Jellyfish, Jelly Fish, Jellies – Jellyfish belong to the phylum Cnidaria in the classes Hydrozoa and Scyphozoa, though the term jellyfish is commonly applied to other aquatic jelly-like creatures. Jellyfish range from about one millimeter in bell height and diameter to nearly two meters in bell height and diameter. Of an estimated 1,500 to 2,000 species in the world, British Columbia has about 75.
Jellyfish have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of their body. Some species of jellyfish actively swim most of the time, while others are mostly passive.
Jellyfish are marine animals consisting of a gelatinous umbrella-shaped bell and trailing tentacles tipped with one time use stingers called nematocysts or cnidocysts that can deliver a painful sting. Each of the thousands, perhaps millions, of nematocysts consists of a coiled, microscopic thread that is often tipped with a barb which shoots out like a harpoon in response to chemical or physical stimuli.