Give general characteristics of phylum coelenterata/cnidaria. 

Explanation

There are about 9000 species of Cnidarians.
The name cnidaria (Knide-nettle or sting cells) is derived from the stinging cell or Cnidoblasts present on the ectoderm of tentacles and body of these animals.
Cnidoblasts 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, hypostome.
Digestion is extracellular and intracellular. Some of the cnidarians, e.g., corals have skeleton composed of calcium carbonate `(CaCO_(3))`.
Cnidarians exhibit two basic body forms called polyp and medusa.
Polyp is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas, the medusae is umbrella shaped and free-swimming like Aurelia or jelly fish.
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).
Examples : Physalia (Portuguese man-of-war).
Adamsia (Sea anemone).
Pennatula (Sea-pen).
Gorgonia (Sea-fan)
Meandrina (Brain coral)
Aurelia (Jelly fish)
Obelia (sea fur)

Examples of Coelenterata indicating outline of their body