No Coelom (Acoelomate)
Lacking a fluid filled body cavity presents some serious disadvantages. Fluids do not compress, while the tissue surrounding the organs of these animals will compress. Therefore, acoelomate organs are not protected from crushing forces applied to the animal’s outer surface.
Organisms showing acoelomate formation include the platyhelminthes (flatworms, tapeworms etc.), the cnidarians (jellyfish and allies), and the ctenophores (comb jellies). The coelom can be used for diffusion of gases and metabolites etc. These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorso-ventral flattening.
Read more about this topic: Body Cavity