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Herd of Elephants


Basis of Classification

Animalia kingdom includes multicellular, eukaryotic, heterotrophic organisms and their cells lack cell walls.

Basis of classification:

Kingdom Animalia is further classified into different phyla based on fundamental features. Such as arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive systems.


Levels of organization:

  1. The cellular level of organization – Porifera

  2. Tissue level of organization – Coelenterata, Ctenophora

  3. Organ level of organization – Platyhelminthes

  4. Organ-system level of organization – Aschelminthes to Chordates.


Circulatory System:

  • Open type: Blood is pumped out of the heart and the cells and tissues are directly bathed in it.

  • Closed type: Blood is circulated through a series of vessels.



  1. Asymmetrical: The body of an organism does not divide into two equal halves at any plane. e.g., Sponges.

  2. Radial symmetry: Any plane passing through the central axis of the body divides the organism into identical halves. e.g., Coelenterata, Ctenophora, Echinodermata.

  3. Bilateral symmetry: If the body of an organism can be divided into identical left and right halves in only one plane. e.g., Annelids and Arthropods and most of the chordates.

Radial Symmetry.png
Radial Symmetry
Bilateral Symmetry.png
Bilateral Symmetry

Germinal layers:

  • Diploblastic: Cells arranged in two embryonic layers i.e. external ectoderm and internal endoderm. (Mesoglea may be present in between ectoderm and endoderm) e.g., Coelentrates. (Cnidarians).

  • Triploblastic: Three layers present in developing embryo i.e., ectoderm, endoderm and mesoderm. e.g., Chordates

Diplo and triploblastic.png
(a) Diploblastic  (b) Triploblastic

Coelom (Body cavity):

  1. Coelom: The body cavity which is lined by mesoderm is called coelom. Animal possessing coelom are called coelomate animals. e.g., Annelids to Chordates.

  2. Pseudocoelom: If the body cavity is not lined by mesoderm, instead, the mesoderm is present in scattered pouches between ectoderm and endoderm. e.g., Aschelminthes.

  3. Acoelom: Body cavity is absent. e.g., Platyhelminthes.

(a) Coelomate (b) Pseudocoelomate  (c) Acoelomate

Metamerism: If the body is externally and internally divided into segments with serial repetition of at least some organs. This phenomenon is called metamerism. e.g., Earthworm.


Notochord: It is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals. e.g., Chordates.


  • Members of this phylum are commonly known as sponges.

  • They usually marine and asymmetrical.

  • Have cellular level of organisation.