Any of numerous marine coelenterates of the class Scyphozoa, some poisonous species of which notably the Portuguese man-of-war, produce a toxin that can be injected into the skin by nematocysts on the tentacles, causing linear wheals.

These fish eating creatures drift on the ocean currents using their poisonous tentacles to catch their pray, although some do live in fresh water. They come in a variety of sizes from about the size of a peanut to more than 7 feet across and 100 feet long! and there are an estimated 2000 species. These guys have no brain, they use their nervous system to detect light and smell. Most of them are very fragile as they are made up of around 95 % water, although they have no brain and are fragile some of them are very dangerous due to the poison they inject via their tentacles. The poison can cause anything from slight discomfort to fatal body complications like heart attacks or suffocation in humans. If you are stung by a jellyfish the best thing to apply to the sting is vinegar as this will neutralize the stinging effect, it is possible to be stung by dead jellyfish that are washed up on the beach or even a severed tentacle, so be careful and always treat jellyfish with respect. Jellyfish populate most of our oceans and live in a variety of waters from shallow coastal waters to very deep ocean waters. Although they usually drift the ocean currents the can also propel themselves through the water by sucking water into there bell shaped frame and then expelling it like a jet.