Active mode of excretion across digestive tissues predates the origin of excretory organs
Fig 4
Evolution of excretory mechanisms.
Illustration of the proposed direction of fluxes in Cnidaria and Xenacoelomorpha and evolution of active ammonia transport and ultrafiltration mechanisms. Cnidaria (e.g., N. vectensis) excrete across their intestinal epithelium (and probably across the epidermis too) via diffusion, whereas in xenacoelomorphs, excretion occurs both via diffusion across the epidermis and gut-associated tissues and via active transport across gut-associated tissues. Ultrafiltration mechanism originated within Nephrozoa. cu, cuticle; me, mesoglea.
doi: https://doi.org/10.1371/journal.pbio.3000408.g004