All of these neurons can also be identified by Golgi impregnations (Figure?3 rightmost). to the retinula (R-cells) axons are characterized by their morphology: five types of descending unipolar neurons and one type of ascending neurons. These cell types are also AZD4017 identified by Golgi impregnations. Mapping of all identifiable chemical synapses AZD4017 indicates that the descending unipolar neurons are postsynaptic to the R-cells and, hence, are second-order neurons. The ascending neurons are predominantly presynaptic and sometimes postsynaptic to the R-cells and may play a feedback role. Conclusions Comparing these results with the compound eye visual system of crustaceans and insects C the only arthropod visual system studied so far in such detail C we found striking similarities in the morphology and synaptic organization of the different neuron types. Hence, the visual system of pycnogonids shows features PBT of both chelicerate median and mandibulate lateral eyes. Electronic supplementary material The online version of this article (doi:10.1186/s12915-014-0059-3) contains supplementary material, which is available to authorized users. and later by Meinertzhagen and O’Neil  in reconstructing synaptic connections in the lamina cartridges of [8,9]. An early attempt to use computerized three-dimensional reconstructions to study the axonal wiring of photoreceptor axons is that by Melzer [11C13]. Furthermore, in recent years, a new generation of three-dimensional-EM tools has been developed [14C16], which includes Serial Block Face Scanning Electron Microscopy (SBF-SEM or simply SBEM) based either on mechanical sectioning [17,18] or milling with a focused ion beam (FIB-SEM, [19,20]). These methods enhanced the potential of three-dimensional-EM considerably and are applied, for example, on nervous tissue [21C23] and to display and count synapses in vertebrates AZD4017 [24C26]. In the present study, we analyze the visual neuropils in the AZD4017 pycnogonid with one of these methods, namely FIB-SEM. The advantages of this cutting-edge method are that compared to serial section TEM, the generation of the image-stack is much faster and without loss, the images are perfectly aligned with a z-resolution down to 5 nm (TEM approximately 70 nm), and the x-y-resolution and contrast compared to TEM are only slightly reduced. The Pycnogonida, or sea spiders, are exclusively marine invertebrates, numbering more than 1,300 species worldwide . Although largely unnoticed due to their cryptic life habits and economic insignificance, sea spiders are common benthic animals occurring from the littoral zone to the deep sea, from tropical to polar waters. The fossil record dates back to the early Paleozoic Era, with the earliest unequivocal records dating back to the Ordovician and Silur [28,29]. It has even been hypothesized that Pycnogonida might date back to the Cambrian . The phylogenetic position of the Pycnogonida has long been controversial and is still under debate. Pycnogonids are placed either within the Chelicerata as sister taxon of the Euchelicerata or as sister taxon of all other Euarthropoda [31,32]. In recent years the debate has shifted in favor of sea spiders being chelicerates [33,34]. Studies of the development and the innervation patterns of the brain show that characters in the nervous program can contribute essential arguments towards the discussion in regards to the phylogenetic placement of ocean spiders [35C39]. Because of this field of analysis in general, looking at the advancement and framework of anxious systems within a phylogenetic framework, two different strategies have been set up: neurophylogeny AZD4017 [40,41] and neural cladistics [42,43]. The sensory elements of the arthropod protocerebrum are in charge of the visual system primarily. Two various kinds of eye are located in arthropods, median and lateral eye. Pycnogonids possess just a periscope-like ocular tubercle with four small eye or ocelli (Richter (Onychophora)  as well as the median rudimentary eyes in (Xiphosura) [51,52]. The architecture from the visual system of sea spiders is easy in comparison to that of several various other arthropods relatively. Taking into consideration the phylogenetic position of pycnogonids as basal chelicerates or arthropods recommended by even.