The authors present the first TEM observations on the head organ, a multicellular structure housing solid inclusions discovered in 1996 by Rota & Erséus in the proximity of the brain in Grania. The cellular components, their spatial organisation and their relationships to the inclusions are described. The ultrastructural data confirm the sensory nature of the organ and corroborate its interpretation as a georeceptor. There are four receptor cells, each giving off a whorl of 50 cilia with (8+1)×2 microtubular pattern and no rootlets. The solid inclusions are opaque to the electrons and have the appearance of amorphous aggregations. They contact the sensory cilia within separate extracellular compartments lined by extensions of glial cells, whose perikarya are located on the lateral aspects of the organ. The head organ is unprecedented in annelids and its structure is probably unique among invertebrates. Cells with similarly modified cilia were known in the central nervous system of enchytraeids and also occur in Grania in the region between the head organ and the cerebral ganglion proper (here called “rostral territory"). The derivation of the head organ from this type of ciliated cells is suggested. Original light microscopy observations on the central nervous system of enchytraeids are also included.