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Aplacophora is a small enigmatic group of worm-like molluscs covered by sclerites. The general morphology of the posterior end, including the gonopericardial system, in many species is well studied using light microscopy. However, knowledge of the ultrastructure of this complex is fragmentary and many questions concerning the excretion of metabolites in this group still remain. The gonopericardial system of Chaetoderma nitidulum Lovén, 1844 (Caudofoveata) from the White Sea was studied using light microscopy and 3D reconstruction from a series of semi-thin sections. The ultrastructure of the gonopericardial system was described in detail for the first time. The gonopericardial complex consists of gonads connected to the pericardium by ciliated paired gonopericardioducts. The pair of gonoducts begins at the lateral sides of the pericardium and opens into the mantle cavity. Gonoducts consist of upper and lower parts that are morphologically differentiated. The walls of the upper gonoducts consist of cells that likely participate in the excretion of metabolites and reabsorption of useful substances. Lower gonoducts are bulk bags with a small cell wall, in which there is a ciliary tract, likely for the excretion of metabolites and reproductive products. Spermatozoa were found at different stages of development in different parts of the pericardium and gonoducts. On the basis of previously published and newly obtained data, the process of excretion by C. nitidulum is proposed. [ABSTRACT FROM AUTHOR]

The deep-sea hydrothermal vent solenogaster Helicoradomenia is covered with calcium carbonate sclerites. Light and electron microscopy reveal varying morphologies of these sclerites. Many sclerites have hollow tips and/or are pitted and etched. Bacteria are found on and in sclerites. Initial sclerite formation occurs in an extracellular crystalline chamber formed by the invagination of a cuboidal basal cell of the columnar microvillus mantle epithelium. As the sclerite grows, it fills the crystalline chamber resulting in direct contact with the microvilli of both the basal cell and neighboring secondary sclerite-forming cells. These cells shape a collar around the base of the growing sclerite. As growth continues, the sclerite-forming cells stretch around the sclerite forming a sheath in which the base of the sclerite resides. Mature sclerites grow through the cuticle into the external environment. The erosion pattern of sclerites reveals a less stable inner medullary region and a harder outer cortical region. This points to a secondary character state, where foremost hollow acicular sclerites develop into solid sclerites. This is in agreement with the systematic position of the genus Helicoradomenia within Simrothiellidae, a taxon typically with hollow sclerites. [ABSTRACT FROM AUTHOR]

The nervous system of invertebrates is considered to be a very conservative organ system and thus can be helpful to elucidate questions of phylogenetic relationships. Up to now, comparative neuroanatomical studies have been mainly focused on arthropods, where in-depth studies on major brain structures are abundant. In contrast, except for Gastropoda and Cephalopoda, the nervous system of representatives of the second largest phylum of invertebrates, the Mollusca, is as yet hardly investigated. We therefore initiated an immunohistochemical survey to contribute new neuroanatomical data for several molluscan taxa, especially the lesser known Caudofoveata, Solenogastres, Polyplacophora, and Scaphopoda, focusing on the cellular architecture and distribution of neurotransmitters in the brain. Antisera against the widespread neuroactive substances FMRFamide and serotonin were used to label subsets of neurons. Both antisera were additionally used in combination with acetylated α-tubulin and the nuclear marker DAPI. This enables us to describe the morphology of the nervous system at a fine resolution and to compare its cellular architecture between different species of one taxon, as well as between different taxa of mollusks. On the basis of these results, the nervous system of caudofoveates seems to be most highly derived within the so-called basal (non-conchiferan) mollusks, and a monophyly of a clade Aplacophora could not be confirmed. In general, the brain as well as the remaining nervous system of the molluscan taxa investigated shows a great variability, suggesting a deep time origin of the diversification of this prominent protostome clade. [ABSTRACT FROM AUTHOR]