The urethra is identified by the letter . Human excretory system. Letters from A to E mark definite structures of this system. Letter A marks vessels that nourish a paired bead-shaped organ. Letter B marks a tube that goes from the bean-shaped organ to a sac in the pelvis. Letter C marks the sac in the pelvis. Letter D marks the bean-shaped organ. Letter E marks a tube that goes from the sac in the pelvis outside the body.