You're a crab. An ordinary crab, going about your business, frolicking in the surf, causing no one any harm except what little scuzzy creatures you hoover up to round out your diet. We all eat someone else. Down the hatch. It's over in a flash.
By chance, purely by chance, a female larva of a certain species of barnacle lands on your back and attaches herself. Unlucky for you, she has found a soft spot in your armor. She stabs through with a hollow needle that is part of her anatomy. Discarding unnecessary bits and pieces of her body, she injects her internal cell mass, hypolike, and, having accomplished her purpose, the rest of her body falls away. The injected cells -- her essential self -- find their way to your abdomen and grow a web of tendrils that soon invade every part of your own anatomy, like a fungus that feeds on rotting wood, except that you are very much alive. You are now eating for two, yourself and your uninvited guest.
But this was a female barnacle, remember, who in her new incarnation lives inside. She needs a mate. She opens a hole to the outside world and waits for a male larva of the same species to happen by. He enters, then seals the hole to keep out other males. (How preciously we hoard our paternity.) In the trysting bower of your body he fertilizes his mate's eggs, or rather he is absorbed by his mate, and she, using him, does the fertilizing herself. She disgorges thousands of larvae, male and female.
You are doomed. Your good old days are finished. No more molting, no more sex, no more crabby fun. From now on everything you do is devoted to the jostling crowd of barnacles that teem inside. Even your own reproductive apparatus has been hijacked. Now, when you should be releasing your own progeny into the sea, it is barnacle larvae that you expel and send on their way.
I know, I know. It isn't fair. A barnacle, for heaven's sake! A barnacle that doesn't even look like a barnacle. Or behave like a barnacle. Selfish genes for whom a nice crustacean like you is just an opportunity. You never had a chance.
It is a stretch to imagine how all of this might have evolved by natural selection. If ever there was an argument for Design, this might be it. But before you jump to that conclusion, dear little crab, you might ask yourself why a Loving Designer so cruelly excluded you from his benevolence.