Phacops rana can be recognized by its large eyes (which remind some observers of a frog's eyes—the specific name rana is a reference to a common frog), its fairly large size (up to 6 inches long), and its habit of rolling up into a ball like a pill bug ("volvation"). In order to protect themselves from predators, Phacops rana would roll into a ball with its hard exoskeleton on the outside as protection. Many other trilobites possessed the same ability, but Phacops rana nearly perfected it. The slightest amount of sediment would trigger their senses, and Phacops rana would be hidden in a tiny shelter made of its own body. Although this safety feature often helped them to evade predators, occasionally it backfired and the trilobite would be buried under heavy sediment. Their fossils can still be found in balled-up positions 400 million years later.
Phacops rana is found both in the northeastern U.S. and in Morocco; North America was attached to the African plate during the Devonian.
The most striking feature of the morphology of Phacops rana and its relatives is their eyes. These differed from the eyes of most trilobites in having comparatively few lenses spaced between deep sclera. The lenses themselves were very rounded instead of largely flat. The eyes were mounted on turret-like structures which could swivel, providing the animal with an almost 360-degree field of view. This type of eye is known as the schizochroal eye.