Walking on Harding Beach in Chatham, MA on Cape Cod, I came across a Horseshoe Crab washed up on shore. Horseshoe Crabs are invertebrates, meaning that they lack a spine and they live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. The entire body of the horseshoe crab is protected by a hard carapace. Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, and are also occasionally used for swimming. As in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills. They are more often found on the ocean floor searching for worms and mollusks, which are their main food. They may also feed on crustaceans and even small fish. Pilot Falcon Fountain Pen with Noodlers Bulletproof Black ink in a Stillman and Birn Beta Sketchbook.