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

Phylum Cnidaria (Coelenterata): General Characters

• Grades of Organization: Tissue grade.
• Symmetry: Radial symmetry.
• Germ Layers: Diploblastic
• Coelom: Acoelomate
• Habit and Habitat:
• Aquatic (mostly marine).
• Sessile/free swimming.
• Solitary/colonial.
• Digestive System:
• Incomplete.
• Intracellular & extracellular digestion.
• Respiratory System: Absent
• Circulatory System: Absent
• Reproduction:
• Polyp reproduces asexually (budding) and medusa sexually.
• Most are separate sexes.
• External fertilization.
• Development is indirect.


• Unique Features:
• Tentacles with cnidoblasts.
• Tentacles: Finger-like structures which surround 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.



Cnidoblast

• 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 show alternation of generation (Metagenesis). It is 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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1. Basis of Classification 2. Porifera 3. Cnidaria 4. Ctenophora 5. Platyhelminthes 6. Aschelminthes 7. Annelida 8. Arthropoda 9. Mollusca 10. Echinodermata 11. Hemichordata 12. Chordata - up to Sub phylum Vertebrata 13. Chordata - Cyclostomata, Pisces 14. Chordata - Tetrapoda

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