 During evolution , darts appear to have been lost secondarily , i.e. , after they had evolved and been functional . Vestigial darts ( ones that exist only in a rudimentary condition ) occur in the family Sagdidae . , and in many Helicoidea , the surrounding organs have also degenerated ( become non @-@ functional ) . The sarcobelum is a fleshy or cuticle @-@ coated papilla which is considered to be a degenerated , previously dart @-@ bearing , organ . 
