Cnidarians: Structure & Life Cycles — NEET Biology
📚 Practice ConceptCnidoblasts 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. Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate. Cnidarians exhibit two basic body forms called polyp and medusa. The former is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, the latter is umbrella-shaped and free-swimming like Aurelia or jelly fish. Those cnidarians which exist in both forms exhibit alternation of generations (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
Which of the following statements accurately describe the skeletal or body support structures found in various animal phyla as per NCERT? S1: The body of sponges is supported by an endoskeleton made of spicules or proteinaceous spongin fibres. S2: Corals, a type of cnidarian, are known to have a skeleton primarily composed of calcium carbonate. S3: The defining characteristic of Arthropods is their tough, chitinous exoskeleton. S4: Echinoderms possess an endoskeleton made of calcareous ossicles, which gives them their spiny body. S5: Molluscs universally possess an external calcareous shell as their primary body support structure.
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