Lophotrochozoan neuroanatomy: An analysis of the brain and nervous system of Lineus viridis(Nemertea) using different staining techniques

Abstract

BackgroundThe now thriving field of neurophylogeny that links the morphology of the nervous system to early evolutionary events relies heavily on detailed descriptions of the neuronal architecture of taxa under scrutiny. While recent accounts on the nervous system of a number of animal clades such as arthropods, annelids, and molluscs are abundant, in depth studies of the neuroanatomy of nemerteans are still wanting. In this study, we used different staining techniques and confocal laser scanning microscopy to reveal the architecture of the nervous system of Lineus viridis with high anatomical resolution.ResultsIn L. viridis, the peripheral nervous system comprises four distinct but interconnected nerve plexus. The central nervous system consists of a pair of medullary cords and a brain. The brain surrounds the proboscis and is subdivided into four voluminous lobes and a ring of commissural tracts. The brain is well developed and contains thousands of neurons. It does not reveal compartmentalized neuropils found in other animal groups with elaborate cerebral ganglia.ConclusionsThe detailed analysis of the nemertean nervous system presented in this study does not support any hypothesis on the phylogenetic position of Nemertea within Lophotrochozoa. Neuroanatomical characters that are described here are either common in other lophotrochozoan taxa or are seemingly restricted to nemerteans. Since detailed descriptions of the nervous system of adults in other nemertean species have not been available so far, this study may serve as a basis for future studies that might add data to the unsettled question of the nemertean ground pattern and the position of this taxon within the phylogenetic tree.

The Nervous Systems of Basally Branching Nemertea (Palaeonemertea)

“…The heteronemertean L. viridis has paired dorsal and ventral lobes, each of which is more or less completely surrounded by a matrix and thus a clearly identifiable morphological unit [24]. Identification of these lobes is difficult, if such an extracellular surrounding is missing.…”

“…In recent years, neuroanatomy, by means of immunohistochemistry, has yielded promising data that contributes to unraveling the phylogeny of certain metazoan groups, e.g [19], [20], [21], [22], [23]. The only representative of nemerteans investigated with this method so far, Lineus viridis [24], probably shows derived characters and data concerning basal branching nemerteans is still lacking. Studies on the phylogeny of Nemertea and the relationship they share with other lophotrochozoans need to focus particularly on basally branching nemertean taxa.…”

Genealogical Correspondence of Mushroom Bodies across Invertebrate Phyla

“…Rather, the arrangement of globuli cells and the initial segments of parallel fibers outside what is interpreted as the blood-brain barrier suggest that the distal parts of the intrinsic neurons directly receive information about the chemical composition of the immediate environment. Nemerteans, Heteronemertea Nemerteans (roundworms) are unsegmented, predatory animals possessing complex brains, comprising of a pair of dorsal and ventral lobes that lie flat in the head capsule [25]. In Lineus viridis, a structure called the cerebral organ, which contains tightly packed somata corresponding to globuli cells, is attached to each dorsal lobe posteriorly.…”

Unravelling theLineus ruber/viridisspecies complex (Nemertea, Heteronemertea)

“…and references therein; McDermott ; von Döhren & Bartolomaeus , ; Beckers et al . ; von Döhren ; von Döhren et al ; Martín‐Durán et al . ).…”