Animal Kingdom


Kingdom Animalia is one of the diverse classification system as it is fundamentally based on many criteria. According to Whittaker, the Animalia kingdom comprises various multicellular eukaryotic creatures. It is one of the major kingdoms among the top-5 kingdom scheme of classification. The classification is based on a number of factors ,some of them are the patterns of circulation, digestion and reproduction types, body symmetry, nature of coelom, cellular arrangement, presence of the notochord, and the varied pattern of segmentation. Let us discuss a few of the most important classifications.


Characteristics of kingdom Animalia

  1. They are multicellular organisms with no green pigment (chlorophyll).

  2. Animalia contains eukaryotic cell types hence they have only cell membrane, but no cell wall.

  3. Organisms in the kingdom Animalia are heterotrophic as they get their food from other organisms.

  4. Animalia has a complex body structure. They have many types of cells, tissues, organs.

  5. The muscles and bones  help them in locomotion by contracting and relaxing their body parts.

  6. Reproduction is often by sexual contact but some lower animals reproduce by the asexual mode.

  7. Organisms under Animalia live by aerobic respiration in which they absorb oxygen and release the co2.


Patterns of Organ System

Organ system is  the systematic assimilation of tissues and organs to serve specific functions. Within the Animalia, different animals have different frameworks of organs as follows.

1. Digestive System 

 There are two forms of digestive frameworks, Complete and Incomplete. In an incomplete Digestive framework, the digestive system has only one opening to the outside of the body. The opening is a solitary in nature as it work as both  mouth and rear-end. One of the example is Platyhelminthes. In the case of animals with Complete Digestive Framework, there are 2 different openings, a mouth and a rear-end. F0r example, Arthropods, and Chordates.

2. Circulatory System 

Based on the circulatory system, animals can be again classified into the closed type and open type. In an open circulatory framework, the blood is pumped out of the heart resulting in a gush of blood against every cell as we see in the arthropods and molluscs. On the other hand, a closed circulatory framework is the one in which the blood moves through different vessels - the arteries, veins, and capillaries instead of a simple movement. Most of the vertebrates come fall under this group.



Criteria for the Classification of organisms

 1. On the basis of body symmetry

Based on the body symmetry, the animals can be categorized as radial and asymmetrical ones. Asymmetrical type of animals have 2 unequal body partitions  (bilateral asymmetry), for example, sponges. Animals that fall under radial symmetry are characterized by the forming a spiral symmetry on any plane that can form 2 indistinguishable parts. Example, Ctenophores, and coelenterates. On the other side, animals that fall under bilateral Symmetry are those whose body can be partitioned into indistinguishable right and left parts. For example, Annelids, Arthropods.


2. On the basis of body Wall of animals

Based on this, animals are organized into diploblastic  and triploblastic types. Diploblastic animals have only 2 layers of the body wall (ectoderm and endoderm) whereas, triploblastic animals have 3 body layers I, e germinal layers ( mesoderm), the external ectoderm, inward endoderm. For example, Platyhelminthes to Chordates.


3. On the basis of the nature of Coelom

Coelom is the body cavity lined by the mesodermal cells. Animals can be classified based on the presence or absence of a body cavity. The 3 main class of animals based on the nature of coelom are Acoelomates, Pseudocoelomates or Coelomates. Acoelomates are the invertebrates with no coelom. For example, flatworms and nemerteans. Pseudocoelomates are those animals in which the body cavity is not formed by the distinctive pattern of mesoderm cells, instead, the mesoderm is scattered between the endoderm and the ectoderms. For example, Aschelminthes. Coelomates have the body cavity lined by typical mesoderm. For example, Arthropods and Chordates. 


4. On the basis of segmentation

 Based on the segmentation, animals are classified as metameric in which the body is segmented from inside to outside, with serial redundancy of few organs. For example, Earthworm.  


5. On the basis of the presence or absence of notochord

Based on the notochord, few animals can be chordates (the animals which have Notochord) and few are Non- Chordates (Animals that lack notochord) for example, Porifera to Echinoderms.  


Levels of Organization of animals

There are five levels: cellstissueorgansorgan systems, and organisms. In organisms such as Sponges, the cells are organized as free cell lumps. The following chart gives you an idea of levels of organization in animals.



System of Classification of Animal Kingdom

Swedish botanist Carolus Linnaeus (1707-1778) has introduced the modern System of Scientific Classification. He considered the animal`s kingdom, phylum, class, family, genus and species to classify as under.

  1. Kingdom – Kingdom includes all organisms exist in the world. For instance, the animal kingdom comprises all the animals in the world.

  2. Phylum is when each kingdom is further divided into smaller subdivisions, phyla, for instance, chordates are a phylum with their notochord.

  3. The chordates are further separated into many classes, for example, Birds, Reptilians, Mammalians, and Amphibians.

  4. Class can be redivided into many families. A family has more than one genus.

  5. A family lead to Genus or genera. Animals originated from the common predecessor looks similar.

  6. Species – A species is the accumulation of similar types of animal, for example, all homosapiens.  ​​​​​​



Read More


Animal Kingdom            


Structure and functions of cell             


Mendel`s experiments


Transgenic animals


Check your understanding 

  1. Distinguish between the complete and incomplete digestive canals.

  2. Write the systematic classification of Animalia.






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