One of the main tenets of evolution is to "survive long enough to produce viable offsprings". To ensure this, we have witnessed bizzare adaptations, e.g male birds of paradise, the peacock etc.
This undersea tubeworm beats it all. The male is basically nothing more than a packet of sperm living in the female's body.
See this National Geographic article.
Previously mentioned here, 30th July 2004: "Osedax rubiplumus and Osedax frankpressi that feed on the bones of dead whales. No eyes, legs, mouths, or stomachs, but colorful feathery plumes and green "roots.""