Criteria for Animal Classification

On the Basis of Number of Germ Layers:

  • Animals in which the cells are arranged in two embryonic layers, an external ectoderm, and an internal endoderm, are called diploblastic animals,  e.g., coelenterates.
  • An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm

Classification of Animals Germ Layers

  • Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals  e.g. Platyhelminthes to Chordates

On the Basis of Animal Body Plan:

  • There are three body plans among animals.
  • Cell aggregate plan:

    • The animals show aggregation of cells with least division of labour.
    • The cells do not form tissues or organs. There is no nervous coordination between them.
    • Examples: Members of phylum Porifera. Sponges. Sponge Asconoid

Body Plan Aggregate Body Plan

  • Blind sac body plan:

    • There is a digestive cavity and a single opening which acts as a mouth and anus.
    • The digestive system is incomplete.
    • Examples: Members of phylum Cnidaria.

Animal Classification Blind Sac Body Plan

  • Tube within tube plan:

    • This body plan is present in all advanced and highly evolved animals.
    • The digestive system is complete and has two openings one at each end.
    • Examples: Members of phylum Annelida onwards.

Animal Classification Tube Within Tube





On the Basis of Body Symmetry:

  • Symmetry means similarity in shape, size and number of parts on the opposite sides of the meridian line.There are three body symmetry plans among animals.
  • Asymmetrical animals:

    • In such animals, there is no plane that passes through the centre and divides them into equal halves.
    • Examples: Certain sponges, snails.

Animal Classification Assymmetric

  • Radially symmetrical animals:

    • When any plane passing through the central axis of the body divides the organism into two identical halves, then the symmetry is called radial symmetry.
    • Examples: Coelenterates, ctenophores and echinoderms, hydra, starfish.

Animal Classification Radial Symmetry

  • Bilaterally symmetrical animals:

    • When a single plane passing through the central axis of the body divides the organism into two identical halves, then the symmetry is called bilateral symmetry.
    • Examples: Fish, Frog, human being.

Animal Classification Bilateral Symmetry

On the basis of Body Cavity or Coelom:

  • It is the space between the body wall and the alimentary canal, which is produced by splitting of mesoderm, during embryonic development.On the basis of nature of the coelom, animals are classified into three types.
  • Acoelomates:

    • In certain animals, there is no body cavity or coelom. Hence they are called acoelomates.
    • The space between the body wall and the alimentary canal is filled with parenchymatous tissue.
    • Examples: Members of phylum Platyhelminthes.

Animal Classification Acoelomates



  • Pseudocoelomates:

    • In certain animals there is false  body cavity or coelom. Hence they are called pseudocoelomates.
    • The space between body wall and alimentary canal is lined by  patches of mesodermal cells.
    • Examples: Members of phylum Aschelminthes.

Animal Classification Pseudocoelomates

  • Coelomates:

    • In triploblastic animals the mesoderm splits into two layers enclosing a cavity called coelom.
    • The outer layer lines the body wall while inner layer covers the alimentary canal.
    • The cavity is filled with coelomic fluid.
    • Examples: all mammals.

Animal Classification coelomates



On the Basis of Body Segmentation or Metamerism:

  • In some animals, the body is externally and internally divided into segments with a serial repetition of at least some organs.
  • The external segmentation matches with internal segmentation.
  • For example, in earthworm, the body shows this pattern called metameric segmentation and the phenomenon is known as metamerism.

Animal Classification Body Segmentation

On the Basis of Body Support:

  • The internal or external framework that provides support to the body is called skeleton.
  • It has two types exoskeleton and endoskeleton.
  • Lower animals like cockroaches have exoskeleton for their protection. While higher animals like fishes, cobra parrots possess both the exoskeleton and endoskeleton.

On the Basis of Presence of Notochord:

  • The notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
  • Animals with notochord are called chordates  e.g. Human being, reptiles, birds3.
  • The animals which do not form notochord structure are called non-chordates,  e.g., Porifera to echinoderms.

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