This is a question regarding my little critters that hijack of their victim. By doing so they get direct access to a steady flow of nutrients through the bloodstream and get shelter with optimal temperature and humidity with the added protection of a ribcage. What's terrifying about these little bugs is that you can't get rid of them, because they've highjacked the one organ that keeps you alive.
What kind of physiology would a heart parasite have?
This question is not asking "would this evolve" as this creature is already very complex, it performs a risky procedure as part of its life cycle on mere instinct, which is not something your typical leech or tick does. A more pertinent question is what kind of body they should have to make this work? They must first burrow into a host without inflicting a life threatening injury. Then they must attach to a still beating heart and drain sustenance through the blood. They might go through multiple life stages like a larva for burrowing and a crab for latching on the heart.
What do you think?
Edit: The definition of hijack is "seizing" something, in this case that would be the heart. The parasite does not replace the heart itself, but laches onto it.