Horseshoe crabs are an arthropod more closely related to spiders and scorpions than crabs and lobsters. They have a three part body: prosoma (head), opisthosoma (heavy shell with legs under it) and the telson (tail). This amazing body structure has been unchanged for over 200 million years. Interestingly enough, this is this Beach Chair Scientist’s favorite animal and there have been numerous posts about Limulus polyphemus. Read more here.