Animal Kingdom


Kingdom Animalia is a group of eukaryotic, multicellular organisms with a heterotrophic mode of nutrition. Animals lack cell walls in their cell structure and they are usually motile

Animals have different structures and forms but there are some characteristic features similar in various individuals based on the arrangement of cells, symmetry, coelom, digestive and circulatory systems. These features are used to classify animals for better understanding

Classification of Animal kingdoms based on their characteristic

Animals do not show a similar pattern of cells as all individuals are multicellular. The classification of animals is based on the organization of cells, tissues, organs, and organ systems


  • Protozoa are microscopic, unicellular, and free−living organisms found in almost all habitats

  • They are usually motile and parasitic

  • Protozoa digest food through osmotrophy that is engulfing food through cell membranes


  • Bryozoans are also known as moss animals. They are a small group of aquatic animals that are sessile, colonial, and moderately invertebrates

  • They capture food with tentacles lined with cilia and pores

  • Bryozoans are found mainly in warm humid conditions


  • Members of the phylum Vertebrates possess notochord during the embryonic stage which later gets replaced by a bony vertebral column in the adult stage

  • Besides the backbone, vertebrates also have a muscular heart with chambers, a kidney for excretion, and appendages

Levels of organization

The cellular level of organization − Animals at this level of organization form loose aggregates of cells. Sponges exhibit a cellular level of organization to perform different functions. These loose cell does not organize together to form tissue.

Tissue level of organization − Animals with this level of organization contain cells that perform the same function forming a tissue together.

  • Diploblastic organization − The embryonic cells are arranged in two layers, outer layer ectoderm, and inner layer endoderm. Coelenterates are a common example.

  • Triploblastic organization − Animals in which embryonic cells are arranged in three−layer i.e. ectoderm, mesoderm, and endoderm. Common examples are Platyhelminthes, chordates, etc

Organ level of organization − animals in which tissues performing the same function organize together to form an organ.

Organ system level of organization − when organs associate together to form an organ system functioning for different physiological roles is exhibited under this category.

Organ system pattern

Different animals exhibit different patterns of complexities in the organ system

The circulatory system is a network of arteries, veins, and capillaries that originate from the heart. The circulatory system is of two types− open and closed systems

  • In an open system, the blood does not restrict to the blood vessels but is pumped into a cavity called hemocoel which is located outside of the heart.

  • In a closed system, the blood is contained inside the blood capillaries and circulates in one direction from the heart to surrounding vessels and then returns to the heart again

The digestive system is the functioning of organs in the digestion and assimilation of food. All animals have a digestive system

  • The digestive system comprises two types−complete and incomplete. The complete system consists of two openings i.e. the mouth and anus. While the incomplete system has only one opening as in the case of Platyhelminthes

  • However, vertebrates possess a very complex digestive system including the gastrointestinal tract, and esophagus extending to the anus through which swallowing, absorption, and assimilation of food take place

Body symmetry Animals can be classified based on their body symmetry. There are three types of symmetry in the body

  • Asymmetrical is a symmetry that does not divide the plane into equal halves. It is a unique feature in phyla Porifera (sponges)

  • Radial symmetry is a symmetry that divides a plane passing through the central axis into two halves. Animals with radial symmetry have only top and bottom surfaces. Phylum like Coelenterates, Ctenophores, and Echinoderms show radial symmetry in their body structure

  • Bilateral symmetry is when the plane divides into identical left and right halves. Animals like Annelids and Arthropods exhibit bilateral symmetry.

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Different types of symmetry

Kingdom Animalia

Phylum Porifera

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  • They are also referred to as sponges. They are multicellular animals with a cellular level of organization

  • They are usually found in marine habitats and have asymmetric body types

  • They have a water canal system through which the water enters Ostia (pores) lined on the body surface and transported into a central cavity (spongocoel) from where it is excreted out through the osculum

  • The transport of water supports the capture of food, exchange of gases, and water excretion

  • Members of Porifera are hermaphrodites.

