The urchin's jaw apparatus, known as Aristotle's lantern, carries five calcite teeth arranged around the mouth on the underside. The teeth are built in layers that wear unevenly, so the biting edge is continually resharpened as it abrades, and they grow continuously from the base to replace what is lost.
Applied to rock, this hardware carves a depression that shelters the animal from surge and from predators. Excavation is slow and depends on rock hardness, being far quicker in sandstone than in granite. On some coasts the cumulative effect of many urchins is a measurable contribution to shoreline erosion.
Urchins are often described as simply eating kelp, but their grazing role is more consequential than that: where predators are absent, dense urchin aggregations can strip kelp forest to bare rock. What is often misreported is the idea that the pits are dissolved chemically. The evidence points to mechanical abrasion by teeth and spines.