Jellyfish aren't actually fish—they're invertebrates, or animals with no backbones. Jellyfish have tiny stinging cells in their tentacles to stun or paralyze their prey before they eat them. Inside their bell-shaped body is an opening that is its mouth.The jellyfish swims by contracting and relaxing a ring of muscles around the bell. The muscles open and close the bell, drawing in water and then forcing it out again to push the jellyfish forward. The lion's mane jellyfish is the biggest jellyfish in the world, with tentacles 118ft (36m) long.Moon jellyfish are saucer-shaped and are up to 16 inches wide in diameter. You can tell the male from the female by its sex glands.

Jellyfish are one big neural network and respond to stimuli. So while they don't feel the same type of pain other animals do, they do know to remove themselves from bad stimuli.They don't have a heart, lungs or a brain either! So how does a jellyfish live without these vital organs? Their skin is so thin that they can absorb oxygen right through it, so they don't need lungs. They don't have any blood so they don't need a heart to pump it.

The jellyfish has six eye clusters. Each contains four very simple eyes consisting of pigment-filled pits to catch light, and a pair of more complex, lensed eyes. At just a tenth of a millimetre across, the lenses are made of material that has variable optical properties.Their eyes can also sense changes in light. All jellyfish species' eye spots give them these abilities – and in some species, like the box jellyfish, their eye spots are very well-developed so they can see and hunt their prey.Throughout their lifecycle, jellyfish take on two different body forms: medusa and polyps. Polyps can reproduce asexually by budding, while medusae spawn eggs and sperm to reproduce sexually.