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The ecomorphological guild “gastromyzophorous” joins tadpoles that inhabit flowing water and have an abdominal sucker which is employed to adhere to substrates. Historically, gastromyzophorous larvae were known in the Bufonidae and Ranidae, but a new sucker-bearing hylid tadpole was recently described from phytotelmons in Brazilian forests. We describe the larval internal anatomy of Phyllodytes gyrinaethes and ask whether its exceptional external morphology is accompanied by derived anatomical internal features that can be related to the special habitat. We also compare it to the anatomy of sucker-bearing tadpoles from other families with a focus on characters exclusive of each lineage and the shared, convergent features. The skeleton of P. gyrinaethes is highly modified relative to that of pond-type hylines and shows a profound restructuring of the oral region, palatoquadrates, and the branchial baskets. Among the muscles, besides the overall reduction in the branchial musculature, the most unusual feature in this species are the enormous, anteriorly oriented mm. levatores mandibulae externus profundus that likely produce the abduction of the two halves of the snout. The presence of the abdominal sucker is coupled with changes in some muscle trajectories and hypertrophy of the subhyoid ligaments, and the sucker connectivity differs in some aspects compared with those of bufonids and ranids (e.g., the presence of massive mm. diaphragmatopraecordialis parallel to the sucker plane). P. gyrinaethes tadpoles, with their combination of both rare and unique morphological features plus their confined microhabitat with exceptional functional and ecological requirements, represent an extreme morphotype within Hylidae and anuran tadpoles in general. Fil: Vera Candioti, María Florencia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - Tucumán. Unidad Ejecutora Lillo; Argentina Fil: Haas, Alexander. Biozentrum Grindel Und Zoologisches Museum Hamburg; Alemania Fil: Altig, Ronald. Mississippi State University; Estados Unidos Fil: Peixoto, Oswaldo. Universidade Federal do Rio de Janeiro; Brasil