The opisthosoma's the posterior portion of the arachnids body behind the prosoma (cephalothorax). The number of segments and appendages on the opisthosoma vary. Scorpions have 13, but the first's only seen during its embryological development. Other arachnids have twelve or less. In general, appendages are absent or reduced, although in horseshoe crabs they persist as large plate-like limbs, called opercula or branchiophores, bearing the book gills, and that function in locomotion and gas excha… (More on Opisthosoma)