Nemertea is an undoubtedly monophyletic group of vermiform unsegmented spiralians. Most species are marine, inhabiting a wide range of interstitial, benthic, or pelagic habitats. There are some representatives that have invaded limnic or moist terrestrial environments. To date, about 1280 species have been described . Nemerteans possess a unique structure, the eversible proboscis, to catch and intoxicate their prey organisms. Most benthic nemerteans hunt actively at night at low tide pursuing their prey animals by following them in their tracks [2–4]. For this purpose they use a number of different sensory organs which are mainly situated in the frontal region of the animals . The most conspicuous sensory organs are the cerebral organs. These spherical structures are closely associated with the brain and have been demonstrated to play a role in chemoreception [2, 6].
Descriptions of the gross anatomy of the central nervous system of nemerteans were first made in the late 19th and early 20th century. According to these authors, the central nervous system of nemerteans consists basically of a pair of cerebral ganglia and a pair of lateral nerve cords. The cerebral ganglia are arranged as dorsal and ventral lobes which are interconnected by a dorsal and a ventral commissure [7–9]. The cerebral ganglia thus enclose the anterior portion of the rhynchocoel.
Due to morphological characters like the acoelomate body organization, the architecture of the nervous system, the sense organs, and the protonephridial excretory structures, Nemertea were traditionally placed close to Platyhelminthes . In contrast, the fate of the trochoblast cells gives some evidence for including nemerteans into Trochozoa . Moreover, recent molecular studies have produced ambiguous results. Even though none of the molecular based studies found support for a relationship between Nemertea and Platyhelminthes, the placement of Nemertea within Lophotrochozoa varies between different studies [12–17]. Therefore, additional data are necessary to unravel the phylogenetic position of nemerteans.
Searching for novel characters, one promising structure is the nervous system. The methodological backbone of a discipline, that is now being termed "neurophylogeny", has been outlined in a number of publications [e.g. [18, 19]. In the last decade neuroanatomical characters have already been used successfully for the inference of phylogenetic relationships within the arthropods [20, 21]. Recently, the neuroanatomy of various lophotrochozoan taxa has been studied using immunohistochemical methods [22–28]. Even though immunohistochemical investigations of the larval nervous system of nemerteans have been published [29–31], actually no data are available for adult nemerteans.
In the present study, we revealed the structure of the central and peripheral nervous system of the nemertean Lineus viridis using antibodies directed against FMRFamide and serotonin. These two antisera are known to label subsets of neurons in all major animal clades and are frequently used in neuroanatomical studies across the animal kingdom. Therefore, these markers facilitate the comparison of nemerteans to other taxa. Since one aim of this study is to describe the nervous system of a representative of nemerteans in detail, we also used DAPI nuclear labelings and the classical histological Azan staining method to obtain a complete view of the nervous system.