This image shows a small preserved specimen of the horseshoe crab Limulus polyphemus.  These members of the Class Merostomata are entirely marine and are commonly found in the shallow waters of eastern North America from Canada to Mexico.  Note that the body is divided into two tagmata, an anterior cephalothorax covered by a tough, leathery carapace and a posterior abdomen that terminates in a spine-like telson.   Horseshoe crabs are "living fossils" that have changed little over the last  200 million years!