The Scyphozoa or "true jellyfish" is a group class within the phylum Cnidaria. There are about 175 species worldwide. Most Scyphozoans are free-swimming marine animals living in the open ocean, however some small species are planktonic, and a few species that are filter feeders, using the tentacles to strain plankton from the water live attached to the ocean floor. Many species live solitary lives others are found in shoals of hundreds to thousands of individuals stretching for dozens of kilometres.
Other similar animals are classified in the Hydrozoa and Cubozoa, two other groups of cnidarians.
Jellyfish range in size from a twelve millimetres to more than two metres in diameter. The largest is Cyanea arctica, which can have tentacles over 40 metres long. Jellyfish have no head or skeleton, and no special organs for respiration or excretion. However, a ring of muscle fibres is present within the mesoglea around the rim of the dome, and the jellyfish swims by alternately contracting and relaxing these muscles. The periodic contracting and relaxing propels the jellyfish through the water, allowing it to escape predation or catch its prey. Marine jellyfish can consist of as much as 99% water
Most species of Scyphozoa have two phases in their life cycle - the planktonic medusa or jellyfish form usually seen in the warm summer months, and an inconspicuous, but longer-lived bottom-dwelling polyp, which seasonally produce new medusae.
In the medusa phase they eat a wide range of crustaceans and fish, which they capture using their nematocysts. The nematocysts are located throughout the tentacles that spread downward from the edge of the umbrella dome, and also cover the four or eight oral arms that hang down from the central mouth.
Food and waste must be passed in and out through the same opening, since like other Cnidarians, they have no digestive tract. Cilia within the digestive cavity transport water, food and gases around.