Introduction 

In biology, there are different types of kingdoms based on the characteristics of the elements such as animal, fungi and Protista kingdom. Out of different kingdoms in the living kingdoms of biology, the animal kingdom is the most extended and developed kingdom. It is separated by two groups basically that are invertebrates and vertebrates. There are different types of animals in the kingdom such as mammals, humans, extinct animals and others. It is one of the diverse kingdoms and draws numerous attention and research at a regular interval of time. Scientists have found a total of 8.7 million species till the present time.

Animal kingdom: Definition 

Every animal possesses unique features. Animals are multicellular, eukaryotic and belong to the kingdom” Animalia”. Animals obtain energy by feeding either on plants or on animals or both. Till now millions of species have been found and scientists believe that numerous species are still unknown and require a deep study on the extinction of the animals. Animals have dominated their presence on earth due to the voluntary mobility, development of organs and muscles and involvement of sheer diversity on earth. After the year 1765, the sponges’ group of the animal were identified and helped in understanding the concept of the animal kingdom. 

Levels of Organisation in Animal Kingdom

• In animals such as molluscs, chordates, arthropods, annelids, organs are enclosed in functional systems
• Every system in organs does the specific functions of physiology
• This pattern can be known as the different levels of organ systems in organisms 

• Though all animals in the kingdom are multicellular, they do not share the same pattern of organs and cells
• Organ levels in the animals are higher and exhibited by Platyhelminthes members of the kingdom
• In different groups of animals, the pattern of the organ system is different for physiological function
• In coelenterates, cell arrangement is very complex
• Cells in the organ system are performing the same functions and are arranged into the structure of tissues
• Therefore, it is known as the tissue level in the organisation
• There are two types of circulatory systems: open type and closed type
• In the open type system blood is pumped outside the heart with cells and tissue closed with tit, while in the closed system, blood is circulated in diameters through a series of vassals

Classification of the Animal Kingdom

Animal kingdoms can be classified based on formation, organ structure and other characteristics. The features that form the basis of classification can be understood by following.

Symmetry: animals can be grouped based on the structure of symmetry, for instance, sponges are asymmetrical. Radial symmetry can be understood as a plane passing from the centre without dividing it into two halves. Coelenterates, Ctenophores possess this type of body plan. Annelids and arthropods have a bilateral system, where the body plane is divided into two equal halves.

Arrangement of cells: arrangement of cells is based on the two embryonic layers. External ectoderm, as well as internal endoderm, is known as diploblastic animals. An undifferentiated layer known as mesoglea can be found between the endoderm and ectoderm.

Coelom: classification of the animal kingdom is done on the basis of its presence and its absence in the gut wall and body wall. The body cavity lined by mesoderm is known as the coelom. The body cavity is not lined with mesoderm in some animals. It is present with scattered pouches between the endoderm and ectoderm. These animals where body cavities cannot be found are called acoelomates

Conclusion

The animal kingdom included varieties of the species living or extinct. It is the most evolved and developed kingdom among other kingdoms such as plants, fungi, and protists. Animal kingdom varies from the other kingdom in terms of organ structure, tissue formation, voluntary mobility and other aspects. There are a total of 8.7 million species that have been found now. The weird kingdom is derived from the scientific name “Animalia”. Classification of the animal is done on the basis of various facets such as an arrangement of cells, segments, notochords, symmetries, coelom, and a pattern of respiratory, reproductive and digestive systems.