Look here. An American marine ribbon worm is giving birth.
Honestly, it doesn't look terribly dramatic: there's a little writhing, and a miniature Prosorhochmus americanus crawls out of its mother's anus and heads off on its merry way. But for these animals, giving birth through the anus is the least weird part of their reproductive process.
First, although this particular worm is perfectly capable of mating with another member of its species, most of the time it makes babies by putting its own sperm on its own eggs.
Second, because those eggs never leave the ovaries, they get fertilised in there and the baby worms — up to three at a time — grow inside the organ. And when the babies are ready to be born, they have a problem. According to Clint Turbeville, an invertebrate biologist at Virginia Commonwealth University, the worms don't have a dedicated reproductive canal that would let the babies out of the ovary.
Instead, explains Serena Caplins, a doctoral student at UC Davis who has studied these animals, each baby worm pushes first through the wall of the ovary and then through its mother's intestinal wall. "Once it's in its mum's intestine, it swims out through its mum's anus."