Jaw shedding and replacement of Diopatra aciculata (Onuphidae, Eunicida), by the same process as arthropod molting, involving apolysis and ecdysis, is described here. These observations suggest that molting has either evolved convergently in eunicidan polychaetes and ecdysozoans or it was present in the last ecdysozoan/lophotrochozoan common ancestor and thus may not represent a synapomorphy of the ecdysozoans.