Phylum – Coelenterata (Cnidaria)

The name Cnidaria- is derived from the word cnidoblast or cnidocytes which contains stinging capsules or nematocytes. This phylum is a group of aquatic or marine organisms. The members of Coelenterata are found in colonies or solitary. Over 11,000 species are found.

Chareceristic feature of coelentrates: 
  • Habitats: They are aquatic, mostly marine, Free-living, or maybe sessile.
  • Body symmetry: These are radially symmetrical animals.
  • Level of organization: They exhibit a tissue level of organization.
  • Diploblastic i.e. the cell found in two embryonic layers as external ectoderm and internal endoderm, undifferentiated mesoglea found between ectoderm and endoderm.
  • Uses of cnidocytes: Cnidocytes present on the tentacles and body, used for anchorage, defense, and prey capture.
  • Digestion: In Coelenterates the digestion is found extracellular and intracellular. A central gastro-vascular cavity having a single opening, the mouth is found on the hypostome.
  • Excretion and respiration through simple diffusion.
  • Skeleton: In some cnidarians like corals, the skeleton is composed of calcium carbonate.
Body forms in Cnidarians

The Cnidarians exhibit two body forms (polymorphisms):

    1. Polyp– Nutritive zooids, are in a cylindrical form and sessile asexual stage.
      • Example: Hydra and Adamsia are examples of this category.
    2. Medusa– Sexual zooid, these are umbrella-shaped and free-swimming.
      • Example- Aurelia or jellyfish.
    Medusa and polyp in aurelia and admasia

    Medusa and polyp in aurelia and admasia


    • Reproduction is mostly asexual through budding, in a few members sexual reproduction is seen.

    Alternation of generation: Some cnidarians exist in both forms, alternation of generation (metagenesis) is found in them.

    • Metagenesis– Polyp produces medusae by asexual reproduction and medusae produce polyps by sexual reproduction. e.g. Obelia

    Holoblastic cleavage. Development is direct or indirect through planula larva (Obelia) or Scyphistoma and ephyra larva (Aurelia)


    Classification of animals- Polyp Medusa - Polymorphism in Cnidaria

    Polymorphism in Cnidaria

    Scientific Classification of Coelenterata

    Kingdom Animalia
    Subkingdom Eumetazoa
    Phylum Coelenterata
    Classification in Coelenterata
    1. Anthozoa
    2. Hydrozoa
    3. Scyphozoa

    Features of Anthozoa:

    • The members are exclusively marine
    • Mesoglea contains fibrous connective tissues and amoeboid cells
    • Medusa stage is not present.
    • Example: Metridium, Xenia, Telesto

      Features of Hydrozoa:

      • Hydrozoa is mostly marine, Few freshwater animals.
      • some are solitary and some colonial
      • The asexual polyp is dominant
      • Medusa possesses true velum
      • Mesoglea is acellualr
      • Example: Obelia, Hydra, Physalia

      Features of Scyphozoa:

      • Scyphozoa are exclusively marine
      • Medusa is dominant
      • Polyp absent
      • Cellular mesoglea
      • Example: Aurelia (jellyfish), Rhizostomata

      Some examples of cnidarians: Physalia (Portuguese man-of-war), Pennatula (Sea-pen), Admasia (Sea anemone), Pennatula (Sea-pen), Meandrina (Brain coral), and Gorgonia (Sea-fan).


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