Tapeworm
Roundworms have no formal respiratory system in there bodies. Thsi is mainly because they are such simple organisms. Roundworms undergo diffusion do move substances in there bodies around. The pressure from constant moving also allows there body to circulate substances.
Tapeworm
Tapeworms use diffusion to get oxygen to the blood in it's body.
EarthWorm
Earthworms also have no respiratory system. They breath thorugh there skin which must be kept moist at all times.Earthworms bodies release mucous and body fluid in order to keep there skin moist if it dries it can not undergo diffussion.