evolution theory, absolute crap I'm sure, very horror
the notion of human sentience being the result of a partially distributed intelligence resulting from tight biointegration (think fishes with worms for tongues, kids, there's never one real example of anything ever) with a arthropod parasite co-ancestor that eventually also integrated into our reproduction, and so tentacle stomachs from hell start tearing out of people suddenly a la alien when they're of certain ages
re: evolution theory, absolute crap I'm sure, very horror
This sort of thing is a part of nature, and a part of our shared evolutionary history: we know that mitochondria are descendants of ancient symbiotic bacteria related to existing bacterial parasites, and that substantial parts of our genome originate from ancient viruses co-opted into functional roles.
There are actually worms with their nutrition and excretion systems entirely replaced by symbiotic bacteria.