Department of Zoology at ANDC/Zoology Museum/Museum specimens/Cnidaria

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Phylum Cnidaria

Date & Time : 18, October 2017 13:15


General Characteristics of Cnidaria

  • Cnidaria includes coral animals, true jellies, sea anemones, sea pens, and their allies.
  • These were earlier called Coelenterates which is no longer recognized as scientifically valid.
  • Phylum cnidaria contains over 10,000 species of animals. These are exclusively aquatic and mostly inhabit marine environments.
  • The name Cnidaria comes from the Greek word "cnidos," which means stinging nettle.  Cnidarians  got this name because their nematocysts eject barbed threads tipped with poison.
  • Cnidarians  are multicellular animals.
  • They contain  hollow body cavity  known as Gastrovascular cavity which is not a true coelom.
  • They may be sedentary or free swimming forms.
  • Body wall consists of 2 layers of cells - Ectoderm/epidermis and Endoderm/gastrodermis. there is a jelly like layer called, Mesoglea, is present in between two layer.
  • They show the phenomena of polymorphism and exist in two basic forms- Polyp and Medusa.

(i) Polyp

a) It is ssesile and asexual zooid.

b) They are approximately cylindrical in shape and elongated at the axis of the body.

c) In solitary polyps, the aboral end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly.

d) The oral end contains the mouth and is surrounded by a circlet of tentacles.

(ii) Medusa

a) It is free swimming and sexual zooid.

b) Medusae vary from bell-shaped to the shape of a thin disc, scarcely convex above and only slightly concave below.

c) The upper or aboral surface is called the ex-umbrellar surface and the lower surface is called the sub-umbrellar surface.

d) The mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin, called the velum.


  • Cnidarians possess nematocysts which serves the function of paralysing the prey by injecting prey .
  • Reproduction is both asexual and sexual. The eggs develops into a cilliated larva known as planula.

Classification of Cnidaria

The cnidarians are mainly classified into three classes:

  • Class Hydrozoa
  • Class Scyphozoa
  • Class Anthozoa


Class Hydrozoa

  • Generally solitary animals, though mostly colonial in polyp stages
  • Have both, polyp and medusa stage dominant in their life cycles
  • Crespedote medusa (having true velum)
  • Gastrovascular cavity present (but not well developed)
  • No stomodaeum, neither nematocyst nor any mesenteries are present in the gastrovascular cavity
  • The sex cells are found in the epidermis
  • Mesoglea is non-cellular

Examples: Hydra, Obelia, Millepora, Physalia, Sertularia


Class Scyphozoa

  • Solitary animals
  • Mostly are marine forms
  • Medusa stage dominant; however polyp stage may be present(in reduced form as a larval stage - scyphistoma larva; or even absent)
  • Acrespedote medusa (no true velum)
  • Mesoglea is cellular and very thick.

Examples: Aurelia, Pilema, Periphylla


Class Anthozoa

  • Marine animals
  • May be solitary or colonial
  • Only polyp stage present; no medusa stage
  • The gastrovascular cavity is well developed.
  • Gastrovascular cavity has mesentries which are muscular in nature beset with nematocysts
  • Gonads are formed in the gastrodermis (in the mesenteries)
  • Mesoglea is cellular
  • Calcareous skeleton may be present
  • Many anthozoans form corals and reefs

Examples: Tubipora, Metridium, Alcyonium, Gorgonia

A Few Examples

1. Hydra

2. Obelia

3. Tubularia

4. Sertularia

5. Millepora

6. Aurelia

7. Metridium

8. Madrepora

9. Tubipora

10. Alcyonium

11. Gorgonia