Most bilaterian animals excrete toxic metabolites through specialized organs, such as nephridia and kidneys, which share morphological and functional similarities and define this major clade as the nephrozoa. In contrast, the excretory mechanisms in other (non-nephrozoan) animals are largely unknown, making the reconstruction of ancestral excretory mechanisms problematic. In this study, Andrikou et al. investigated the excretory mode of members of the Xenacoelomorpha, the sister group to Nephrozoa, and Cnidaria, the sister group to Bilateria. They show that although a cnidarian (a sea anemone) seems to use diffusion as its main excretory mode, two xenacoelomorph animals (an acoel and a nemertodermatid) use both active transport and diffusion mechanisms. The authors propose that digestive-associated tissues functioned as excretory sites before the evolution of specialized organs in nephrozoans. The image shows the localization of the rhesus glycoprotein (an ammonia transporter; magenta) in syncytium-affiliated cells in the gut of the acoel worm Isodiametra pulchra. The image is a projection of merged confocal stacks, also showing microtubules (green) and nuclei (blue). Andrikou et al.

Image Credit: Carmen Andrikou