In Annelida, as well as in other invertebrate taxa, the nervous system is considered to be a very conservative organ system. Immunohistochemical investigations [use of anti-5-HT (serotonin), FMRFamide, and acetylated α-tubulin antibodies] in combination with laser scanning microscopy enable more detailed reanalyses of known structures and detection of new characteristics that are useful for phylogenetic analyses. One hypothesis enabled by such studies is outlined for the evolution of arrangements of the dorsal circumesophageal roots in polychaetes and oligochaetes. These roots are not a unique feature of polychaetes; they also occur in oligochaetes. According to the Articulata hypothesis of metazoan relationships, the specific structure of the rope-ladder-like nervous system is, among others, an autapomorphic characteristic that unifies Annelida and Arthropoda. Recent studies applying the techniques mentioned here, however, demonstrate that the annelidan bauchmark (central nervous system of the trunk), in contrast to the arthropod pattern, is highly variable in terms of the number and position of connectives and the number of commissures per segment. The variability of the neuronal architecture as well as a hypothesis on how it evolved will be introduced with the aid of regeneration and developmental studies. Furthermore, it is shown that hitherto unknown nerves are present in the peripheral nervous system.