Mastigiidae, Ulmaridae, Pelagiidae, Chiropsalmidae, Rhizostomatidae - Jellyfishes

True jellyfishes belong to the phylum Cnidaria, pronounce "ni-dare-ee-a" (the "C" is silent). Their translucent free-floating bodies are usually bowl-shaped, semi-circular or squarish in shape. They are gelatinous and contain about 90% water. The main body is usually called a "bell", "medusa" or "umbrella". Jellyfishes have no brain, heart, bones, eyes, etc. They basically have a mouth and anus, four stomachs, gonads, and a nerve network. From the underside of the bell extends four to eight oral arms or flaps which are used to capture their prey. Tentacles with stinging cells (cnidocysts or neumatocysts) usually extend from the margin of the bell. They swim by opening and closing their bell. The largest jellyfish, the lion's mane, can be up to 2.1 m (7 ft) wide with tentacles as long as 40 m (130 ft). The deadliest jellyfish, the sea wasp, has killed more people than any other marine organism.

Some countries consider jellyfishes a delicacy.