Most urchins are grazers, scraping animal and plant material off surfaces as they move slowly along. Algae, bryozoans, sponges, and even sea cucumbers may be food to an urchin. A complex jaw apparatus called the Aristotle's lantern allows grazing urchins to eat just about anything. The lantern contains five teeth which are used for chewing, but can also be pushed out through the mouth for scraping. Feeding by urchins makes a star-shaped imprint from the lantern. When fossilized, the imprint leaves a record of urchin activity that can be interpreted by paleontologists. Some urchins, such as sand dollars and heart urchins, have evolved to burrow. While grazing urchins tend to have sharp spines that protect them from predators as they move along the seabed, burrowing urchins do not. Some burrowers lack the Aristotle's lantern, but others use the lantern and strong teeth to crush and ingest sand grains.

Echinoderms stand out as the only organisms on Earth with five arms or other elements spaced evenly around a central point. This pentaradial symmetry (penta = “five;” radial = “around a central point") is obvious in some echinoderms, such as sea stars or brittle stars that have five arms. It’s more subtle on others, such as the five rows of tube feet on a sea cucumber or five grooves on a sand dollar. This unusual symmetry of adult echinoderms is not found in juveniles. In fact, echinoderm larvae have two-sided (bilateral) symmetry like humans, and must undergo a metamorphosis to become pentaradial adults. Body plans of animals during their development often say something about their ancestry. The bilateral bodies of developing echinoderms are a reminder that, aside from other back-boned animals (vertebrates), echinoderms may be our closest relatives. Unlike most living echinoderms, many fossil echinoderms did not have pentaradial symmetry.