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Phylum Cnidaria (Coelenterata): General Characters and more

Updated on April 18, 2016

Phylum Cnidaria (formely known as coelenterata) includes about 9000 species.Some are found in fresh water, but the grater number are marine. Coelenterates (Gr. koilos, hollow+enteron,gut).are the simplest of eumetazoans. This interesting group of animals take its name from the large cavity in the body that serves as the intestine.They include the hydra, jellyfish, sea anemones and corals.

Jelly fish
Jelly fish

General Characters

  • They are aquatic, mostly marine and a few are fresh water
  • Tissue grade of organization: organs and organ system are absent.
  • They are diploblastic .
  • Body is formed of two layers of cells, viz., an outer ectoderm and an inner endoderm with a non cellular jelly like layers of mesoglogea in between .
  • Radial symmetry with oral and aboral ends, no definite head.
  • A central cavity called gastrovascular cavity which opens to exterior by the mouth. Anus is absent.
  • Digestion is both intracellular and extracellular.
  • Presence of stinging cells or cnidoblast in the ectoderm. The cnidoblast cells possess a capsule called nematocyst containing poisonous fluid. The cnidoblast cells are defensive cells used to kill or paralyses other preys and enemies.
  • They are acoelomates. i.e. there is no separation between the digestive cavity and the general body cavity.
  • Respiration and excretion occur by diffusion through the body surface.
  • Nervous system is primitive and formed of a network of nerve cells and fibres throughout the body.
  • Reproduction is by both sexual and asexual means. Fertilized eggs may develop into a larva called planula.
  • An alternation of generations or metagenesis is found in the life history i.e. the asexual polyploid generation alternates with the sexual medusoid generation

Polyp and Medusa body forms

Cnidarians have two types of body forms, viz., polyp and medusa.

  • The polyp is cylindrical and usually fixed. It represents the asexual stage.
  • The medusa is umbrella like and usually free swimming. It represents the sexual phase.

Polyp vs Medusa

Phylum Cnidaria is divided into three classes:

Class: Hydrozoa: Polyp is pre dominant, but the life cycle shows both polyp and medusa stages.

Examples: Hydra, Obelia and Physalia


Polymorphism in Cnidarians

Cnidarians show the phenomenon of polymorphism, i.e . presence of two or more morphological different types of individuals are present in the colony of same species.The different individuals are called zooids .

  • A division of labour is seen in the colony because each type of zooid is specialized to perform a specific function .
  • Physalia and Halistemma are example of polymorphic colonies

Class : Scyphozoa

Dominant stage is the free swimming medusa stage.

Example: Aurelia

Class : Anthozoa

Polyp stage is the only zooid and medusa is completely absent.

Examples: Sea anemone and corals like Fungia, Favia and Madrepora.

Sea anemone
Sea anemone


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