Opisthosoma

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The opisthosoma is the posterior portion of the arachnids body behind the prosoma (cephalothorax). 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.