Cnidarians are classified into four main groups: the almost wholly sessile Anthozoa ( sea anemones , corals , sea pens ); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa , a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War .