Scyphozoa

The Scyphozoa is a class of the phylum Cnidaria, sometimes referred to as the "true jellyfish". There are 200 species. The name Scyphozoa comes from a Greek word which refers to the cup shape of the jellyfish.
The medusa form is the dominant life form, as opposed to the polyp. They eat plankton, small crustaceans and fish larvae, which they capture using stinging cells called nematocysts. The nematocysts are attached to the tentacles that hang down from the edge of the umbrella dome. Scyphozoans have an internal jelly-like material. They have no hard parts, no head, and no specialized organs for respiration or excretion. 
Unlike other types of jellyfish, scyphozoans lack a "velum", a circular membrane which propels other jellyfish through the water. Scyphozoans move through the water by contracting and relaxing the muscles of their umbrella.