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This is a pre-print version of an article published in Organisms Diversity and Evolution. The final authenticated version is available online at https://link.springer.com/article/10.1007%2Fs13127-020-00447-y Open access read-only publisher version of the article can be found here: https://rdcu.be/ciNpI (Provided by the Springer Nature SharedIt content-sharing initiative) The post-print has open access through the first author's web page https://mariaherranzm.com/publications This project received funding from the European Union's Horizon 2020 research and innovation programme, under the Marie Sklodowska-Curie grant agreement No 797140 Abstract Comprehensive morphological investigations covering the diversity of metazoan lineages are needed to obtain a complete picture of organ system evolution. Despite the increased amount of studies on lesser-known phyla during the last decades, the gap in knowledge for these lineages is still remarkable. This is the case for kinorhynchs, or mud dragons, where only a few genera, mainly belonging to the major clade Cyclorhagida, have undergone detailed morphological examinations. Here, we explore the neuroanatomy and myoanatomy in the other major clade, Allomalorhagida, through studies of Dracoderes abei and Pycnophyes ilyocryptus by immunochemistry, CLSM, and computational 3D reconstruction. The current phylogenetic position of Dracoderes as a sister group to all the remaining allomalorhagids makes it a key taxon for understanding the evolution of organ systems within Kinorhyncha. Clear segmental arrangement of muscles and nerves was found in the trunk of D. abei and P. ilyocryptus, excluding modifications observed in the posteriormost segments. When comparing current and previous studies of allomalorhagids and cyclorhagids, the nervous system shows a conserved pattern across kinorhynchs, while the musculature shows significant variation among genera. Segmentation in kinorhynchs is restricted to the trunk, including mesoderm (muscles) and ectoderm derivated structures (nerves ...

This is a pre-print of an article published in Organisms Diversity and Evolution. The final authenticated version is available online at: https://link.springer.com/article/10.1007/s00435-021-00519-3 The post-print has open access through the first author's web page https://mariaherranzm.com/publications Open access to read-only published article: https://rdcu.be/ciOg9 (Provided by the Springer Nature SharedIt content-sharing initiative) This project received funding from the European Union's Horizon 2020 research and innovation programme, under the Marie Sklodowska-Curie grant agreement No 797140 Abstract Aberrant kinorhynchs show several modifications deviating from the typical kinorhynch body plan, including a modified introvert with very elongated and flexible scalids, a weakly developed neck, and a slender trunk with less distinct segmentation. How these aberrant external features are reflected in the inner anatomy and how their aberrant body plan evolved are not understood. Here, we provide a comprehensive and comparative myoanatomical study of three putatively, distantly related worm-like species: Cateria styx, Franciscideres kalenesos and Zelinkaderes yong. Despite the weak external segmentation of the trunk, the studied species show a distinct segmental arrangement of the musculature. However, this arrangement is shifted posteriorly with respect to the external segmentation, because the extremely thin and flexible cuticle is lacking the apodeme-like cuticular thickenings (pachycycli) where the longitudinal muscles usually attach. The muscular arrangement in the three species is overall similar, yet, C. styx shows most resemblance to the allomalorhagid F. kalenesos, whereas the cyclorhagid Z. yong differs in several ways. This suggests a closer relationship of C. styx to Allomalorhagida. Whereas most kinorhynchs prefer muddy sediments, both the allomalorhagid and cyclorhagid worm-like kinorhynchs are mainly found in sandy environments, suggesting that a flexible, slender body evolved at least twice ...