Phylum Coelenterata (Cnidaria) They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals The name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocysts) present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and for the capture of prey Cnidarians exhibit tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, mouth on hypostome. Digestion is extracellular and intracellular.

Phylum Colenterata has been divided into three classes: Hydrozoa, Scyphozoa and Anthozoa.
Example of Coelenterata | Phylum Cnidaria |


Only polyps (Hydra) or both asexual polyps and sexual medusae present (Obelia).

1. Class Hydrozoa: Polyp is pre dominant, but the life cycle shows both polyp and medusa stages.

Examples of Hydrozoa
  • Hydra
  • Obelia (Sea fur)
  • Physalia (Portuguese man of war)
  • Velella (Sail by wind)
2. Class Scyphozoa: Medusoid stage dominant. Polypoid stage reduced or absent.

Example of Scyphozoa
  • Aurelia (Jelly fish)
  • Periphylla
  • Rhizostoma

3. Class Anthozoa (Actinozoa): Only polypoid stage. Medusoid stage absent.

Examples of Anthozoa:
  • Gorgonia (Sea fan)
  • Metridium (Sea anemone)
  • Alcyonium (Dead man’s finger)
  • Fungia (Mushroom coral)
  • Pennatula (Sea pen or Sea feather)
  • Corallium (Red coral)
  • Astraea (Star coral)
  • Madrepora (Stag horn coral)
  • Tabipora (Organ pipe coral)
  • Meandrina (Brain coral)
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