Jellyfish sting: The injection into the skin of venom from the stinging unit (nematocyst) of the jellyfish. The jellyfish tentacles can extend for several feet and are lined with venom-filled cells (nematocysts). One tentacle may fire thousands of nematocysts into the skin on contact. The pain can be severe, particularly in the first hours after an attack, and itching is common. The victim may have weakness, nausea, headache, muscle pain and spasms, tearing and nasal discharge, increased perspiration, changes in pulse rate, and chest pain. Welting may persist for weeks at the site, and scarring may remain. Even dead jellyfish are capable of leaving a painful mark. Those who get serious stings may require oxygen or cardiorespiratory assistance. There is no antivenom for the stings of North American jellyfish, but there is antivenom for the stings of some Australian species.