Theory:

Human beings are surrounded by so many varieties of organisms.
There were around 11 crore different organisms recorded and described. Still, every year new organisms are reported. These organisms live in different habitats.
They may live in water, land, air or even inside the human and animal body!
 
For studying these varied type of organisms Swedish botanist, Carolus Linnaeus developed a system - taxonomy of animals.
Taxonomy: According to it the animals are classified in terms of genus, species. Latin nomenclature is used universally for this classification.
Classification of living organisms
Classification involves ordering and grouping the animals based on their similarities, differences and relationships.
The hierarchy of classification is shown below:
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species
Kingdom: Highest category, animals grouped in have common characters.
For example, microorganisms, plants, animals.
 
Phylum: classes that are interrelated constitute a phylum.
For example, in animals, birds, reptiles, fishes and mammals are grouped into different phylum.
 
Class: In class, different categories of the same phylum is grouped.
For instance, different varieties of bats, rats, monkeys are grouped inside the class.
 
Order: In order, animals that have the most common similarities are further grouped.
Take, for instance, monkeys. In monkeys order, we may classify baboons, apes and even man because all these animals have similar features.
 
Family: In the family, further similar characters make the members of the family classified together.
For example, cat family Felidae has not only cats but also leopard and tiger together.
 
Genus: In the genus, further similar category of animals are grouped.
For example, Indian wolf (Canis pallipes) and Indian jackal (Canis aureus) are grouped under the same genus Canis.
 
Species: In this level of classification, the animal under classification becomes unique. This is the lowest or last category in the taxonomy.
For example, the Indian parakeet (Psittacula eupatria) and green parrot (Psittacula krameri) are different species of birds. These species cannot interbreed and thus are so unique.
Basis of classification
Within the animal kingdom, the further classification is done as a part of taxonomy, using the following characteristics:
  • Level of organisation
  • Symmetry
  • Germ layers
  • Coelom
Level of organisation:
Cellular arrangement defines this level of organisation. For instance, unicellular and multicellular organisms.
 
Symmetry:
Symmetry defines the arrangement of body parts inside the body.  There are two types of symmetry, such as radial symmetry and bilateral symmetry.
  • Radial symmetry: Radial arrangement of body parts surrounding the central axis. Animals of radial symmetry can be cut into two equal parts around the central axis in any direction.
    Example: Hydra, Jellyfish and starfish.
  • Bilateral symmetry: Animals of bilateral symmetry can be cut into two equal halves only in one direction along the central axis. In other directions, it can not be cut into two equal (identical) halves.
    Example: Frog, butterfly, octopus, spiders and insects.
Germ layers:
Germ layers define the formation of different organs when the embryo becomes adult. Some organisms have two germ layers - ectoderm and endoderm. These organisms are called 'diploblastic animals'. Organisms which have three germ layers-ectoderm, mesoderm and endoderm are called 'triploblastic animals'.
 
Coelom:
Coelom defines the presence or absence of body cavity in animals. Some animals do not have body cavity like tapeworm. Some animals have pseudo (or false) cavity like roundworm. Some animals have cavity like earthworm or frog.

In this cavity, the digestive organs are found.
 
Coelom types.JPG
Classification of animal kingdom based on fundamental features
animal classification.JPG
 
In addition to the above, the whole of the animal kingdom can be divided based on the presence or absence of spinal cord (backbone) or notochord:
  • Invertebrate are a group of animals that do not have spinal chord.
  • Vertebrate are a group of animals that have a spinal cord.
The Vertebrate can further be classified also like warm-blooded and cold-blooded animals.