The horseshoe crab, Limulus polyphemus, is more closely related to chelicerates such as spiders, scorpions, ticks and mites than it is to true crabs and other crustaceans. Horseshoe crabs are considered to be "living fossils" that have evolved little in the past 250 million years. Limulus is an ancient genus which has probably existed since the Silurian period (440 to 410 million years ago), and shows little morphological change from the now extinct genus Paleolimulus that lived about 200 million years ago. Limulus polyphemus is believed to be the closest living relative of trilobites (Shuster 1982).Like all chelicerates, members of the Order Xiphosurida have a two-part body consisting of a prosoma, or head region; and an opisthosoma, or abdominal region. The prosoma contains 6 pairs of legs, all of which bear claws except the last pair. The prosoma also contains 2 types of eyes: 2 compound eyes, or ommatidia, are located on either side of the head; and 2 simple eyes, or ocelli, are located in the center of the head. The opisthosoma contains an additional 6 pairs of appendages which aid in respiration, reproduction, and locomotion. The first pair of abdominal appendages form a genital operculum which houses the genital pores. The remaining 5 pairs of appendages are modified into a series of overlapping plates which function as gills. The underside of each plate is highly folded into leaf-like folds, or lamellae, which provide the actual surface for gas exchange. Due to their morphology, the abdominal plates have become known as book gills. In addition to their respiratory function, the opisthosomal appendages also function as paddles in locomotion. A long spine, called a telson, is located behind the opisthosoma and gives this order its name: Xiphos being Greek for "sword", and uros meaning "tail."