There are five classes of jellyfish. The true jellyfish or Scyphozoa, also known as a medusa, is a hollow, transparent creature that looks like an inverted bowl or umbrella. There are also four other groups of ocean creatures which have a similar medusa form for all or part of their life cycle: stalked jellyfish, box jellyfish, comb jellies, and hydrozoa.
Scyphozoa, or true jellyfish, are found in all the oceans of the world, both floating close to the surface and swimming in deep waters. Scyphozoa generally have two life stages. In the summer, they are seen floating in the ocean in their medusa form, which may range in size, depending on the species, from two centimeters to two meters across. In the winter, they become bottom-dwelling polyps, which produce new medusae in the spring. Medusae have tentacles which hang down from a central dome and contain nematocysts, the stinging cells that they use to stun their prey. The stings of some species of jellyfish are poisonous to humans.
There are about 50 species of stalked jellyfish or Stauromedusae. They have a trumpet-shaped body with upward pointing tentacles and do not alternate between polyp and medusa life phases.
There are probably 20 species of box jellyfish, or Cubozoa, which live mainly in tropical and sub-tropical oceans. They are cube-shaped, and have a more developed nervous system than other jellyfish. Unlike other jellyfish, they have true eyes, complete with retinas, corneas and lenses. They are almost transparent and umbrella-shaped, can move rapidly and actively hunt small fish. Their tentacles contain cnidocytes which inject venom into their prey. Some species are extremely poisonous and can be fatal to humans.
There are also around 150 species of comb jellies, or Ctenophora, varying in size from one millimeter to one and a half meters in diameter. Although different species exhibit a variety of body shapes, a typical comb jelly is egg-shaped, and has a pair of fringed tentacles covered with sticky cells, known as colloblasts, which it uses to capture its prey. The combs from which it gets its name are rows of cilia, hairlike extensions along their bodies that are used for swimming.
Hydrozoa is a large and diverse group which includes approximately 3,800 species that inhabit both salt and fresh water. While many hydrozoa have a similar life cycle to the true jellyfish, alternating between polyp and medusa forms, not all Hydrozoa have a medusa or jellyfish form, and may spend all their lives as bottom-dwelling polyps. Hydrozoan medusae are very small, and may range in size from half a centimeter to six centimeters in diameter. Their bodies are domed umbrellas to which their tentacles are attached, and most only have four tentacles.
Being stung by a jellyfish is a hazard of ocean swimming, particularly in warmer waters. The stings of most jellyfish will result in nothing more than an irritating rash. However, beach-goers should take careful heed of advisories concerning the presence of more venomous jellyfish species.