In which group in the course of evolution did the excretory system first appear?

The excretory system first appeared in flatworms. Flatworms are non-segmented invertebrates with a soft body, without body cavities and specialized organs of blood circulation and respiration. Thus, oxygen and nutrients pass through the body through a diffusion process. Most of the known flatworm species are parasitic and some can cause significant harm to humans and livestock. Flatworms are common in undercooked foods and foods imported from high-risk areas.