How does the structure of organisms change during the transition to parasitism?

Parasitism is a type of relationship between organisms, one of which benefits, the other harms. The parasite does not kill its host, because it is unprofitable for him. These creatures have gone through a long evolutionary path to adapt to a new habitat – constantly increased acidity of the environment, defense mechanisms of the host-carrier, competition. To survive in these conditions, parasites are maximally simplified in almost all organ systems. The only thing that is well developed in them is the reproductive system; since only a fraction of the eggs of the parasites can survive, the body must produce a large number of offspring. The parasites have atrophied sense organs – sight and hearing are useless for these creatures. They have a dense skin shell and special devices – suction cups – necessary to stay in the host’s body.