Most jellyfishes have two life-stages, a pelagic medusa or jellyfish stage and a bottom-dwelling polyp stage. The medusa stage is usually short and seasonal. In this stage they catch their prey using stinging cells located at their tentacles. Some species however, use their tentacles to filter plankton from the water. The smallest ones have a diameter of 2 cm, while the very familiar lion's mane jellyfish may reach a diameter of 2 meters.
Scyphozoa formerly included the box jellyfishes and the stalked jellyfishes, but these are currently regarded as separate classes. This leaves us with only three orders of Scyphozoa:
- Coronatae, commonly called crown jellyfishes.
- Semaeostomeae, including some of the most frequently encountered jellyfishes in the North-East Atlantic Ocean.
- Rhizostomae is an order of jellyfishes without tentacles or other structures around the bell's margin.