Hemi was ten years old, and low water at the harbour wall lasts about forty minutes. He had a rule that he had made himself. You look in every pool before you touch the wall, because the pools keep their water long after the sea has gone down.

The pools were full of shells. Empty whelks and periwinkles, washed up along the edges. He lined five of them up on a flat stone in order of size. Then he measured the mouth of each one with the wooden ruler he kept in his boot. Six millimetres. Eight. Ten. Twelve. Fourteen. Then he put his hand into the pool, and something walked into his hand.

It was small and brown and it went sideways, and it was carrying a house. Hemi stood very still. The house was a shell, a proper spiral shell, and the animal was walking about inside it and dragging it behind. It went about two of its own body lengths, then stopped and gripped the shell with its big claw.

He knew the blue shore crab that lives under the stones of the harbour wall. He had caught one in a bait bucket. It had a hard blue shell all over its back, wider than it was long. It walked on eight legs, with two claws in front. He wanted to take it home and show his sister. His mother had said at breakfast that a bucket is not a sea. He could borrow the animal until the tide turned, and not one minute past it. So he picked the shell up.

The animal came out. There was no hard shell under the shell. There was a soft brown body, narrow and long, with a hard little shield at the front and nothing at the back at all. Two claws, and the left one was much bigger than the right. Four legs that walked. Four more, small, folded up under the tail like fingers. He counted them again. Two, four, four. Ten.

The blue shore crab in the bucket had ten too. Its shell went right over the top of all ten, and past the back of them. This one wore a borrowed shell on its front end, and it did not like being held. He put it back in the pool quickly. It went down into the deep end and sat there with its claws over the opening.

The ten millimetre shell was lying on the stones with silt in its mouth. The animal had to get all the way in, claws first, and keep its soft back covered the whole time. A mouth too tight and it cannot get in at all. A mouth too wide and it goes right down inside and cannot come back out.

Hemi sat on the wall with his ruler and waited. When it came up again it was wearing the eight millimetre shell. He moved the shells along the stone. He put the eight where the twelve had been, and put the twelve at the end. It went to the ten and tried it. It turned around inside twice, and came out, and went back to the eight. But it kept looking at the twelve.

So he put the twelve in the water, just under the surface, and let go of it. The animal went in. It turned around twice. Then it came out and stood in the eight again with its claws across the mouth. After a while it went to look at the twelve one more time.

At twenty minutes to turn he had five shells and one animal. Then he had four shells and one animal. The eight millimetre shell was drifting along the bottom of the pool, empty, with a bubble of air turning inside it. He wrote his list on his arm in pencil. Six, eight, ten, twelve, fourteen. Then he counted the animals in the pool before he left. Six in the open, and one under the weed, so seven. He counted them again from the top of the wall and got seven.

The water had gone over the rim of the top pool by then, so what he counted was not what was in the pool. It was what he could still see. Then the one he had been watching walked into his hand, wearing the fourteen. His biggest shell was never the one it wanted. He left the row of shells on the flat stone for the next animal and went up the ramp in the rain. At the top he read his arm again.

Eight, ten, twelve, fourteen. And then sixteen. Sixteen was not a number he had. He had not gone looking for it. He put the pencil in his pocket and went home.