Imagine you're a crab. You're lingering on the shoreline, the warm sun on your back, cool water in your gills. You've reached a large size, dodging the many dangers of youth. Life is going well. But today you begin to feel strange as if there is something growing inside you, and in fact, something is.
Its roots are crawling through your tissue, your gut, your brain. It's a rhizocephalan barnacle, and it's about to take you over.