Scyphozoans are true jellyfish. They also have long tentacles that are covered in stinging cells. The Scyphozoa is what most would consider when they think of a jellyfish.They range in size from a tiny twelve millimeters to an enormous two meters (or more!) across. The largest jellyfish is the Cyanea Arctica, which can have tentacles that reach over 40 meters long. Despite their often enormous size, jellyfish lack a head, skeleton, and special organs for respiration and excretion. Their life cycle involves an alternation between sessile polyp and a free-swimming medusa stage.