach to the intestinal wall. Symptoms include diarrhea, stomach pain, and anemia.

Note: Most roundworm infections can be treated with anti-parasitic drugs and controlled through improved sanitation.

3. Phylum Annelida

This phylum includes all “true worms”, such as earthworms, marine worms (clam worms, sea mouse, tubeworm), and freshwater leeches.

General and Distinctive Features

• Body and internal organs are segmented.
• Covered by a thin, moist, non-chitinous cuticle.
• Presence of chaetae (bristles) on segments.

The earthworm has a long cylindrical body pointed at both ends. The anterior end contains the mouth, and the posterior end has the anus. Earthworms lack eyes but are sensitive to light, which they avoid.

• The soft, moist skin is protected by a cuticle and mucus layer.
• Each segment (except the first two) has four pairs of chaetae.
• The clitellum is a thickened segment that contains eggs.

Importance of Earthworms in Soil Fertility

• Their burrowing breaks up soil, facilitating root growth.
• Burrowing aerates the soil, providing oxygen essential for root respiration.
• Burrows improve water drainage, preventing waterlogging.
• They drag leaves and plant material into the soil, adding organic matter (humus).

4. Phylum Arthropoda

Phylum Arthropoda includes grasshoppers, spiders, crabs, and many other joint-legged animals.

Distinguishing Characteristics

• Possess jointed appendages.
• Have an exoskeleton made of chitin.

The phylum is divided into five classes:

1. Insecta (e.g., grasshopper)
2. Crustacea (e.g., crab)
3. Arachnida (e.g., spider)
4. Diplopoda (millipedes)
5. Chilopoda (centipedes)

Class Insecta

Distinguishing Characteristics:

1. Body divided into three regions: head, thorax, and abdomen.
2. One pair of antennae.
3. Three pairs of jointed legs.
4. Most adults have wings.

Insects vary widely:

• Some have wings (grasshoppers), others do not (termite workers).
• Size varies from large (beetles) to small (mosquitoes).
• Habitats include water (water scorpion), land (cockroach, housefly, grasshopper), and parasitic lifestyles.
• Wing number varies: one pair (housefly) or two pairs (grasshoppers). Some have hard outer wings and soft inner wings (beetles).
• Pollinating insects feed on pollen and nectar, playing a vital role in plant reproduction.
• Soil-dwelling insects aid decomposition by feeding on dead plant material.
• Some insects are disease vectors, such as mosquitoes (malaria, yellow fever, filariasis) and tsetse flies (sleeping sickness).
• Some insects are pests, damaging crops and stored products.
• Termites consume living plants and dead wood, damaging timber and furniture.
• Houseflies transmit diseases by carrying germs from feces and decaying matter to food.

Class Arachnida

Arachnids include spiders, scorpions, ticks, and mites.

Distinguishing Characteristics

• Four pairs of jointed legs.
• Head and thorax fused into a cephalothorax, with a separate abdomen.
• One pair of chelicerae (anterior appendages).

Arachnids vary in body form and habitat. Some, like scorpions, have a narrowed posterior abdomen. They can be aquatic or terrestrial.

Economic Importance of Spiders: They control pest insect populations, although some species are venomous.

Class Crustacea

Distinguishing Characteristics:

1. Two pairs of antennae.
2. Biramous (two-branched) appendages.

• Crustaceans vary in habitat and form; many are marine (crabs, prawns, lobsters), while others live in freshwater or on land.
• Some have a hard exoskeleton covering the anterior body.
• Aquatic crustaceans may have a telson at the posterior end aiding swimming.
• Many crustaceans are important food sources for humans.

Classes Chilopoda and Diplopoda

These two classes are similar but differ in body structure and feeding habits.

Chilopoda (Centipedes)

• Body flattened dorsoventrally.
• One pair of legs per segment.
• Carnivorous feeders.

Diplopoda (Millipedes)

• Body rounded.
• Two pairs of legs per segment.
• Herbivorous feeders.
• Millipedes can damage crops by feeding on roots and other parts.

Note: Both centipedes and millipedes are soil-dwelling animals.

5. Phylum Chordata

Distinguishing Characteristics

• Presence of a notochord during embryonic development, often replaced by a vertebral column in adults.
• Hollow dorsal nerve cord located above the gut.
• Gill slits present at least during embryonic stages.
• Post-anal tail present behind the anus.

Classification within this phylum considers features such as gills, fins, mammary glands, skin type, presence of tadpoles, wings, feathers, eggshell hardness, and beak structure.

Classes of Phylum Chordata

1. Class Chondrichthyes
2. Class Osteichthyes
3. Class Amphibia
4. Class Reptilia
5. Class Aves
6. Class Mammalia

Class Chondrichthyes

This class includes cartilaginous fish such as sharks, skates, and rays. Most are marine, with body shapes ranging from dorsoventrally flattened to spindle-shaped.