  • They reproduce asexually and sexually through fragmentation and formation of gametes

  • Fertilization occurs inside the body and development is indirect

  • Common examples are Spongilla and Sycon

Phylum Cnidaria

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  • They are free−living or sessile, aquatic, and radially symmetric animals.

  • They have cnidoblast (stinging capsule) on their tentacles and body. The cnidoblasts provide anchorage and defence mechanisms

  • They exhibit tissue level and diploblastic organization

  • They have internal and external digestive systems with a single opening mouth on the hypostome

  • Medusa and polyp are two basic body forms exhibited by Cnidarians

  • Examples are Phylasia and Adamsia

Phylum Platyhelminthes

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  • Members of Platyhelminthes are more complex and structured than the earlier phyla

  • They have dorsally flattened bodies, hence known as flatworms

  • They are endoparasitic and found inside animals like human beings. Some of them directly absorb nutrients from their host

  • They are bilaterally symmetric, triploblastic animals with organ level of organization

  • They have no true internal body to accommodate well-developed organs for proper functioning

  • The fertilization occurs inside the body and the development occurs through multiple larval stages

  • Examples are Fasciola and Taenia

Phylum Annelida

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  • They can be terrestrial or aquatic, free−living or parasitic with a closed circulatory system

  • They show organ level of organization and have bilateral symmetry

  • The body surfaces are divided into segments and metameres

  • They have circular muscles that help in movement

  • The reproduction is sexual in the members of Annelida

  • Examples are Nereis and Hirudinaria.

Phylum Arthropoda

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  • It is the largest phylum including insects in Kingdom Animalia

  • They exhibit organ level of organization and bilateral symmetry

  • They have an external hardcore called exoskeleton with joint legs

  • Arthropods have a head, thorax, and abdomen in their body covered with a chitinous layer

  • They excrete through Malpighian tubules

  • The fertilization is generally internal and the development may be direct or indirect

Phylum Mollusca

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  • They are the second−largest phyla of animals

  • They may be terrestrial or aquatic with an organ level of organization

  • They have bilateral symmetry, triploblastic organization, and true body

  • Calcerous shell covers the body of most arthropods

  • The body is segmented into the head, visceral hump, and muscular foot.

  • Fertilization is internal and development is indirect

  • Examples are Pila, Chaetopleura, etc.

Phylum Echinodermata

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  • Animals have calcerous covering on their body

  • They are found in marine environments with an organ level of organization

  • Larval staged animals have bilateral symmetry while adults have radial

  • They have a water vascular system that supports movement, food gathering, and respiration

  • They do not have an excretory system.

  • Echinoderms reproduce through sexual modes and fertilize externally

  • Examples are Asteria and Echinus

Phylum Protochordata

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  • These animals are bilaterally symmetric and triploblastic

  • They possess a true body with the presence of a notochord at some stage in their life cycle

  • They are generally found in marine habitats

  • Examples are Herdmania and Amphioxus

Phylum Chordata

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  • Human beings, birds, and other animals constitute this phylum

  • The common feature of all animals belonging to this phylum is the presence of notochord.

  • They possess a single, hollow, dorsally placed central nervous system

  • There is a presence of a post−anal tail which helps in locomotion

  • A pharynx is penetrated by gill slits.

  • The phylum is further divided into subphyla Urochordata, Cephalaochordata, and Vertebrata


The kingdom Animalia is the largest kingdom comprising all animals. These animals are classified on several bases like level of organization, symmetry, coelom, segmentation, and notochords. Moreover, other fundamental characters classify animals into specific phyla. These phyla are Porifera, Cnidaria, Ctenophora, Platyhelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Protochordata, and Chordata


Q1. Who proposed the five−kingdom classification?

Ans. R H Whitaker proposed the five−kingdom classification based on cell structure, nutrition, body, and reproduction

Q2. How many species of animals are classified in the Animal kingdom?

Ans. About 1.7 species are described in the kingdom Animalia.

Q3. Branchiostoma belongs to which subphylum?

Ans. Cephalochordata. Branchiostoma is invertebrates with notochord but no backbone. They may be the closest relative to vertebrates.


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