Give general characteristics of phylum Coelenterata (Cnidaria).

(N/A) $\rightarrow$ There are about $9000$ species of Cnidarians.
$\rightarrow$ The name Cnidaria (Knide-nettle or sting cells) is derived from the stinging cells or Cnidoblasts present on the ectoderm of tentacles and the body of these animals.
$\rightarrow$ Cnidoblasts are used for anchorage, defense, and for the capture of prey.
$\rightarrow$ Cnidarians exhibit tissue level of organization and are diploblastic.
$\rightarrow$ They have a central gastro-vascular cavity with a single opening, the hypostome.
$\rightarrow$ Digestion is extracellular and intracellular. Some of the cnidarians, e.g., corals, have a skeleton composed of calcium carbonate $(CaCO_{3})$.
$\rightarrow$ Cnidarians exhibit two basic body forms called polyp and medusa.
$\rightarrow$ Polyp is a sessile and cylindrical form like Hydra, Adamsia, etc., whereas the medusa is umbrella-shaped and free-swimming like Aurelia or jelly fish.
$\rightarrow$ Those cnidarians which exist in both forms exhibit alternation of generation (Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
$\rightarrow$ Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan), Meandrina (Brain coral), Aurelia (Jelly fish), Obelia (Sea fur).