Sea urchinEchinoidea

The mouth is on the bottom, and it can chew stone.

Turn an urchin over and the pale five-part structure in the middle is Aristotle's lantern, named for the description he left of it, which compared the thing to a horn lantern. It is a muscular frame carrying five teeth that meet at a point, and they grow continuously from the top while the tips wear away, layered so that wear keeps the edge sharp rather than blunting it. Urchins use it to scrape algae off rock, and some use it on the rock itself: the common burrowing urchin of the reef flats grinds out a hollow in the limestone and spends its life in a socket it cut to fit.

The spines are not the fixed armor they look like. Each one sits in a ball and socket joint with its own collar of muscle, and the animal aims them. Pass a shadow over a long-spined urchin and the spines swing to point at it before you have finished moving. They are brittle and barbed in many species, which is why they snap off in a wound and stay there.

Between the spines is the part almost nobody notices. The surface is covered in pedicellariae, minute jaws on flexible stalks, usually three-bladed, that close on anything settling on the test: larvae, debris, the tube feet of a hunting sea star. In some species they carry venom glands and are the real defense, the spines being mostly a fence to slow an attacker down long enough for the jaws to reach it.

What the group does to a reef is grazing. Urchins crop the fast algae that would otherwise smother settling coral, and their value shows up clearly when they vanish: a disease that killed nearly every long-spined urchin across the Caribbean was followed by reefs there turning to weed. One local species goes about it while wearing its lunch, holding shell fragments and scraps of algae over itself with its tube feet.