Crustaceans have hard outer shells (exoskeletons), which help them to adapt to life on the rocky shore. The shell protects them against the waves as well as predators, such as birds or sharks.
The exoskeleton, which is made mostly out of calcium and the protein chitin, offers protection for the crustaceans. However, it also provides lots of support for the attachment of muscles. The crustacean is able to freely move its body with the segmented exoskeleton, which creates joints for ease of movement.