part of adult posterior hindgut adult circular muscle cell adult posterior hindgut circular muscle cell fly_anatomy.ontology FBbt:00003539 Muscle cell of the circular muscle of the adult posterior hindgut, located as an outside layer to the inner longitudinal fibers. It is a single layer of thick, closely set circular fibers. Muscular fibers are 3um thick and 10um wide in the rectal sac, and 3-5um thick in the posterior rectum. adult posterior intestine circular muscle cell adult hindgut muscle cell