Sea urchins are special. They have spines protruding from their bodies, each with separate ball-and-socket joints, which they use to move and defend themselves. They have rows of tiny holes along the plates of their hard outer shell. I thought they were eyes, but it turns out that urchins haven’t got eyes. The mouth is underneath, in the middle. People catch them in nets and scoop out the insides. I never tried them. They would remove the spines, wash the shells and set them out to dry. The result was their amazing skeletons: purple, green, blue, brown, dark, light, each striped in a certain way. Rough on the outside. Covered with tiny humps aligned in wedge-like segments with alternating rows of holes. Inside it’s even better. A smooth pearlescent surface with all the colours of the rainbow. Fine as lace. If you hold the urchin up to your eye like a spyglass, you see the dome with oculi where light pours in, and the scale changes.