Animal Kingdom

The Animal Kingdom are organisms whose cells have a nucleus enclosed within a nuclear envelope, multicellular organisms that they take food from external sources. Animals can also be divided on the basis of the development of other features as well as the developmental origin of their mouth. The classification of the animal kingdom also helps in allot a position in a systematic way and can be well placed newly discovered species.

Basics Of Classification Of Animal Kingdom

The Animal Kingdom is split into parts on the basis of differences in structure and form of different animal features common to various individuals in relation to the patterns of cells, body symmetry, patterns of digestive, circulatory, nature of coelom, or reproductive systems.

Levels of Organisation

  • On the basics of tissue level of organization
  • On the basics of the organ level of organization
  • On the basics of organ system level of organization
  • On the basis of the circulatory system of Organization
    • Open type: The blood is pumped out of the heart and the cells and tissues are directly bathed in it.
    • Closed type : The blood is circulated through a series of vessels of varying diameters.


Radial symmetry
  • Asymmetrical: Any plane that passes through the center does not divide them into equal halves that they have lacking symmetry between the two same parts.
  • Radial symmetry: When any plane passing through the central axis of the body divides the organism into two identical halves that they have symmetry between the two same parts.
  • Bilateral symmetry: The body can be divided into identical left and right halves in one plane only.

Diploblastic and Triploblastic Organisation

Diploblastic and Triploblastic Organisation
  • Diploblastic Animals: Animals in which the cells are arranged in two embryonic layers and the layers are called as external ectoderm, and an internal endoderm.
  • Triploblastic Animals: Those animals in which the developing embryo has a third germinal layer called mesoderm which is found between the ectoderm and endoderm.


  • The body cavity that is lined by mesoderm is called coelom.
  • Animals which possessing coelom are called coelomates.
  • The body cavity is not lined by mesoderm.
  • The mesoderm is present as scattered pouches in between the ectoderm and endoderm are called pseudocoelom.
  • The animals possessing pseudocoelom are called pseudocoelomates, e.g., aschelminthes.
  • The animals in which do not contain body cavity are called acoelomates, e.g. Platyhelminthes.


  • The body is externally and internally divided into segments.
  • It has a serial repetition of at least some organs.
  • Example – Earthworm, the body shows this pattern called metameric segmentation.


  • The notochord is a mesodermally derived rod-like structure.
  • It is formed on the dorsal side during embryonic development in some animals.
  • E.g., Porifera to echinoderms



Phylum (Porifera)

Phylum (Porifera)
  • It is commonly known as sponges.
  • It is generally marines animal.
  • The phylum is mostly asymmetrical animals
  • It is made up of many cells that are said to be multicellular animals and have a cellular level of organization.
  • Pathway of water transport is helpful in food gathering, removal of waste, and respiratory exchange.
  • Sponges reproduce asexually by fragmentation.
  • Sexually by the formation of gametes.
  • The fusion of haploid gametes, egg, and sperm, to form the diploid zygote is internal and development is indirect having a larval stage which is form and structure of an organism distinct from the adult.
  • Examples of the phylum are
    • Sycon (Scypha)
    • Spongilla (Freshwater sponge)
    • Euspongia (Bath sponge).

Phylum – Coelenterata (Cnidaria)

Phylum – Coelenterata (Cnidaria)
  • Coelenterata is an aquatic animal.
  • They are mostly marine, free-swimming.
  • They are radially symmetrical animals.
  • Coelenterata has a central gastro-vascular cavity with a single opening, mouth on hypostome.
  • Digestion takes place outside a cell and intracellular.
  • Some of the cnidarians have a skeleton that constitutes a part of calcium carbonate.
  • Cnidarians exhibit two basic body forms called
    • Polyp
    • Medusa
  • Cnidarians that exist in both forms exhibit alternation of generation i.e., polyps produce medusae asexually and free-swimming sexual form the polyps sexually.
  • Examples
    • Physalia (Portuguese man-of-war)
    • Adamsia (Sea anemone)
    • Pennatula (Sea-pen)
    • Gorgonia (Sea-fan)
    • Meandrina (Brain coral).

