Jellyfish are animals of the phylum Cnidaria, class Scyphozoa, order Semaeostomeae and family Cyaneidae. The phylum Cnidaria is an incredibly ancient group of invertebrate animals, stretching back hundreds of millions of years. Jellyfish are among the oldest types of multicellular animals. True jellyfish have a number of distinguishing characteristics by which they are classified.
Jellyfish are universally planktonic in their adult stage, meaning that they have only minimal ability to propel themselves through water, and are carried along freely by ocean currents. Jellyfish have minimal sensory capabilities and no nervous systems, with each part responding only to its immediate environment without any coordination with the rest of the organism. The body of a jellyfish is composed of a gel-like substance known as mesoglia, which is about 90 percent water, surrounded by a skin only a single cell in thickness.
Jellyfish have distinctive weapons, known as nematocysts, for hunting prey. These are venom-bearing barbs found in great numbers in their tentacles. These weapons are shared only by jellyfish and related groups, such as sea anemones. Jellyfish use one of the major cnidarian body plans, a free-floating bell with trailing tentacles, known as a medusa, while anemones use the other body plan, a sessile stalk with upward-pointed tentacles, known as a polyp.