All members of the phylum Cnidaria have stinging cells called cnidocytes. The stinging cells can be ejected by harpoons called nematocysts. Cnidarians are mostly all marine carnivores, feeding mostly on small prey such as small fish and crustaceans. Captured prey are pushed through the mouth into the gastrovascular cavity, where extracellular digestion occurs followed by phagocytosis.
Cnidarians possess radial symmetry and can be divided like a pizza, all slices being equal.
Cnidarians have two basic body plans, the polyp and the medusa, and most cnidarians alternate between these lifecycles, with one body plan being dominant. Phylum scyphozoa are the jellyfish. The jellyfish are characterized by their gelatinous medusa, that dominates their lifecycle. This group includes one of the largest invertebrates in the world, Cyanea capillota . Cyanea can exceed 2 meters in diameter.