AbstractThe entire nervous system of the smallest annelid hitherto known, the dwarf male of the highly dimorphic speciesDinophilus gyrociliatus, has been reconstructed by means of TEM investigations of serial ultrathin sections. Altogether there are 68 neurons, 40 of which have a sensory function. The structure and distribution of them is described. The receptor endings of the 20 sensory cells of each side are located either in two groups — the anterior receptor group and the posterior receptor group — or are singly positioned in the integument. Structural differences of the apical portion of the dendrites enables four types of receptors to be distinguished: three types with emergent cilia and one type with non‐emergent cilia. Neurons with emergent cilia can be monociliated collar cells as well as mono‐ or multiciliated cells without collar. Special vesicle‐in‐vesicle structures, are located close to the basal portion of the cilia in some of these cells. The non‐emergent cilia border closely to a neighbouring epidermal cell and contain a prominent intraciliary vesicle. The function of receptors is discussed with regard to a comparison with receptors in other polychaete species, structural specializations and their distribution pattern on the ani