What structure do sea urchins use to scrape algae from rocks?

Sea urchins use Aristotle's lantern to scrape algae from rocks.

Aristotle's lantern is a complex jaw and tooth apparatus located on the underside of a sea urchin. It contains five calcareous teeth that protrude through the animal's mouth and are operated by muscles and supporting skeletal elements. The structure allows many sea urchins to graze efficiently on algae coating hard surfaces.

The name comes from Aristotle, who described the apparatus in ancient biological writings and compared its appearance to a lantern. Its presence is especially obvious in regular sea urchins, although related echinoids show modified forms.

A radula performs a broadly similar scraping role in many molluscs, but it is a ribbon-like structure made of microscopic teeth. An operculum is a shell or covering structure, while a siphon moves water or fluid.