Animal Kingdom Classification: General Characteristics, Body Symmetry, Level of Organisation, and More

The animal kingdom includes all multicellular, eukaryotic, heterotrophic organisms placed under the Kingdom Animalia. Animal kingdom classification is based on features such as body organisation, symmetry, digestive and circulatory systems, and the presence or absence of a backbone.

Basis of Animal Kingdom Classification

Animals are classified based on several structural and functional features. These criteria are important in school biology and competitive exam preparation.

Major Criteria Used for Classification

• level of organisation

• body symmetry

• type of digestive system

• type of circulatory system

• nature of the body cavity

• segmentation

• notochord

• backbone or vertebral column

These criteria help divide animals into non-chordates and chordates, and further into many phyla and classes.

Levels of Organisation in the Animal Kingdom

One important basis of classification is the level of organisation.

Cellular Level of Organisation

In this level, the body is made of loosely arranged cells. Cells may perform different functions, but they do not form proper tissues.

Tissue Level of Organisation

In this type, similar cells group together to form tissues. Each tissue performs a particular function.

Organ Level of Organisation

At this level, tissues combine to form organs. Each organ has a specific role in the body.

Organ System Level of Organisation

In more advanced animals, different organs work together to form organ systems. Each system performs a distinct physiological function.   

Body Symmetry in the Animal Kingdom

Another major basis of classification is body symmetry.

Asymmetrical Animals

These animals cannot be divided into equal halves by any plane. A common example is a sponge. 

Radial Symmetry  

In these animals, the body can be divided into similar parts through many planes passing through the central axis. This type of symmetry is seen in groups like cnidarians and adult echinoderms.

Bilateral Symmetry

In bilaterally symmetrical animals, the body can be divided into equal left and right halves by only one plane. Most higher animals show bilateral symmetry.

Organ System Patterns in the Animal Kingdom

The body systems of animals are also used as classification criteria.

Digestive system

Animals may show either a complete or an incomplete digestive system.

A complete digestive system has two openings, a mouth and an anus. This is found in arthropods and chordates.

An incomplete digestive system has only one opening, which acts as both the mouth and the anus. This is seen in simpler groups such as platyhelminthes.

Circulatory system

There are two main types of circulatory systems.

An open circulatory system is one in which blood is pumped out of the heart and is not always confined to blood vessels. This is found in molluscs and arthropods.

A closed circulatory system is one in which blood flows through arteries, veins, and capillaries. This is found in annelids, vertebrates, and some other advanced groups.

Animal Kingdom Classification

The broad classification of the animal kingdom can be understood by dividing animals into major groups based on body complexity and the presence of a backbone.

At a broad level, animals may be discussed under:

• Protozoa

• Bryozoans

• Non-chordates

• Vertebrates

Standard phylum-wise classification

The animal kingdom is commonly studied under the following major phyla:

1. Porifera

2. Cnidaria or Coelenterata

3. Ctenophora

4. Platyhelminthes

5. Aschelminthes or Nematoda

6. Annelida

7. Arthropoda

8. Mollusca

9. Echinodermata

10. Hemichordata

11. Chordata

Simple Flow Chart for a Better Understanding

Protozoa

Protozoa are unicellular eukaryotic organisms that show some animal-like features, such as movement and ingestion of food. Although they are single-celled, they were traditionally studied along with animals because of their motility and predatory behaviour.

Protozoa obtain food in two major ways:

• by absorbing nutrients through the cell membrane

• by engulfing food particles through phagocytosis, often using pseudopodia

Protozoa represent the simplest form of animal-like life and are important in understanding the early stages of biological organisation.

Bryozoans

Bryozoans are commonly called moss animals. They are filter feeders that use a crown of ciliated tentacles to collect food particles from water. Most of them live in aquatic habitats, especially marine waters, though some may be found in freshwater and cold environments.

Most bryozoans are colonial. The individual units in a colony are called zooids. These zooids are not completely independent. In a colony, some zooids are mainly responsible for feeding and excretion.   

Bryozoans are useful as examples of colonial animal organisation.

Vertebrates

Vertebrates are animals that possess a backbone or vertebral column. They are the most advanced members of the animal kingdom and show an organ-system level of organisation.

The vertebrates include:

• fishes

• amphibians

• reptiles

• birds

• mammals

Important Features of Vertebrates

Vertebrates have a well-developed nervous system, a closed circulatory system, and advanced organ systems. They vary greatly in size, habitat, and structure.

Major Groups of Vertebrates

1. Fishes

These include jawless fish, cartilaginous fish, and bony fish. They are mainly aquatic and use gills for respiration.

2. Amphibians

These animals live both on land and in water. They usually return to the water to breed.

3. Reptiles

Reptiles are generally covered with scales or scutes. Most are terrestrial and breathe through lungs.

4. Birds

Birds are covered with feathers and have streamlined bodies adapted for flight, though not all birds fly.

5. Mammals

Mammals may be terrestrial, aquatic, or aerial. The presence of mammary glands and hair characterises them.

Why is the Classification of the Animal Kingdom Important?

The number of known animal species is extremely large. Some animals share close similarities, while others differ greatly in body plan, habitat, and physiology. Classification helps organise this vast diversity into meaningful groups.

• makes the study of animals systematic

• helps identify similarities and differences

• shows evolutionary relationships

• makes exam revision easier

• helps build concept clarity for NEET and board exams

Difference Between Non-Chordates and Chordates

Major Classification Criteria at a Glance

Why is the Animal Kingdom Important in Biology?

The topic of the animal kingdom is important because it helps students understand:

• diversity of animal life

• evolutionary progression

• structural complexity

• Basic body plan differences

• classification principles used in zoology

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