Though I have a lot of affection for Parasyte, my experience is colored by having been a huge fan of the manga for years. I recognize that adaptations need to make changes and that doesn’t bother me. There are some things that I think were improved by having done so, but at the same time, there are others that absolutely did not bother me in the manga that just felt flat when translated into the animated medium.
Parasyte is the story of Shinichi Izumi, a high school senior who is the victim of a failed attempt by a parasyte organism to take over his brain. Instead of burrowing into his brain, the parasyte landed in his hand and matured partially up his arm. The result is that he and Migi, the parasyte, are two sentient beings that share one body, with Migi controlling Shinichi’s right hand and arm while he is awake.