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An animal built the wrong way round

How Does a Starfish Grow a New Arm?

Cut an arm off a starfish and it grows back — and in some kinds, the arm grows a whole new starfish.

No brain and no bloodHundreds of tiny feetunderneathSeawater pumped roundinstead of bloodArms that grow back
Almost nothing about it is built the way you are.

  1. Step 1 of 5

    It walks on hundreds of little feet.

    Five arms, movingslowly Hundreds of tube feetunderneath

    Turn a starfish over and every arm has grooves packed with tiny tube feet. Each one is a small water balloon with a sucker on the end, stretched out and pulled back by water pressure.

  2. Step 2 of 5

    Seawater does the job of blood.

    You A heart pumping bloodA starfish A canal of seawaterinstead

    There is no heart and no blood. A starfish pumps filtered seawater through a set of canals, and that water both moves the feet and carries oxygen around the body.

  3. Step 3 of 5

    There is no front and no brain.

    One arm starts to lead The othersfollow it A different arm takes over No front,ever

    A starfish has no head, so it has no front. A ring of nerves runs round the middle with a branch down each arm, and whichever arm decides to lead becomes the front for a while.

  4. Step 4 of 5

    A lost arm grows back.

    1 An arm is tornoff2 The wound sealsover3 A small budgrows into afull arm

    Lose an arm to a predator and the wound seals, then a small bud appears and grows over months into a full-sized arm. It is slow, but it costs a starfish far less than being eaten.

  5. Step 5 of 5

    It pushes its stomach out to eat.

    1 Grips a shelland pulls2 Slides itsstomach outinto the gap3 Draws thedigested mealback in

    A starfish cannot swallow a mussel, so it does the opposite. It pulls the shell open a crack, slides its own stomach out through its mouth and into the gap, and digests the meal where it lies.

    In some kinds, a severed arm with a piece of the middle attached will grow four new arms and become a whole second starfish.

The short version

A starfish runs on pumped seawater, has no brain or front, and can regrow an arm it has lost.

Try it yourself

Wet a rubber sucker and press it to a tile. The grip you feel is one tube foot — a starfish has hundreds working together.

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