Phylum :Coelenterata (Cnidaria)

This phylum includes organisms commonly known as jelly fishes, sea anemones, sea pens etc. The name ‘cnidaria’ implies the presence of specialized cells called cnidoblasts (cnidocytes) on the body.

Habitat: Cnidarians are aquatic and mostly marine. They are either sedentary or free-swimming forms.



(Sea anemone)


(Portuguese man-of-war)




(Brain coral)



Body plan: They exhibit tissue level of organization and show blind sac body plan. There is a central gastro-vascular cavity with a single opening – the mouth or hypostome. A whorl of tentacles is found encircling the mouth. It helps in capturing food.

Symmetry: Cnidarians are mostly radially symmetrical.

Germlayer: They have a diploblastic body with an outer layer of epithelio-muscular cells and an inner lining of gastro-endodermal cells. A layer of mesoglea is found between these two layers.

Body characteristics:

  • Cnidarians can exhibit in two forms – Polyp (asexual phase) and Medusa (sexual phase).
  • The polyp is sessile and has a cylindrical shape (Eg.Adamsia).
  • Medusa is umbrella shaped and free – swimming (Eg: Aurelia).
  • Hence, there is metagenesis (alternation of generation) between these two forms in the life cycle of cnidarians (except in Hydra) i.e., polyps produce medusae asexually and medusae form the polyps sexually.

  • The body and tentacles bear numerous stinging cells called cnidoblasts which contain stinging capsules known as nematocysts.
  • These cells help in anchorage, defence and in capturing prey.
  • Some cnidarians such as corals have skeletons made up of calcium carbonate.


Physiology: Respiratory, excretory and circulatory systems are absent. Digestion can be intracellular or extracellular.

Reproduction: Asexual reproduction takes place by budding and sexual reproduction by formation of gametes.

Embryo Development: Development of the embryo is indirect involving two larvae – planula larva and ephyra larva.


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