Unique stinging devices called nematocysts discharge threads to capture prey and fend off predators. An outer epidermis covers the body, and an inner gastrodermis lines the digestive cavity. A nerve net running through both layers coordinates the animal's response to stimuli. Some jellyfishes also have sensory cells and contractile cells. A jelly-like mesoglea lies between the outer and inner body layers.
Jellyfishes have abundant mesoglea helpful in providing buoyancy and in swimming.
Polyps have little mesoglea but use the water in their guts as a hydrostatic skeleton. Physalia consists of a collection of polyps and medusae specialized for feeding, defense, and reproduction. The life cycle of a cnidarian may have a polyp and medusa stage, or it may have just the polyp. The medusa is usually the sexual form with either epidermal or gastrodermal gonads. The zygote develops into a swimming larva called a planula.