The opisthosoma is the posterior portion of the arachnids body behind the prosoma. The number of segments and appendages on the opisthosoma vary. In general, appendages are absent or reduced, although in horseshoe crabs they persist as large plate-like limbs, called branchiophores, bearing the book gills, and that function in locomotion and gas exchange. In most chelicerates the opisthosomal limbs are greatly reduced and persist only as specialized structures, such as the silk-producing spinnerets of spiders or the pectines of scorpions. Orders See text. ... The cephalothorax is an anatomical term used of arachnid and malacostracan arthropods for the first major body section. ... Binomial name Limulus polyphemus The horseshoe crab (Limulus polyphemus) also known as King Crab, is an arthropod that is more closely related to spiders than crabs. ... A spinneret is a spiders silk spinning organ. ... Superfamilies Pseudochactoidea Buthoidea Chaeriloidea Chactoidea Iuroidea Scorpionoidea See classification for families. ...