Uncommon for chelicerates, horseshoe crabs have two compound eyes. The main function of these compound eyes is to find a mate. In addition, they have two median eyes, two rudimentary lateral eyes, and an endoparietal eye on their carapace and two ventral eyes located on the underside by the mouth. Scientists believe the two ventral eyes aid in the orientation of the horseshoe crab when swimming. Each individual has six pairs of appendages. The first pair, the chelicerae, are relatively small and placed in front of the mouth. They are used to place food in it. The remaining five pairs of legs are placed on either side of the mouth and are used for walking/pushing. These are the pedipalps (first pair) and the pusher legs (remaining four pairs). Most of the appendages have straight, scissor-like claws, but in males the first and second pair of walking legs have strongly hooked "scissors", which are used for grasping the female during mating. Located behind their legs are book gills. These gills are used for propulsion to swim and to exchange respiratory gases.