Cycloneuralia (Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera)

Abstract

The taxon Cycloneuralia includes Nematoda, Nematomorpha, Priapulida, Kinorhyncha and Loricifera. The name-giving autapomorphy of this taxon is the so-called cycloneuralian brain. In this type of brain the neuropil forms a ring around the anterior intestinal system, while the somata of the brain nerve cells form clusters anterior and posterior of this ring neuropil. In the ventral region of the brain, a paired or unpaired ventral nerve cord arises. Further longitudinal neurites or neurite bundles, as well as entire or partial circular neurites connecting the longitudinal ones, are present in most taxa. This can be interpreted as a remnant of an orthogonal pattern in the trunk region. In nematodes, the nervous system is postulated to express little variation throughout the taxon, but there is an extreme heterogeneity in the intensity, with which nervous systems are known. For the ‘model nematode’ Caenorhabditis elegans, each single neuron is known, while data especially from free-living nematodes are lacking. Available data support nematodes having a comparable general architecture of the nervous system, but some variation in detail. Nematomorphs show the greatest differences in the nervous system, probably due to their parasitic mode of life during most of the life cycle. Although available data do not correspond with any one particular shape of the brain, it certainly does not form a ring of equal thickness and is therefore not cycloneuralian. Scalidophora (Priapulida, Kinorhyncha, Loricifera) have a cycloneuralian brain and correspond in the innervation of the scalids representing cuticular structures on the introvert that include receptor cells.

Assessing segmental versus non-segmental features in the ventral nervous system of onychophorans (velvet worms)

“…Yang et al [7] hypothesised that the ancestral ecdysozoan possessed an unpaired ventral nerve cord (bottom left diagram in Fig. 11), which was retained in most cycloneuralians, a subclade of ecdysozoan animals [72]. Therefore, a separation of the ventral nerve cord would have occurred in the LCA of panarthropods, as all three panarthropod subgroups show paired ventral nerve cords [3, 11, 50], as well as independently within the cycloneuralian subgroup Loricifera [73, 74].…”

Reconstructing the anterior part of the nervous system of Gordius aquaticus (Nematomorpha, cycloneuralia) by a multimethodological approach

“…As general markers of neural structures, we used three different antibodies, which are all well established in invertebrate neuroscience and known to label their corresponding antigen (e.g., Hartenstein, ; Schmidt‐Rhaesa and Henne, ). Two antibodies both labeling α‐tubulin, one of the major elements of axonal processes were used in a 1:1 combination to increase the intensity of labeling (Gozes, ; Mayer et al, ).…”

“…As terms and definitions of particular structures within the nervous system vary among different authors (e.g., “ganglion”), we will follow the neuroanatomical nomenclature based on the glossary by Richter et al () as well as Schmidt‐Rhaesa and Henne ().…”

“…The few available descriptions of Nematomorpha nervous systems are mostly based on histological or ultrastructural studies. Common features are a brain in the anterior head region and a prominent ventral nerve cord (VNC) that emerges from the posterior part of the brain (Schmidt‐Rhaesa and Henne, ). In Nectonema species, a second, thinner longitudinal neurite bundle is present dorsally (called “dorsal nerve cord”; Schmidt‐Rhaesa, ).…”

“…However, relationships within Ecdysozoa are still in debate and so far, molecular investigations do not support a monophyletic taxon Cycloneuralia (e.g., Dunn et al, ; Hejnol et al, ; Paps et al, ). From a morphological point of view, the structure of the brain (a uniform and equally thick ring‐shaped neuropil surrounding the esophagus/pharynx with clusters of associated somata anterior and posterior to the neuropil, see Richter et al, ) speaks in favor of Cycloneuralia as a monophylum (for detailed reviews see Schmidt‐Rhaesa and Rothe, ; Schmidt‐Rhaesa and Henne, ), with only the nematomorph brain being different in structure, which may be an adaption to the parasitic lifestyle.…”