Sea urchins, parrot fish, and limpets and chitons (both mollusks) feed on coralline algae. They also feed on chitons, limpets, snails, barnacles, echinoids, and even decapod crustacea. "O. bimaculatus" is known to prey on crustaceans, snails, chitons, limpets, and bivalves. Morphologically, many mollusks (such as limpets and chitons) have low-profile, hydrodynamic shells. Amphipods and planarians exist under rocks, along with various algaes, chitons, and some gastropods. This discipline also hosts a smaller global collection of dried gastropods, chitons, and bivalves. Animals which prey on chitons include humans, seagulls, sea stars, crabs, lobsters and fish. External bioeroders include sea urchins (such as "Diadema") and chitons. Shells of chitons are made up of eight overlapping calcareous valves, surrounded by a girdle. These predatory chitons have enlarged anterior girdles. They are dressed in chitons and cloaks and have brightly colored veils on their head. Native Americans of the Pacific coasts of North America eat chitons. It includes all the living and extinct genera of chitons. However, chitons lack a cerebral ganglion. The same is true of many chitons.