Animal Kingdom Introduction



The animal kingdom, also known as Kingdom Animalia, is a major group or kingdom in the classification of living organisms. It encompasses a wide variety of multicellular organisms that are heterotrophic, meaning they obtain their nutrients by consuming other organisms or organic matter.


Basis of classification

The classification of animals in the animal kingdom is based on several criteria. These are the main basis of classification:


  • Level of Organization: Animals can be classified based on their level of organisation of cells, tissues, organs, and organ systems. The two main levels of organisation are:
    • Cellular: Animals in this group are composed of cells only and do not have distinct tissues. For example, sponges.
    • Tissue: Animals in this group have specialised tissues that perform specific functions. This includes most animals, such as insects, fish, birds, and mammals.
    • Organ-system: Group of tissues forms organs, each specialised for specific function. This level shows various complexities across the different groups of animals.


  • For example-

Incomplete digestive system of platyhelminthes whose single opening serves as both mouth and anus whereas a complete digestive system has two openings - mouth and anus.

Similarly, the circulatory system can be of two types- open and closed.


  • Open circulatory system- Blood is pumped out of the heart and the cells and tissues are directly bathed in it.
  • Closed circulatory system- Blood is circulated through the arteries, veins and capillaries.


  • Symmetry: Animals can be categorised based on their body symmetry. There are three types of symmetry:
    • Radial Symmetry: Animals with radial symmetry have body parts arranged around a central axis, like spokes on a wheel. Examples include jellyfish and sea anemones.
    • Bilateral Symmetry: Animals with bilateral symmetry have a distinct left and right side. They can be divided into two similar halves along a sagittal plane. Most animals, including humans, exhibit bilateral symmetry.
    • Asymmetry: Animals with asymmetry lack any specific symmetry. For example, sponges have an irregular and asymmetrical body structure.


  • Diploblastic or Triploblastic: This classification is based on the number of embryonic germ layers in the animal's body.
    • Diploblastic: Animals with two germ layers (ectoderm and endoderm) are called diploblastic. They lack a middle layer called mesoderm. Examples include cnidarians like jellyfish and corals.
    • Triploblastic: Animals with three germ layers (ectoderm, endoderm, and mesoderm) are called triploblastic. Most animals, including arthropods, mollusks, vertebrates, and worms, are triploblastic.

  • Coelom: Coelom refers to the body cavity present in many animals. It is a fluid-filled space between the body wall and the digestive tract. Animals can be categorised based on the presence or absence of a coelom:
    • Acoelomates: Animals without a coelom, i.e., the body cavity is completely absent. Flatworms (e.g., planarians) are examples of acoelomates.
    • Pseudocoelomates: Animals with a pseudocoelom, which is a body cavity that is partially lined with mesoderm. Roundworms (e.g., nematodes) are examples of pseudocoelomates.
    • Coelomates: Animals with a true coelom, which is a body cavity completely lined with mesoderm. Most animals, including mammals, birds, reptiles, and arthropods, are coelomates.


  • Segmentation: Some animals exhibit segmentation, where their body is divided into repetitive segments with similar structures.Segmentation can be found in various groups, including annelids (e.g., earthworms), arthropods (e.g., insects), and vertebrates (e.g., humans).


  • Notochord: The presence or absence of a notochord is another basis of classification in the animal kingdom.
    • Notochord present: Animals with a notochord during some stage of their life cycle. This includes chordates, such as vertebrates (fish, reptiles, birds, mammals) and some invertebrates (e.g., lancelets).
    • Notochord absent: Animals that lack a notochord. This includes most invertebrates, such as arthropods, molluscs, and cnidarians.