Horseshoe-crab
Guides
Chelicerata
Chelicerates
Chelicerata is a major subphylum of arthropods characterized by the presence of chelicerae—pincer-like or fang-like appendages that are the first pair of structures before the mouth. The group includes horseshoe crabs, sea spiders, and arachnids (spiders, scorpions, mites, ticks, harvestmen, and allies). Chelicerates diverged from other arthropods by the mid-Cambrian period, approximately 508 million years ago. With over 77,000 described living species and estimates suggesting hundreds of thousands more undescribed species, chelicerates represent one of the most diverse animal groups after insects. Most living species are terrestrial air-breathers, though marine lineages persist.
Limulus
Atlantic Horseshoe Crabs
Limulus is a genus of horseshoe crabs in the family Limulidae, containing one extant species, Limulus polyphemus, commonly known as the Atlantic horseshoe crab. The genus also includes at least one confirmed fossil species, Limulus coffini, from the Late Cretaceous of the United States. Several other fossil species have been assigned to Limulus historically, though many have since been reclassified to other genera. The genus is distinguished from other horseshoe crab genera primarily by geographic distribution and subtle morphological features.