Classification of the Phylum Cnidaria

There are differences between single celled animals the Protozoa and multicellular animals- the Metazoa. Typical metazoan characteristics are first found in Cnidaria, tissue grades of construction first appeared in Cnidaria. From taxonomic point of view members of the Phylum Cnidaria are regarded as first metazoan animal.

Hyman (1940) presented the classification of Coelenterata. Recently the name Cnidaria in place of Coelenterata is used because Ctenophora has been separated from it and given the status of a Phylum.


A group of diploblastic animals with radial symmetry, without organ system and anus and having single tubular gastrovascular cavity and with tentacles and nematocysts is recognized as Cnidaria.

General Characters of Cnidaria

  1. Cnidarians are multicellular animals and first metazoan with tissue grade of construction.
  2. Most of the Cnidarians are marine, though some are fresh water forms.
  3. Body radially symmetrically, some are biradial.
  4. A single coelenteron or gastrovascular cavity is present which opens through mouth, anus is absent.
  5. Small and narrow tentacles surround the mouth in circlet. Tentacles are used to procure food and for defence.
  6. Body is covered by epidermis, gastrovascular cavity is lined by endodermis and in between the two there is a layer of non cellular matrix known as mesoglea.
  7. Body diploblastic and so coelom is present.
  8. The colonies show polymorphism with polyp and medusa as basic Zooids. Polyp represent asexual form and medusa represents sexual zooid.
  9. A special type stinging cells known as nematocysts are present. These cells help in defence and food capture.
  10. Ciliated planula larva is found.
  11. In the life cycle alternation of generation and metagenesis are found.

Classification of Phylum Cnidaria with reasons and examples

The basis of classification is the adult forms whether polyp or medusa.

Phylum Cnidaria has been classified into three Classes-

Class I: Hydrozoa

Class II: Scyphozoa

Class III: Anthozoa

Class I- Hydrozoa


  1. Fresh water members of this class live singly, but the marine forms live singly or form polymorphic colony.
  2. Though only polyp or medusa is fund but sometimes both polyp or medusa may be fond in the same animal.
  3. Gastrovascular cavity is directly connected with mouth.
  4. Alternation of generation is found in the life cycle.
  5. Ciliated planula larva is found.
  6. No stomodeum, velum present in medusa.
  7. Radial canals in medusa are not branched.
  8. Gametes develop from ectoderm.

Class II- Scyphozoa


  1. Larger medusa, free swimming, all are marine.
  2. Polyp absent or reduced.
  3. Gastrovascular cavity branched.
  4. Well developed nematocyst very active.
  5. Gastric tentacles and gametes developed from endodermis.
  6. Mesoglea thick, fibrous and contains amoebocytes.
  7. Sense organs as tentaculocyst.
  8. Many tentacles hang from the velum.

Class III- Anthozoa


1.      Well developed stomodeum.

2.      Gastrovascular cavity subdivided by complete or incomplete septum.

3.      Mesenteries carry nematocyst and gonad.

4.      Mesoglea thick and cellular.

This class has two subclasses as:

Sub Class I – Octocorallia


  1. Colonial marine forms.
  2. Polyp with 8 hollow, pinnate tentacles.
  3. 8 complete unpaired mesenteries.
  4. Polyps are dimorphic.

Sub Class II – Hexacorallia

Characters :-

  1. Solitary or colonial marine forms.
  2. Tentacles are mesenteries are in multiple of six.
  3. Tentacles are simple, unbranched, hollow and conical.
  4. Polyps are usually monomorphic.

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