Metrostomata

The Merostomata includes two very different groups of marine life, the eurypterids and the horseshoe crabs. Eurypterids are now extinct. the horseshoe crabs are a very old group, but only 5 species exist today.
Members of this class have a large shield that covers the cephalothorax. The compounded eyes are reduced. The second pair of appendages, the pedipalps, resemble walking legs. They have a long, spike-like stinger called a telson that projects from the back-end of their bodies.