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.

  • Food gathering, respiratory exchange and removal of wastes occurs through water canal system.

  • Digestion is intracellular.

  • Ostia (minute pores on body), spongocoel (body cavity) and osculum help in water transport.

  • Spongocoel is lined by choanocytes (collar cells).

  • The body is supported by a skeleton made up of spicules and spongin fibres.

  • Sexes are not separate hence they are hermaphrodites.

  • Fertilisation internal, development is indirect (i.e., has a larval stage distinct from adult stage).

e.g., Sycon (Scypha), Euspongia. Spongilla

(a) Sycon (b) Euspongia  (c) Spongilla


  • Also called Cnidarians because they have cnidoblasts or cnidocysts.

  • They usually marine and radially symmetrical.

  • Have tissue level of organisation, diploblastic animals.

  • Food gathering, anchorage, and defense occur through cnidoblasts present on tentacles.

  • Digestion is extracellular and intracellular.

  • They have a gastro-vascular cavity and an opening, mouth on hypostome.

  • The body wall is composed of calcium carbonate (corals).

  • Exhibit two body forms; polyp and medusa. e.g., Hydra, Aurelia.

  • Alternation of generation between body forms called Metagenesis occurs in Obelia. e.g., Physalia, Adamsia, Pennatula, Gorgonia, Meandrina.

Aurelia (Medusa)


  • Also called as sea walnuts or combjellies.

  • They are exclusively marine, radially symmetrical.

  • Have tissue level of organisation and are diploblastic animals.

  • Digestion is both extra and intracellular.

  • Body has eight external rows of ciliated comb plates for locomotion.

  • They Show Bioluminescence (the property of living organism to emit light).

  • Sexes are not separate, only sexual reproduction occurs.

  • External fertilization, Indirect development.

e.g., Ctenoplana, Pleurobranchia.

Adamsia (Polyp)