In the shallow waters of the ocean, hiding between the individual grains of sand, live flatworms known as Paracatenula. With neither a stomach nor internal organs, they survive using through a symbiosis with bacteria — in a relationship older than any ever found before. To gain energy, the worms essentially need the bacteria to feed them. The bacteria oxidizes sulfur compounds in the waters around the worms, in a food-sharing relationship generally found only among the rugged life that's rooted in deep sea vents.
Here's the really crazy thing: 33%-50% of the worm's body volume is comprised of bacteria. At that point, you have to start asking "who's inhabiting whom?"
The researchers argue that this symbiotic relationship is at least 500 million years old — and the worms have being passing on the bacteria to their offspring the entire time. That means the relationship was formed when flatworms were first evolving, in the world's oldest known animal-bacteria association.
Image: "This is the flatworm Paracatenula urania from the Belize Barrier Reef in the West Caribbean." by Harald Gruber-Vodicka