English: (a) Illustrated phylogenetic relationship between Nephrozoa, Xenacoelomorpha, and non-bilaterians [13]. Excretory organs or specialized excretory cells/tissues using active transport and ultrafiltration are so far only reported in the group of Nephrozoa. (b) Cartoon depiction of the structural components of metanephridia (podocyte, duct, tubule) in comparison to protonephridia (terminal cell, duct, tubule) and summary of the expression domains of orthologous selected genes in relation to their components. (c) NH3 cellular transport. NH3 is secreted into the lumen fluid via parallel H+ and NH3 transport. This involves passive diffusion through the cell membrane (dashed lines), facilitated diffusion via the Rh, active transport via the NKA, the hyperpolarization-activated cyclic nucleotide-gated HCN, and AQ as well as the generation of H+ gradient by a v-ATPase and the CA, which transforms CO2 into H+ and HCO3−. Vesicular ammonia-trapping mechanism is also illustrated. at, active transport; AQ, aquaporin transporter; CA, carbonic anhydrase; cd2ap, CD2-associated protein; HCN, K+[NH4+] channel; NH3, ammonia; NKA, Na+/K+[NH4+] ATPase; Rh, Rhesus glycoprotein; slc, solute carrier transporter; ul, ultrafiltration; v-ATPase, vacuolar H+-ATPase proton pump; zo1, zonula occludens 1. Animal depictions are from phylopic.org and are not copyright protected (Public Domain Mark 1.0 license).