Jellyfish can vary greatly in size and length. Some jellyfish are hardly millimetres across, but the longest one, the Arctic lion's mane jellyfish, has tentacles that can grow up to 100 feet in length.
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The smallest jellyfish are the peculiar creeping jellyfish in the genera Staurocladia and Eleutheria, which have bell disks from 0.5 mm to a few mm diameter.