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. 