What is "opisthosoma"

n. The posterior abdomen of the chelicerates, a phylum including horseshoe crabs, :wikipedia:spider, :wikipedia:acari.

The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata ( arachnids, horseshoe crabs and others). Although it is similar in most respects to an abdomen (and is often referred to as such), the opisthosoma is differentiated by its inclusion of the respiratory organs ( book lungs or book gills) and the heart.