Merostomata - horseshoe crabs

There are 5 living species in this order. The horseshoe crabs you see today are little changed from the horseshoe crab you might have seen 250 million years ago. Horseshoe crabs have three body segments, the prosoma, the opisthosoma, and the telson . The prosoma is the horseshoe-shaped front part of the horseshoe crab's body. The horseshoe crab has ten eyes on the top of the prosoma and six pairs of appendages on the underside of the prosoma. Five pairs of appendages are for walking and one pair of chelicera help the horseshoe crab move food into its mouth. The opisthosoma or abdomen connects to the prosoma with a hinge. The telson or tail is attached to the opisthosoma. Some people think that the telson is a stinger, but it is not. The horseshoe crab uses the telson to steer itself and to flip itself over if it gets turned on its back!