The Horseshoe Crab, Limulus polyphemus, is often called a living fossil as it is one of the world's oldest surviving organisms. It actually isn't a crab but more closely related to terrestrial scorpions and spiders. Its body is comprised of three main parts: the prosoma - its head region, the opisthosoma - its abdominal region, and the telson - its spine-like tail. It also has five pairs of jointed legs and a pair of pincers. Like some other inverts, it will occasionally molt is exoskeleton. 

 Its long tail helps it steer as it swims and right itself when upended onto its back. In the home aquarium it is best to keep the Horseshoe Crab in a tank with a deep sand bed as they like to burrow and search for food. This makes them great sand aerators. They are effective scavengers that will feed on worms, algae, and other meaty organisms found in the sediment. At its small size, the Horseshoe Crab is reef safe but may become more aggressive as it get larger.