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. 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. Some species are extremely poisonous and can be fatal to humans. Their tentacles contain cnidocytes which inject venom into their prey. They are almost transparent and umbrella-shaped, can move rapidly and actively hunt small fish. Unlike other jellyfish, they have true eyes, complete with retinas, corneas and lenses. They are cube-shaped, and have a more developed nervous system than other jellyfish. There are probably 20 species of box jellyfish, or Cubozoa, which live mainly in tropical and sub-tropical oceans. They have a trumpet-shaped body with upward pointing tentacles and do not alternate between polyp and medusa life phases. There are about 50 species of stalked jellyfish or Stauromedusae. The stings of some species of jellyfish are poisonous to humans.
Medusae have tentacles which hang down from a central dome and contain nematocysts, the stinging cells that they use to stun their prey. In the winter, they become bottom-dwelling polyps, which produce new medusae in the spring. 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. Scyphozoa generally have two life stages. Scyphozoa, or true jellyfish, are found in all the oceans of the world, both floating close to the surface and swimming in deep waters. 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. The true jellyfish or Scyphozoa, also known as a medusa, is a hollow, transparent creature that looks like an inverted bowl or umbrella.
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