Found on the shore in Kiawah, South Carolina
male Atlantic Horseshoe Crab (Limulus polyphemus) on the beach at Assateague Island National Seashore, MD, 11/23/10; note the book gills on the opisthosoma [abdomen] between the tail and legs. The mouth is the dark opening between the rear legs. The chelicerae are the two small pincer-appendages between the front legs. The front legs are different from the next three pairs, indicating a male. Not a real crab, or even a crustacean, it's more closely related to spiders and arachnids, but has its own Class, the Merostomata. The genus dates to the Triassic, roughly 230 mya +/- 18 m (think early Age of Dinosaurs). Photo number: D071815 (uncropped, but resolution is reduced for faster web use; 10 MP original available by request).
Outer banks, NC
Merostomata is the name given to a grouping of the extinct Eurypterida (sea scorpions) and the Xiphosura (horseshoe crabs). The term was originally used by James Dwight Dana to refer to Xiphosura only, but was emended by Henry Woodward to cover both groups.