Jellyfish use their tentacles to sting. Most are harmless to humans but stings from some species, such as the box jellyfish, can be very painful and sometimes kill. Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. They are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis.
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