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There were many empty shells of these crabs on the beach. These crabs, which can become quite large, live in shallow ocean waters on soft sandy or muddy bottoms. They occasionally come on shore to mate. Behind their legs, they have book gills, which exchange respiratory gases and are also occasionally used for swimming, which is usually done upside down. A true endoskeleton is absent, but the body has an endoskeletal structure with cartilaginous plates that support the book gills.
Huntington Beach State Park