1. Basis and Importance of Classification

A. Basis for Classification

Classification is necessary to systematically study the wide variety of organisms. The broadest divisions are based on fundamental characteristics, and subsequent smaller groups are based on dependent characteristics.

The main bases for classification are:

Presence or Absence of Nucleus: This makes a basic distinction in the body designs of organisms (prokaryotic vs. eukaryotic).
Cellularity: Organisms being unicellular or multicellular.
Level of Organisation: How the body is developed and its parts are organised.
Mode of Nutrition: Whether the organism is autotrophic (produces its own food, e.g., plants) or heterotrophic (gets food from outside, e.g., animals).
Evolutionary Relationship: Classification is closely related to evolution, where characteristics that came into existence earlier are more basic.

B. Importance of Classification

It makes the study of a wide variety of organisms systematic and easier.
It projects a picture of all organisms and their interrelation with each other.
It provides a base for the study of other branches of biology, such as ecology, which deals with an organism's inter-relation with its environment.
It helps to establish a hierarchy of groups of organisms.

2. The Five Kingdom Classification (Whittaker)

The widely accepted five-kingdom classification includes Monera, Protista, Fungi, Plantae, and Animalia

Kingdom	Characteristics	Examples
Monera	Unicellular, Prokaryotic (no defined nucleus or organelles). May or may not have a cell wall. Nutrition is either autotrophic or heterotrophic.	Bacteria, blue-green algae (cyanobacteria), Mycoplasma.
Protista	Unicellular, Eukaryotic (defined nucleus). Use appendages (cilia, flagella) for locomotion. Nutrition is either autotrophic or heterotrophic.	Unicellular algae, diatoms, protozoans, Amoeba, Euglena.
Fungi	Multicellular at certain stages, Eukaryotic, and heterotrophic. They are saprophytes (use decaying organic material as food). Have cell walls made of chitin.	Yeast, mushrooms, Aspergillus.
Plantae	Multicellular, Eukaryotic with cell walls. They are autotrophs and use chlorophyll for photosynthesis.	All plants.
Animalia	Multicellular, Eukaryotic without cell walls. They are heterotrophs. Most are mobile.	All animals.

3. Classification of Kingdom Plantae

The plant kingdom is classified based on having well-differentiated components, specialised tissues for transport, and the ability to bear seeds.

Group	Key Characteristics	Examples
Thallophyta	Plant body is not well-differentiated (thallus-like). Commonly called algae and are predominantly aquatic.	Spirogyra, Ulothrix, Chara.
Bryophyta	Called amphibians of the plant kingdom. Body is differentiated into stem and leaf-like structures. No specialised tissue for water conduction.	Moss (Funaria), Marchantia.
Pteridophyta	Plant body differentiated into roots, stem, and leaves. Has specialised tissue for conduction of water.	Marsilea, ferns, horse-tails.
Gymnosperms (Phanerogams)	Bear naked seeds (gymno = naked, sperma = seed). Usually perennial, evergreen, and woody.	Pines, deodar, Cycas.
Angiosperms (Phanerogams)	Bear seeds that develop inside an organ modified to become a fruit (angio = covered). Also called flowering plants.	All flowering plants.

Differences between Monocots and Dicots (Angiosperms)

Angiosperms are divided based on the number of cotyledons (seed leaves):

Monocots (Single Cotyledon): Have fibrous roots, parallel venation in leaves, and vascular bundles that are scattered. Flowers are trimerous (multiples of three).
Dicots (Two Cotyledons): Have a tap root system, reticulate venation in leaves, and vascular bundles arranged in rings. Flowers are pentamerous or tetramerous.

4. Classification of Kingdom Animalia

Animals are classified based on the extent and type of body design differentiation.

