Phylum Cnidaria (Greek, cnide = nettle; Latin, aria = like).  The members of this phylum are jellyfish, sea anemones and corals and hydroids.  

Cnidarians are characterized by their body forms  (polyp and medusa).  Polyps have a tubular structure with a mouth and tentacles (arm like structures) around the mouth (such as hydra).  Medusa forms have umbrella like bodies with a mouth located in the center of the body and tentacles are hanging from the edge of the body (such as jellyfish).  

The tentacles of cnidarians are equipped with stinging cells.  The stinging cells are called nematocysts, used to capture pray and for defense.