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Phylum - Coelenterata / Cnidaria

Phylum - Coelenterata / Cnidaria


  • Phylum Coelenterata is also called as phylum Cnidaria due to the presence of cnidoblast cells
  • It includes about 9000 species.

Topics covered

  • General characteristics of phylum Cnidaria
  • Classification of phylum Cnidaria
  • Example

General characteristics of phylum Cnidaria -

Detailed explanation:

  • Habit and habitat
  • These are aquatic, mostly marine and few are also found in freshwater (e.g- Hydra).
  • Sessile or free-swimming.
  • Level of Organisation
  • Tissue level of Organisation.
  • Symmetry
  • Radially symmetrical.
  • Germ layers
  • These are diploblastic, which means their body is derived from two embryonic germ layers i.e., ectoderm and endoderm. Undifferentiated mesoglea is present between them.
  • Body wall
  • Body wall is made up of layers-
  • Epidermis- Outer ,ectodermal layer having Interstitial cells which are totipotent in nature and form nerve cells
  • Gastrodermis- Endodermal layer, present around gastrovascular cavity
  • Body plan
  • These have blind-sac body plans.
  • Coelom
  • These are acoelomates.
  • Central cavity called gastrovascular cavity present.
  • Digestion
  • Incomplete digestive systems present i.e., they have a central gastro-vascular cavity which acts as both mouth and anus.
  • Digestion is both extracellular and intracellular.
  • Nutrition is holozoic.
  • These are carnivorous in nature.
  • Respiration
  • Respiration takes place by the general body surface through diffusion.
  • Excretion
  • Excretion takes place by the general body surface through diffusion.
  • These are ammonotelic i.e., ammonia is the nitrogenous waste.
  • Skeleton
  • Usually absent; soft bodied animals.
  • But in corals body is covered by horny or calcareous exoskeleton.
  • Nervous system
  • A primitive type of nervous system is present in them which consist of a network of non-polar nerve cells or neurons and their processes.
  • Statocyst is seen for the first time, which acts as a sense organ for balance.
  • Reproduction
  • Both sexual (formation of gametes) and asexual reproduction (budding) is found in them.
  • Asexual buds and sexual parts/gonads develop from the interstitial cells.
  • Power of regeneration is present.
  • Development is indirect in them which means larval stages are formed. For eg,
  • Larvae of Obelia - planula (free living)
  • Larva of Aurelia - Ephyra
  • Special characteristics

i. Polymorphism

  • These show polymorphism i.e., exhibit two body forms which are polyp and medusa.

a. Polyp-

  • A cylindrical and sessile form which may be colonial and solitary.
  • Their mouth directed upwards. Example - Hydra, Adamsia, etc.

b. Medusa-

  • An umbrella shaped and free swimming. They are always solitary.
  • Their mouth directed downwards. Example - Aurelia or jellyfish.
  • If both the forms i.e polyp and medusa are found in a species then it shows metagenesis.

ii. Metagenesis

  • The process of alternation of generations, in which polyp produce medusae by asexual reproduction (budding) and medusae produce polyps by sexual reproduction. Example - Obelia
  • Both polyp and medusae phases are diploid


iii. Cnidocytes -

  • Also known as cnidoblast.
  • These consist of a hypnotoxin filled stinging capsule or nematocyst.
  • These are present on the body and the tentacles.
  • Function- i.anchorage (attachment), ii.defense, iii. capturing the prey.



Classification of phylum Cnidaria -

Detailed explanation:

Cnidarians are divided into 3 classes, on the basis of development of zooids or dominance of medusoid or polypoid phase in the life cycle -

1. Hydrozoa

  • Either only polyps are present or polyps and medusae both are present.
  • Examples: Hydra, Obelia (sea-fur) and Physalia(Portugese man of war)

2. Scyphozoa

  • Only medusae forms are present in them.
  • Class of Jelly fishes.
  • Examples: Aurelia(Jelly fish)

3. Anthozoa

  • Only polyp forms are found in them.
  • Medusa form is absent in them.
  • Class of corals and sea anemones
  • Examples : Gorgonia (Sea fan), Adamsia(Sea anemone), Alcyonium (dead man's finger), Pennatula ( Sea pen), Corallium (red coral), , Meandrina (Brain coral), Madrepora (stag-horn coral), Tubipora (organ pipe coral).
  • Australian barrier reef is formed by members of this class, i.e., the corals.


Detailed explanation:

1. Hydra -

  • They are found in freshwater.
  • Some species of Hydra are found in symbiotic association with algae called Zoochlorella and it gives green colour to Hydra.


2. Physalia -

  • They are commonly known as Portuguese Man of war.


3. Aurelia -

  • They are commonly known as jellyfish.
  • They are carnivorous.


4. Adamsia -

  • They are commonly known as “The Sea anemone”.
  • They show commensalism with Eupagurus prideuxi (hermit crab).
  • Commensalism is a relationship between two organisms in which one gets benefited while the other neither gets benefits nor harmed.
  • It gets transported to other places by hermit crabs that lives inside the shell.


Frequently Asked Questions: FAQs

Q1. What is the difference between extracellular and intracellular digestion ?

  • Extracellular digestion is the breakdown of complex food molecules into small molecules that occurs outside the cell while intracellular digestion is the breakdown of complex food molecules into small molecules inside the cell or within the vacuoles.

Q2. Write the function of cnidoblast cells.

  • Cnidoblast cells also known as cnidocytes are found in them that contain the stinging capsules or nematocysts. They are used for anchorage (attachment), defense and for the capture of prey. They are found on the tentacles and the body.

Q3. Why are Coelenterates also known as Cnidarians ?

  • Coelenterates are also known as Cnidarians due to the presence of the cnidoblast cells.

Q4. Write the name of any one freshwater Coelenterate.
Ans :

Q5. Define metagenesis.
Ans :

  • Metagenesis is the process of alternation of generations in which polyp produce medusae by asexual reproduction (budding) and medusae produce polyps by sexual reproduction.
  • Example - Obelia?

Q6. Write the common name of Physalia.
Ans :

  • Physalia is commonly known as the “Portuguese Man of War”.
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