Most described nemerteans are marine benthic hunters. Thus, they possess a well developed nervous system. Basically, the nervous system comprises a four lobed brain, two main lateral medullary cords, some minor nerves, various nerve plexus and sensory structures. The brain and lateral nerve cords are composed of a central neuropil and a layer of neuronal cell somata, which are homogeneously distributed around the neuropil. Formerly, nemerteans fell into three major taxa, the Palaeo- Hetero- and Hoplonemertea. But this classification has been controversially discussed in recent years. Especially the monophyly of palaeonemerteans is matter of debate. But there is a consensus that Hetero- and Hoplonemertea are monophyletic and form an evolutionary lineage called Neonemertea. In recent years, neuroanatomical studies by immunohistochemistry have yielded promising data to contribute to unraveling the phylogeny of the invertebrate metazoans. For Nemertea, however, only one species has been investigated, which most likely shows derived characters states. Therefore the nervous system of 16 species of different evolutionary lineages of nemerteans was reinvestigated using different techniques. Azan staining was chosen to reveal the overall anatomy of the nervous system, additionally immunohistochemical methods were applied to reveal the details. A careful analysis of the nervous system showed considerable differences in the structure of the peripheral nervous system of even closely related species. A number of 61 characters concerning the morphological elements of the nervous system resulted from this investigation. The analysis yielded one most parsimonious tree. According to this result hoplonemerteans and heteronemerteans are monophyletic and form a monophyletic taxon called Neonemertea. Whether Palaeonemertea are mono- or paraphyletic remains uncertain due to the unknown outgroup conditions. Compared to other recently published results, the nervous system, although being just one organ system, turned out to be a valuable morphological system to infer nemertean phylogeny
