Invertebrate nervous systems are highly disparate between different taxa. This is reflected in the terminology used to describe them, which is very rich and often confusing. Even very general terms such as 'brain', 'nerve', and 'eye' have been used in various ways in the different animal groups, but no consensus on the exact meaning exists. This impedes our understanding of the architecture of the invertebrate nervous system in general and of evolutionary transformations of nervous system characters between different taxa. We provide a glossary of invertebrate neuroanatomical terms with a precise and consistent terminology, taxon-independent and free of homology assumptions. This terminology is intended to form a basis for new morphological descriptions. A total of 47 terms are defined. Each entry consists of a definition, discouraged terms, and a background/comment section. The use of our revised neuroanatomical terminology in any new descriptions of the anatomy of invertebrate nervous systems will improve the comparability of this organ system and its substructures between the various taxa, and finally even lead to better and more robust homology hypotheses.

/pdf/invertebrate-neurophylogeny-suggested-terms-and-definitions-2942dww1nc.pdf

Invertebrate neurophylogeny: suggested terms and definitions for a neuroanatomical glossary

The Articulata hypothesis – or what is a segment?

The trochophora concept and the literature on cleavage patterns and differentiation of ectodermal structures in annelids (“polychaetes”) and molluscs are reviewed. The early development shows some variation within both phyla, and the cephalopods have a highly modified development. Nevertheless, there are conspicuous similarities between the early development of the two phyla, related to the highly conserved spiral cleavage pattern. Apical and cerebral ganglia have almost identical origin in the two phyla, and the cell-lineage of the prototroch is identical, except for minor variations between species. The cell-lineage of the metatrochs is almost unknown, but the telotroch of annelids and the “telotroch” of the gastropod Patella originate from the 2d-cell, as does the gastrotroch in the few species which have been studied. The segmented annelid body, i.e. the region behind the peristome, develops through addition of new ectoderm from a ring of 2d-cells just in front of the telotroch. This whole region is thus derived from 2d-cells. Conversely, the mollusc body is covered by descendants of cells from both the C and D quadrants and a growth zone is not apparent. This supports the notion that the molluscs are not segmented like the annelids, and that the repeated structures seen in polyplacophorans and monplacophorans do not represent a segmentation homologous to that of the annelids. J. Exp. Zool. (Mol. Dev. Evol.) 302B: 35–68, 2004. © 2004 Wiley-Liss, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jez.b.20001

Trochophora larvae: cell-lineages, ciliary bands, and body regions. 1. Annelida and Mollusca.

High-resolution fate map of the snail Crepidula fornicata: the origins of ciliary bands, nervous system, and muscular elements.

Despite the large variation in adult bodyplan phenotypes, a worm-shaped morphology is considered plesiomorphic for both Lophotrochozoa and Bilateria. Although almost all larval and adult lophotrochozoan worms have serially arranged ring muscles in their body wall, a comparison of their ontogeny reveals no less than six different developmental pathways that lead to this homogenous arrangement of ring muscles. However, in all taxa, with the exception of chaetodermomorph molluscs and the segmented annelids, ring muscle development starts with synchronous formation of certain pioneer myocytes, which is thus considered basal for Lophotrochozoa. Recent studies on spiralian neurogenesis revealed remnants of ancestral segmentation in echiurans and sipunculans, thus confirming molecular phylogenetic studies that propose a close relationship of these three taxa. Larval entoprocts exhibit a mosaic of larval and adult molluscan characters and, among other apomorphies, share with polyplacophoran Mollusca a complex larval apical organ and a tetraneurous nervous system, strongly suggesting a monophyletic assemblage of Entoprocta and Mollusca. The term Tetraneuralia is proposed herein for this lophotrochozoan clade. Overall, formation of the lophotrochozoan neuromuscular bodyplan appears as a highly dynamic process on both the ontogenetic and the evolutionary timescales, highlighting the importance of insights into these processes for reconstructing ancestral bodyplan features and phylogenetic relationships.

https://www.journals.uchicago.edu/doi/pdf/10.1086/BBLv216n3p293

Shaping the Things to Come: Ontogeny of Lophotrochozoan Neuromuscular Systems and the Tetraneuralia Concept

Neurogenesis in the chiton Mopalia mus- cosa (Gould, 1846) was investigated by applying differen- tial interference contrast microscopy, semithin serial sec- tioning combined with reconstruction techniques, as well as confocal laser scanning microscopy for the detection of fluorescence-conjugated antibodies against serotonin and FMRFamide. The ontogeny of serotonergic nervous struc- tures starts with cells of the apical organ followed by those of the cerebral commissure, whereas the serotonergic pro- totroch innervation, pedal system, and the lateral cords develop later. In addition, there are eight symmetrically arranged serotonergic sensory cells in the dorsal pretro- chal area of the larva. FMRFamide-positive neural ele- ments include the cerebral commissure, specific "ampul- lary" sensory cells in the pretrochal region, as well as the larval lateral and pedal system. In the early juvenile the cerebral system no longer stains with either of the two antibodies and the pedal system lacks anti-FMRFamide immunoreactivity. Outgroup comparison with all other molluscan classes and related phyla suggests that the cord-like, nonganglionized cerebral system in the Polypla- cophora is a reduced condition rather than a primitive molluscan condition. The immunosensitivity of the pedal commissures develops from posterior to anterior, suggest- ing independent serial repetition rather than annelid-like conditions and there is no trace of true segmentation during nervous system development. Polyplacophoran neurogenesis and all other available data on the subject contradict the idea of a segmented molluscan stem spe- cies. J. Morphol. 253:109 -117, 2002. © 2002 Wiley-Liss, Inc.

Neurogenesis in the mossy chiton, Mopalia muscosa (Gould) (Polyplacophora): evidence against molluscan metamerism.

Towards a sound definition of Skeneidae (Mollusca, Vetigastropoda): 3D interactive anatomy of the type species, Skenea serpuloides (Montagu, 1808) and comments on related taxa

\n The genus Lodderena Iredale, 1924 has been classified in the Skeneidae by most recent authors. However, this family, originally characterized by their minute size, lack of nacre in the teleoconch and a rhipidoglossate radula, is currently considered to be polyphyletic assemblage, and preliminary molecular systematic data suggest exclusion of Lodderena from Skeneidae. In order to shed light on the systematic position of this genus, we provide a detailed description of the anatomy and histology of the type species, Lodderena minima (Tenison-Woods, 1878), and of L. ornata (Olsson & McGinty, 1958). The anatomical data confirm the vetigastropod-trochoid nature of Lodderena but exclude the genus from Skeneidae. Skeneidae are mainly characterized by a propodial penis, a hermaphroditic condition with separated testis and ovary, and a large receptaculum in the mantle roof. In contrast, Lodderena species lack both a copulatory organ and receptaculum, and have a true ovotestis. We also provide a critical review of nominal species in the genus. Based on molecular and morphological evidence, we exclude Lodderena from Skeneidae, instead treating it as a genus incertae sedis within Trochoidea. We discuss the implications of small size for functional morphology and reproduction.

/pdf/skeneimorph-but-not-skeneid-systematics-and-anatomy-of-nkwnmmzqfz.pdf

Martin Brückner

Papers

Neurogenesis in the mossy chiton, Mopalia muscosa (Gould) (Polyplacophora): evidence against molluscan metamerism.

Towards a sound definition of Skeneidae (Mollusca, Vetigastropoda): 3D interactive anatomy of the type species, Skenea serpuloides (Montagu, 1808) and comments on related taxa

Skeneimorph but not skeneid: systematics and anatomy of Lodderena Iredale, 1924 (Vetigastropoda: Trochoidea)