![]() Supposedly, sensory ciliated cells in the pharynx and the gut were documented for the first time. Neurites of the stomatogastric nervous system were found lining the pharynx and connecting to a prominent buccal ganglion. Their intra-epidermal, unsegmented nervous systems comprise an anterior brain and three to five ventral and two to four dorsal longitudinal nerves, connected by few transverse commissures. The nervous system of representatives from all major groups of Gnathostomulida was here mapped using confocal laser scanning microscopy and immunohistochemistry. Yet, very few nervous system studies have been conducted on this lineage of microscopic, jaw-bearing worms, limiting our understanding of the evolution of this organ system in Spiralia. Within Spiralia, Gnathifera may represent the deepest branching lineage comprising the jaw worms Gnathostomulida and their sister group Micrognathozoa + Syndermata. This simplicity is secondary and is caused by two factors-the parasitic life style and miniaturization of free‐living sexual stages. linei may present a good example of a highly simplified Bilaterian with fully functioning nervous and muscular systems. Free‐living males and females of the orthonectid I. The general neuromuscular organization corresponds to the pattern of small‐sized annelids, suggesting their possible phylogenetic affinity. Compared to females, the organization of the nervous system is modified and its progenetic origin seems unlikely. Immunohistochemistry revealed six serotonin‐like cells at the anterior part of the body, and two backward lateral longitudinal nerves, merging at the posterior end. The whole muscular system consists of four outer longitudinal and eight pairs of inner semicircular muscle fibres. Here, we describe the musculature and serotonin‐like immunoreactive (SLIR) nervous system of male adults of Intoshia linei (Orthonectida) using immunohistochemistry and confocal laser scanning microscopy. Orthonectida is a small group of parasites, which, according to recent studies, may be phylogenetically close to Annelida. Reduction of the stomatogastric nervous system in the males correlates with the loss of the entire digestive tract and associated morphological structures. We propose that dwarfism of monogonont non-feeding males is the result of a specific case of heterochrony, called “proportional dwarfism” as they, due to their inability to feed, retain a juvenile body size, but still develop a complex neural architecture comparable to adult females. However, the nervous system in males is more compact and lacks a stomatogastric part.Ĭomparison of the neuroanatomy between male and normal-sized feeding females provides a better understanding of the nature of male dwarfism in Monogononta. The general nervous system architecture is similar between males and females and shows a similar level of complexity. Here, we provide a CLSM-based description of the nervous system of both sexes of Epiphanes senta, a freshwater monogonont rotifer. Thus far, male musculature of only two species has been described with confocal laser scanning microscopy (CLSM) and it remains unknown how dwarfism influences the neuroanatomy of males on detailed level. The morphology of monogonont feeding females is relatively well described, however data on male anatomy are very limited. Most monogonont species display cyclical parthenogenesis, where generations of asexually reproducing females are interspaced by mixis events when sexual reproduction occurs between mictic females and dwarf, haploid males. Monogononta is a large clade of rotifers comprised of diverse morphological forms found in a wide range of ecological habitats. Analyses of the structure of all the orthonectids studied so far suggest that reduction and simplification of the free-living males and females are the dominant mode of evolution in orthonectids. Instead, a pair of nerves with three pairs of serially organized nerve cells runs posteriorly from the ganglion along the lateral sides of the body. In males, the cerebral ganglion is significantly smaller, and the body plexus is absent. The nervous system of females is represented by a well-developed cerebral ganglion and a nerve plexus in the body. The muscular system is formed by four outer longitudinal muscular bundles and several inner transversal muscles. Here, we studied the nervous and the muscular systems in the male and female orthonectid Rhopalura litoralis using confocal laser scanning microscopy and immunohistochemistry. Recent molecular studies demonstrated that Orthonectida belongs to Annelida however, the lack of morphological data does not allow to follow the evolutionary pathway from free-living annelids to parasitic orthonectids. Orthonectida is an enigmatic group of parasitic invertebrates with an unclear taxonomic position.
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