No brain, no heart, no bones
How Does a Jellyfish Live With No Brain?
A jellyfish has been drifting through the sea for longer than there have been trees, without a single one of the parts you would call essential.
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Step 1 of 5
It is mostly water.
Take a jellyfish out of the sea and leave it, and there is almost nothing left. About ninety-five parts in a hundred are plain water — the animal is a thin sheet of cells holding a shape.
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Step 2 of 5
A net of nerves does the deciding.
Instead of a brain, a jellyfish has a net of nerves spread evenly through its bell. There is no boss and no centre — a touch anywhere sets off a response nearby, and that is enough.
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Step 3 of 5
It swims by squeezing.
A ring of muscle round the rim pulls the bell shut, pushing water out behind. Then the bell springs back to its own shape by itself, ready to squeeze again.
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Step 4 of 5
Every sting is a loaded spring.
The tentacles are covered in cells holding a coiled harpoon under enormous pressure. Brush against one and it fires in less than a millionth of a second — one of the fastest movements in nature.
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Step 5 of 5
It was here before the trees.
Jellyfish were drifting about hundreds of millions of years before the first tree, the first dinosaur or the first flower. Doing without a brain has worked out extremely well.
A stinging cell keeps working after the jellyfish is dead. That is why a piece washed up on the beach can still sting you.
The short version
A jellyfish gets by with a net of nerves instead of a brain, seawater instead of blood, and water pressure instead of bones.
Try it yourself
Squeeze an open umbrella shut quickly under water in the bath. The push you feel is exactly how a jellyfish swims.
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