ANIMAL KINGDOM Part-8, NCERT Class-11

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Phylum-Coelenterata(Cnideria)

• Aquatic, Mostly marine, Sessile or free swimming 
• Radially symmetrical animals
• Cnideria name is derived from Cnidoblast or Cnidocytes 
• Cnidoblast cells containStinging capsule or NEMATOCYST 
   Structure of Nematocyst cell
• Cnidoblast cells are present on tentacles and body and used for anchorage, defense, prey capturing.
• Cniderians have a central gastro-vascular cavity with a single opening.
• Mouth in Cniderians is present of HYPOSTOME.
• Extracellular and Intracellular digestion.
• Some Cniderian live Coral have a skeleton composed of calcium carbonate (CaCO3)
• Two basic body form is shown named polyp and medusa.
• Polyp stage is Sessile, Cylindrical
e.g. Hydra, Adamsia etc.
• Medusa stage is umbrella-shaped and free-swimming 
e.g. Jelly Fish (Aurelia)
• Cniderans may or may not posess both of the stages in their life cycle. If both forms are present in their life cycle, then they exhibit alteration of generation or METAGENESIS.
In this process, polyp asexually produce medusa and Medusa sexually  produce polyp.
e.g. Obelia

Example:
• Physalia (Postuguese-man-of-war)
• Adamsia (Sea anemone)
• Pennatula (Sea-pen)
• Gorgonia (Sea-fan)
• Meandrina (Brain-coral)

Example of Cnideria in Medusa phase : Aurelia

Example of Cnideria in Polyp phase : Adamsia

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