Classification of Phylum Cnidaria
Posted on : 24-01-2017

Phylum Coelenterata/Cnidaria includes about 10,000 known species. It is classified into three classes namely Hydrozoa, Scyphozoa and Anthozoa.

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The following are the general characters of each of them,

Class I:

  • These are mostly marine animals but some may also live in fresh water.
  • They are chiefly colonial. Some forms may also appear solitary.
  • Medusa stage is absent in few animals. Sometimes both polyp and medusa stages are present in few animals of this class. Medusa is craspedote (presence of velum)
  • Coelenteron of the polyps of this class is undivided
  • Mesoglea is acellular
  • Cnidocytes are restricted to the epidermis
  • Gonads also occur in the epidermal region
  • Their colonies are polymorphic with different types of zooids like gastrozooids (feeding type), dactylozooids (defensive type) and gonozooids (reproductive type)

Class II: Scyphozoa (Gr. skyphos=cup, zoon=animal)

  • All the animals belonging to this class are marine in nature
  • Medusa stage is predominant in this class. Medusa is acraspedote (No velum)
  • Mouth is surrounded by four oral arms.
  • Mesoglea is cellular and contains amoebocytes
  • Cnidocytes occur in the epidermis and also in the gastrodermis region
  • Gonads occur in the gastrodermal region.
  • Polyps are solitary or may also exist in colonies. Polyp stage is syphistoma (body is divided by septa). This syphistoma produces juvenile medusa called as ephyrae by the process of strobilation. Finally this ephyra grows into the sexual adult medusa.
  • This class includes Jelly fish

Class III: Anthozoa (Gr. anthos=flower, zoon=animal)

  • All the animals of this class are marine
  • They may be solitary or colonial
  • All are sedentary polyploid forms. The medusa stage is absent
  • Mouth is oval and is surrounded by a whorl of tentacles resembling a flower like structure. Hence the name of the class.
  • The mouth leads into tubular pharynx called stomodaeum that in turn opens into coelenteron. Coelenteron is divided into radial compartments by vertical septa called as mesenteries.
  • Cnidocytes occur in epidermal as well as gastrodermal region
  • Gonads occur in the gastrodermis.

  1. Into how many classes is the Phylum Cnidaria classified? What are they?
  2. Give the feature of the class Anthozoa.
  3. What is the meaning of the term Hydrozoa.

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