The horseshoe crabs are a successful family of widespread merostomate arthropods from the Late Ordovician-Holocene of the entire world. They were named in 1819 by William Elford Leach. They are one of the few animals to be called "living fossils" (due to surviving up until now), and are the closest living relatives of the eurypterids.
Horseshoe crabs are unique arthropods. They have a large, rounded cephalothorax, 4 pairs of walking legs (each leg ending in a claw), 2 small arms, a large abdomen with book gills under it, small eyes, and a long, spear-like telson used for flipping themselves back on their feet. Their bodies are covered in a hard, chitinous exoskeleton.
Horseshoe crabs are predators, preying mainly on worms and mollusks, but also snacking on small crustaceans and fish. The claws on their legs are used for catching and crushing prey, as well as passing them into its mouth.
Horseshoe crabs have an interesting lifestyle. Whenever they contract a disease, they coagulate all of the blood in their body, suffocating the disease. As well as this, they are capable of swimming (in an upside down position) using their legs, telson, and book gills, and whenever they find themselves helpless, they use their long telson to flip themselves back into their regular position. However, the most interesting thing about them is that they regularly walk on the seafloor, but during mating season, they crawl out of the sea to mate and lay eggs; often times, this may help scientists understand why ancient arthropods (possibly eurypterids) took their first steps onto land during the Silurian.