Friday, October 23, 2020

Animal Kingdom - Notes | Class 11 | Part 3: Cnidaria

 4. ANIMAL KINGDOM

 PHYLUM CNIDARIA (COELENTERATA): GENERAL CHARACTERS

  1. Grades of organization:
    • Tissue grade.
  2. Symmetry:
    • Radial symmetry.
  3. Germ layers:
    • Diploblastic
  4. Coelom:
    • Acoelomate
  5. Habit and habitat:       
    • Aquatic (mostly marine).
    • Sessile/free swimming.
    • Solitary/colonial.
  6. Digestive system:          
    • Incomplete.
    • Intracellular & extracellular digestion.
  7. Respiratory system:
    • Absent
  8. Circulatory system:
    • Absent
  9. Reproduction:
    • Polyp reproduces asexually (budding) and medusa sexually.
    • Most are separate sexes.
    • External fertilization.
    • Development is indirect.
  10. Unique features:
    • Tentacles with cnidoblasts.
    • Tentacles: Finger-like structures which surrounds the mouth of coelenterates. Used for food capture & defense.
    • Cnidoblasts (Cnidocytes): These are stinging cells (present on the tentacles and the body) with a poison-filled capsule called nematocyst. Cnidoblast is used for anchorage, defense and to capture prey.
    • Gastro-vascular cavity (coelenteron) with an opening (mouth) on hypostome.
    • Polyp & Medusa: 2 types of body forms in cnidarians.
    • Polyp is cylindrical sessile form. E.g. Hydra, Adamsia.
    • Medusa is umbrella like, free-swimming form. E.g. Aurelia (Jelly fish).
    • Some shows alternation of generation.
    • Alternation of generation (Metagenesis): The phenomenon in which polyps produce medusae asexually and medusae form the polyps sexually. E.g. Obelia.
    • Corals have skeleton (CaCO3).
Examples
    • Hydra
    • Obelia
    • Aurelia
    • Physalia (Portuguese man of war)
    • Adamsia (Sea-anemone)
    • Pennatula (Sea pen)
    • Gorgonia (Sea fan)
    • Meandrina (Brain coral).

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