The largest and most familiar crustaceans belong to Decapoda. The 10,000 species of crabs, shrimps, crayfishes, lobsters, and their relatives are decapods. The first three segments of the decapod thorax are fused with the head to form a cephalothorax and their appendages are maxillipeds. The remaining five pairs of thoracic appendages bear simple or chelate walking legs.