Description:
Scyphozoa is a class in the Phylum Cnidaria that is commonly known as the true jellyfish. All true jellies live in the ocean, unlike some other animals commonly referred to as jellyfish. They are almost exclusively carnivorous, and a few filter plankton from the water. It is difficult to trace the origins of Scyphozoa because they can consist of up to 98% water, but it is known that they have existed since at least the Cambrian era. They contain nematocysts (stinging cells) and because of this are generally unwelcome at beaches. Stalked jellies and box jellies are not considered to be in this Class.
Physical Features:
Jellies in their medusa form have a usually transparent bell at the top which contains most of the organs with long tentacles extending out from it. These tentacles contain the nematocysts that give the jellies their famous sting. In addition to tentacles, jellies have oral arms that look like kelp leaves that also have stinging cells. The tissue of the jellyfish is made of mesoglea, a substance that appears jelly-like, giving the jelly its name. Scyphozoans also have a polyp form that is bottom dwelling. Scyphozoans actually stay longer in polyp form than in medusa form.