Distinguishing Characteristics

1. Skeleton made of cartilage.
2. Body covered with placoid scales.
3. Caudal fin with two lobes of different sizes.
4. Each gill pair is in a separate compartment.
5. Gill slits are visible; no operculum (gill cover).
6. Mouth and two nostrils are centrally located.
7. Body temperature varies with the environment (ectothermic/cold-blooded).

Some species are important food sources for humans.

Disadvantage

1. Some species may pose a threat to humans as predators.

Class Osteichthyes

This class consists of bony fish.

Distinguishing Characteristics

• Skeleton made of bone.
• Body covered with overlapping bony scales.
• Mouth terminally placed; nostrils on dorsal surface.
• Gills located in a common chamber covered by an operculum.
• Tail fin lobes of varying size.
• Most have an air sac (swim bladder) aiding buoyancy.
• Body temperature varies with environment (ectothermic).

Note: Members vary in form, size, and habitat. Most have scales; some lack them. Found in both seawater and freshwater. Some have lungs instead of gills.

Advantages

1. Important food source for humans.
2. Provide employment through fishing industries.
3. Fish oil has medicinal uses.
4. Fish meal is used as fertilizer.

Class Amphibia

This class includes toads, frogs, salamanders, and limbless amphibians. Amphibians spend part of their life in water and part on land. Most lay eggs in water, and their young (tadpoles) live in water, while adults live on land.

Distinguishing Characteristics

1. Skin is moist and lacks scales.
2. Life cycle includes a larval stage called a tadpole.
3. Larvae have external and internal gills.
4. Heart has three chambers: two atria and one ventricle.
5. Gaseous exchange occurs via gills in tadpoles and lungs, skin, and mouth lining in adults.
6. Eggs are covered with jelly-like substance and laid in water.
7. Body temperature varies with environment (ectothermic).

Note:

• Amphibians vary greatly in size, form, and habitat.
• Most have limbs; some are limbless and resemble snakes or worms.
• Some have webbed toes; others do not.
• Amphibians are never found in salty water.

Advantage: They feed on insects and pests that could damage crops.

Class Reptilia

Reptiles are believed to have evolved from amphibians. The main groups include snakes, lizards, turtles, and crocodiles.

Distinguishing Characteristics

1. Dry skin covered with horny scales.
2. Heart with two atria and partially divided ventricles; crocodiles have a four-chambered heart.
3. Body temperature varies with environment (ectothermic).
4. Lay eggs on land with soft shells.

Note:

• Reptiles vary greatly in size, form, and habitat.
• Some are limbless (snakes), others have limbs (lizards, crocodiles).
• Some have protective shells (tortoises, turtles).
• Some are aquatic; others are terrestrial.
• Other reptiles include tortoises and chameleons.

Disadvantages

• Some reptiles are venomous and can attack or kill humans (e.g., snakes, crocodiles).

Class Aves

Most birds are capable of flight, although some species are flightless (e.g., ostrich, penguin).

Distinguishing Features

1. Body covered with feathers.
2. Forelimbs modified into wings.
3. Mouth modified into a beak, varying according to feeding habits.
4. Lay hard-shelled eggs.
5. Maintain a constant body temperature (homeothermic/warm-blooded).

Note:

• Birds vary in size, form, and habitat.
• Some are large (ostrich), others small (sunbird).
• Body is spindle-shaped and streamlined for flight.
• Most have hollow bones to reduce weight.
• Birds with webbed feet can swim (e.g., ducks).

Advantages

1. Provide food through eggs and meat.
2. Feathers are used for making cushions and pillows.

Class Mammalia

This class includes bats, rats, cows, mice, and humans. Mammals have highly developed brains and complex behaviors.

Distinguishing Characteristics

1. Have hair or fur covering all or part of their bodies.
2. Possess mammary glands for feeding young.
3. Teeth are heterodont, with different types and shapes.
4. Have a diaphragm to aid breathing.
5. Red blood cells lack nuclei.
6. Viviparous: embryos develop internally with maternal support.
7. Have sweat glands in the skin.
8. Maintain constant body temperature (homeothermic/warm-blooded).

Note:

• Mammals vary greatly in size, form, and habitat.
• Some are very large (elephants), others very small (mice, rats).
• Some have wings (bats).
• Many have external ears (pinna).
• May have horns, hooves, claws, or nails.
• Heart has four chambers.

Most mammals live on land, while some are aquatic.

Advantages

1. Provide food sources such as cattle, sheep, goats, and rabbits.
2. Supply materials like leather and manure.
3. Support eco-tourism through wildlife parks and game reserves.

Disadvantages

1. Some mammals prey on humans and livestock.
2. Can cause crop damage.