Group	Key Characteristics	Examples
Porifera	Organisms with holes (pores) all over the body. Non-motile and attached to a solid support. Minimal differentiation and division into tissues. Pores lead to a canal system for food and oxygen circulation.	Sponges (Sycon, Spongilla).
Coelenterata	Water-living animals. Body made of two layers of cells (outside and inner lining). Some live in colonies (corals); others have a solitary life span (Hydra).	Jellyfish, sea anemones, corals, Hydra.
Platyhelminthes	Flatworms (flattened dorsiventrally). Triploblastic (three cell layers) and bilaterally symmetrical. No true internal body cavity (coelom). Most are parasites.	Tapeworm, planaria, liver fluke.
Nematoda (Aschelminthes)	Bilaterally symmetrical and triploblastic. Body is cylindrical (roundworms). Have a pseudo-coelom (sort of body cavity). Many are parasitic.	Filarial worms, roundworm, pinworms.
Annelida	Bilaterally symmetrical, triploblastic, and have a true body cavity (coelom). Extensive organ differentiation in a segmented fashion. Found in fresh water, marine water, and land.	Earthworms, leeches.
Arthropoda	Largest group of animals. Have jointed legs (arthropod means 'jointed legs'). Bilaterally symmetrical and segmented. Open circulatory system.	Prawns, butterflies, houseflies, spiders, scorpions, crabs.
Mollusca	Bilateral symmetry and little segmentation. Reduced coelomic cavity. Have a kidney-like organ for excretion and a foot used for moving around.	Snails, mussels, octopus.
Echinodermata	Spiny skinned organisms (echinos = hedgehog, derma = skin). Exclusively free-living marine animals. Have a peculiar water-driven tube system for moving. Skeleton made of hard calcium carbonate structures.	Starfish, sea urchins.
Protochordata	Bilaterally symmetrical, triploblastic, and coelomate. Show a notochord (rod-like support structure) at least at some stages. All are marine animals.	Balanoglossus, Herdmania, Amphioxus.
Vertebrata	Possess a true vertebral column and internal skeleton. Have a notochord, dorsal nerve cord, are triploblastic, and have paired gill pouches.	Fish, amphibians, reptiles, birds, and mammals.

Vertebrata Classes

Class	Key Characteristics	Examples
Pisces (Fish)	Exclusively aquatic. Skin covered with scales/plates. Use gills to obtain oxygen. Cold-blooded, two-chambered heart. Lay eggs.	Sharks (cartilage skeleton), Tuna, Rohu (bone and cartilage skeleton).
Amphibia	Live on land and in water (amphibious). Cold-blooded. Three-chambered heart. Respiration through lungs (on land) and moist skin (in water). Lay eggs in water.	Frogs, Toads, Salamanders.
Reptilia	Cold-blooded. Have scales. Breathe through lungs. Most have a three-chambered heart (crocodiles have four). Lay eggs with tough coverings on land.	Snakes, turtles, lizards, crocodiles.
Aves (Birds)	Warm-blooded. Four-chambered heart. Forelimbs modified into wings. Outside covering of feathers. Lay eggs. Hollow and light bones.	White Stork, Ostrich, Pigeon, Sparrow.
Mammalia	Warm-blooded. Four-chambered heart. Have mammary glands to produce milk to nourish young ones. Skin has hairs, sweat glands, and oil glands. Most give birth to young (viviparous).	Whale, Human, Cat, Rat, Bat.

5. Nomenclature

Need for Naming: A systematic scientific name resolves the difficulty of identifying an organism across different languages and regions.
Binomial Nomenclature: The system used today was introduced by Carolus Linnaeus in the eighteenth century.
Scientific Name: An organism is identified by its genus and species name.
Conventions for Writing:
1. The name of the genus begins with a capital letter.
2. The name of the species begins with a small letter.
3. When printed, the scientific name is given in italics.
4. When written by hand, the genus name and species name must be underlined separately.

Summary

Kingdom	Cell Type	Multicellularity	Cell Wall	Mode of Nutrition
Monera	Prokaryotic	Unicellular	Present in some	Autotrophic or Heterotrophic
Protista	Eukaryotic	Unicellular	Present in some	Autotrophic or Heterotrophic
Fungi	Eukaryotic	Multicellular at stages	Present (Chitin)	Heterotrophic (Saprophytes)
Plantae	Eukaryotic	Multicellular	Present	Autotrophic (Photosynthesis)
Animalia	Eukaryotic	Multicellular	Absent	Heterotrophic