Often found under large rocks or in small depressions, the purple sea urchin (Fig. 4-17) is one of the most common inhabitants on the rocky shore. Its abundance is increased on exposed rock surfaces along the open coast. They are vegetarians that feed upon particulate matter by grazing with a complex series of mouthparts called Aristotle's Lantern. In many areas, the urchins are collected for their gonads which many people consider to be a delicacy. This practice has severely depleted their numbers in some areas.