It has been suggested instead that the "gill tract" was an organ for breathing air, perhaps actually being a lung, plastron or a pseudotrachea. Plastrons are organs that some arthropods evolved secondarily to breathe air underwater. This is considered an unlikely explanation since eurypterids had evolved in water from the start and they would not have organs evolved from air-breathing organs present. In addition, plastrons are generally exposed on outer parts of the body while the eurypterid gill tract is located behind the "blatfüssen". Instead, among arthropod respiratory organs, the eurypterid gill tracts most closely resemble the pseudotracheae found in modern isopods. These organs, called pseudotracheae, because of some resemblance to the tracheae (windpipes) of air-breathing organisms, are lung-like and present within the pleopods (back legs) of isopods. The structure of the pseudotracheae has been compared to the spongy structure of the eurypterid gill tracts. It is possible the two organs functioned in the same way.