(a) The posterior and often elongated region of the body of an arthropod, behind the thorax or cephalothorax; e.g. the tagma following the thorax of a crustacean, including the telson, and consisting of seven or fewer segments; or the complete, usually unsegmented part of the body of an arachnid or merostome, following the cephalothorax. Cf: opisthosoma; pygidium. (b) The third joint of the shell of a nasselline radiolarian. Pl: abdomens or abdomina. Adj: abdominal.