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Context

• Caribbean box jellyfish are barely a centimeter long and have no brain.

Details

• Cnidaria is a phylum under the kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
• Cnidarians -- the animal group that includes jellyfish, sea anemones, and coral -- are brainless, instead getting by with a "dispersed" central nervous system.
• Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey.
• Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick.
• Cnidarians mostly have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes.
• Both forms have a single orifice and body cavity that are used for digestion and respiration.
• Not all cnidarians reproduce sexually, but many species have complex life cycles of asexual polyp stages and sexual medusae stages.
• Cnidarians are distinguished from all other animals by having cnidocytes that fire harpoon-like structures and are usually used mainly to capture prey. In some species, cnidocytes can also be used as anchors.

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