Phylum Coelenterata (Cnidaria)
- The name Cnidaria is derived from the cnidoblasts or cnidocytes , which contain the stinging capsules or nematocytes present on the tentacles and the body.
- Cnidarians exhibit tissue level of organisation and are diploblastic, digestion is extracellular and intracellular, and have a central gastro-vascular cavity with a single opening, hypostome.
- Cnidarians like corals have a skeleton composed of calcium carbonate and exhibit two basic body forms called polyp and medusa, polyp is sessile and cylindrical form like Hydra, whereas, the latter is umbrella-shaped and free-swimming like Aurelia.
- Cnidarian which exist in both polyp and medusa forms exhibit alternation of generation called as
- Polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
Examples- Physalia (Portuguese man-of-war), Adamsia (Sea anemone).
Fig. Sea anemone