Skip to main content

Phylum Cnidaria (Coelenterata): General Characters and more

  • Author:
  • Updated date:

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

More about Animal Kingdoms

  • General Characters of Phylum Ctenophora
    General characteristics of Ctenophora with examples
  • Coral and Coral Reefs
    Corals are animals belonging to the phylum Coelenterata. The corals in general have diverse shapes and sizes, but majority are colonial with very small polyps. A coral reef is a complex living community
  • Phylum Porifera: General Characters and more
    The Phylum Porifera include more than 5000 animal species, mostly marine, but includes about 150 fresh water inhabitants also. They found in all seas, wherever there are rocks, shells, submerged timbers, or coral to provide a suitable substratum.


Denisa Domondon on August 28, 2020:

concise and easy to understand, thank you for this

Scroll to Continue

yitagesu rega on January 09, 2018:

so nice

Ikram on October 07, 2017:

Very nice

suparna bhunia on August 06, 2017:

It's really beautiful

Farhad Ullah on August 03, 2017:

Thanks for uploading this valuable information

Jahnavi_gss on July 23, 2017:

All of this is told in The best way a student can understand.

Pratiksha on July 17, 2017:

It's nice ha! !!!!!

Danish jalil on January 07, 2017:


aapeksha on July 07, 2016:

it gives more information while compare to other slide.

makuka aj on January 14, 2016:

dat is helpful! big up

ganesh on November 03, 2015:

very nice method to explain

Umar Farooq on October 14, 2015:

This is very benifit for study

maheen on October 09, 2015:

very nice explanation

DHIRAJ KUMAR on September 27, 2015:


Jayaprada on September 24, 2015:

realy good

sachin on September 19, 2015:

Nice explanation

saumya on September 04, 2015:


praveen on August 19, 2015:


alok on July 03, 2015:

V. Good

shelly gurjar on June 24, 2015:

sweet and simple explanation, just go through it

zainab on June 15, 2015:

fantastic explanation

SP SHARMA on May 09, 2015:

Thanks . It really nice

Alya rey on February 18, 2015:

Thanks :-) it's so good

fati khan on January 21, 2015:

Good explanation

Saud on December 09, 2014:


darshan kumar.t on November 19, 2014:


mahi on November 18, 2014:

Good ....

peter oulo oracha on October 29, 2014:


Taj sangha on September 13, 2014:

Superb explained topic I like dis explanation so much....well said guys.. :)

madhura on July 17, 2014:

Nice explanation

arman srivastav on November 10, 2013:

well done!!! this topic is explained very well.

Related Articles