An account of a light and electron microscope study of the setae of Disoma multisetosum, Pectinaria belgica (Polychaeta sedentaria) and Echiurus echiurus (Echiuroidea) is given. The results are compared with those of previous researchers, and the possibility of using the ultrastructure of the "annelid seta" as a phylogenetic-systematic instrument is preliminarily discussed.