Phylum (Ctenophora)

Phylum (Ctenophora)
  • It is commonly known as sea walnuts or comb jellies
  • The tissue level of Ctenophora is organization.
  • The body bears eight external rows of ciliated comb plates, which help in move oneself from one place to another place.
  • Digestion is both taking place outside a cell and intracellular.
  • The process by which new individual organisms takes place only by sexual means.
  • Examples
    • Pleurobrachia
    • Ctenoplana

Phylum (Platyhelminthes)

Phylum (Platyhelminthes)
  • They are called flatworms.
  • They have a directed from the back to the front flattened body.
  • Flatworms have affected both sides i.e. bilateral symmetrical, triploblastic, and acoelomate animals.
  • They are the organ level of organization.
  • Flame cells help in the maintenance of constant osmotic pressure in the fluids and excretion.
  • Fertilization is internal and development is through many immature forms of insect stages.
  • Examples
    • Taenia (Tapeworm)
    • Fasciola (Liver fluke).

Phylum (Aschelminthes)

Phylum (Aschelminthes)
  • Aschelminthes are also called roundworms.
  • They are circular in cross-section area.
  • They have an organ-system level of body organization.
  • They are bilateral i.e. the body can be divided into identical left and right halves in one plane only.
  • An excretory tube removes the unnecessary materials from the body fluids through the body cavity of an organism through the pore.
  • Fertilization is internal process.
  • The development of an organism may be direct or indirect.
  • Examples
    • Ascaris (Round Worm)
    • Wuchereria (Filaria worm)
    • Ancylostoma (Hookworm).

Phylum (Annelida)

Phylum ( Arthropoda)
  • Organ-system level of body organization and bilateral symmetry.
  • The body surface is distinctly marked out into segments or metameres and, hence, the phylum name Annelida.
  • They possess longitudinal and circular muscles that help in locomotion.
  • The closed circulatory system is present.
  • Nephridia help in osmoregulation and excretion.
  • Examples: Nereis, Pheretima (Earthworm), and Hirudinaria (Bloodsucking leech).

Phylum ( Arthropoda)

Phylum ( Arthropoda)
  • The largest phylum of Animalia which includes insects.
  • Two-thirds of all named species on earth are arthropods.
  • Organ-system level of organization and bilaterally symmetrical.
  • The body is covered by a chitinous exoskeleton.
  • The circulatory system is of open type.
  • Excretion takes place through malpighian tubules.
  • Fertilization is usually internal.
  • Examples: Economically important insects – Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect) Vectors – Anopheles.

Phylum (Mollusca)

Phylum (Mollusca)
  • Second-largest animal phylum.
  • Molluscs are terrestrial or aquatic having an organ-system level of organization.
  • They are bilaterally symmetrical.
  • The body covered by a calcareous shell.
  • Its body is unsegmented with a distinct head, muscular foot, and visceral hump.
  • The anterior head region has sensory tentacles.
  • Examples
    • Pila (Apple snail)
    • Pinctada (Pearl oyster)
    • Sepia (Cuttlefish)
    • Loligo (Squid)
    • Octopus (Devilfish).

Phylum (Echinodermata)

  • This is also called as Echinodermata.
  • They have an endoskeleton of calcareous ossicles.
  • All are marine with the organ-system level of organization.
  • They are radially symmetrical but larvae are bilaterally symmetrical.
  • The digestive system is complete with a mouth on the lower side.
  • The anus on the upper side.
  • A distinctive feature is the presence of a water vascular system which helps in moving from one place to another place, capture, and transport of food and respiration.
  • The excretory system is absent.
  • Reproduction is sexual.
  • Examples
    • Asterias (Starfish)
    • Echinus (Sea urchin)
    • Antedon(Sea lily)
    • Cucumaria (Sea cucumber).

Phylum (Hemichordata)

  • Primarily considered as a sub-phylum under phylum Chordata.
  • Consists of a small group of worm-like marine animals.
  • They have an organ-system level of organization.
  • They are bilaterally symmetrical.
  • The shape of the body is cylindrical.
  • The body is composed of an anterior proboscis, a collar, and a long trunk.
  • The circulatory system is of open type.
  • Excretory organ is proboscis gland.
  • Fertilisation is external.
  • Examples
    • Balanoglossus
    • Saccoglossus.

Phylum (Chordata)

  • It is characterized by the presence of a notochord, a dorsal hollow nerve cord, and paired pharyngeal gill slits.
  • It is bilaterally symmetrical.
  • They possess a post-anal tail.
  • The circulatory system is closed.
  • Phylum Chordata is divided into three subphyla are
    • Urochordata
    • Cephalochordata
    • Vertebrata

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