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.

it also supports my hypothesis. if you rest first you can squeeze the clothespin more times. from the data i collected the average of the eight people concluded exercise makes your muscles more fatigued and less able to squeeze the clothespin compared to resting before squeezing.