Medusozoa is the scientific name for what are commonly and incorrectly referred to as jellyfish. They are free-swimming carnivorous marine animals consisting of a gelatinous umbrella-shaped bell and trailing tentacles. Medusozoa (or jellies) are found in all ocean waters at all depths, but most commonly in coastal waters. They are the oldest multi-organ animals, dating back more than 500 million years. While they have multiple organs, these organs tend to be, for the most part, relatively primitive. The umbrella mass is covered on the aboral surface by a thin close-fitting layer called the epidermis. Beneath the umbrella mass is another layer of skin, called the gastrodermis. The gastrodermis is attached to the umbrella mass only along the edges. At the center of the gastrodermis is the mouth, where food (primarily plankton, crustaceans, and fish eggs) are ingested. Between the umbrella mass and the gastrodermis is the gastrovascular cavity where digestion takes place. Medusozoa have a loose network of nerves, primarily sensing touch, and some species have primitive light-sensing organs. They move by radially expanding and contracting the umbrella mass, pushing water behind them. Tentacles (over 100 feet long in some species) hang from the edges of the umbrella mass and from the gastrodermis. The tentacles contain stinging structures, called nematocysts, used to pierce the skin and inject venom into prey (or predators). Triggered by physical contact, a lance inside the nematocyst pierces the skin and venom is injected. Different medusozoa have evolved different lances and venom, depending on feeding habits and its particular biosphere. Some of these venoms are particularly painful to humans. The venom of several species can be deadly to humans, causing anaphylaxis (a severe allergic reaction affecting a number of body systems at the same time). Application of a vinegar wash is a common first-aid treatment, followed by careful removal of nematocysts for the affected area and administration of antihistamines. The pain, sometimes excruciating, may last for several hours. Wearing of wetsuits is generally effective in preventing medusozoa attacks.