a little while back i found the shell of a green sea urchin washed up on secret beach. it was only a couple of centimetres wide. when i got it home i found it was very smelly and had the dried remains of the urchin inside. i gave it a scrub and then dug out the inside debris with the tip of a needle. most of it had rotted away. to my amazement the last little bit came out with a tiny white seed head-like structure attached. excitedly i ran for my very old observer's book of the sea and seashore and looked up urchins. and there on the page was a picture of this amazing little thing - 'aristotle's lantern' (mouth of sea urchin).
it's absolutely fascinating. it's the chewing organ of the urchin and is composed of 50 skeletal elements worked by 60 muscles. and my one is just tiny - only a centimetre tall. you can read more about it on the natural history museum website here.