What structures have parasite worms lost? What structures/organs are expanded?

Explanation:

Something that is lacking, is a circulatory system. Instead of a typical circulatory system flatworms exchange oxygen with the air outside through diffusion. This is the process by which gases will pass through a membrane from an area of high concentration to an area of low concentration.

from my research i have to the conclusion that decomposers, such as fungi, bacteria and invertebrates, play a critical role in nutrient cycling and without them the earth's ecosystem may not support life as we know it. elements such as carbon, nitrogen and phosphorus exist in limited amounts on earth, although huge amounts of carbon are being released into the atmosphere from underground sources now by humans. it is still extremely important that other nutrients be retained by the process of re-use. decomposers are organisms that are able to chemically break down dead material and make these elements available for use by other organisms. without this breakdown of dead material, dead bodies would pile up. so decomposers also play a role in clean up also. without them, life might not continue as we know it and dead material would overwhelm the earth.